Scientific Reasons for XI

  

1	Stoichiometric calculations are only possible for irreversible reactions
2	Why the actual yield is less than theoretical yield?
3	23 g of sodium and 238 g of Uranium have equal numbers of atoms.
4	–273.15°C is the lowest possible temperature.
5	Rates of diffusion of N2 and CO are same
6	Lighter gases diffuse more rapidly than heavier gases
7	H2 and He are ideal at room temperature and ordinary pressure while SO2 is non-ideal.
8	High pressure and low temperature makes the gas non-ideal/Real gases deviate from ideal behaviour
9	Heat is evolved when a liquid (or gas) condenses.
10	Different gases move with different velocities at the temperature although their K.E is same
11	The surface of a liquid appears like a stretched sheet of rubber trying to contract
12	A freely falling drop of a liquid is spherical.
13	Mercury does not wet glass surface.
14	A liquid (water or organic liquid) rises in capillary tube.
15	Level of mercury falls in capillary tube.
16	Liquid rises in a capillary tube.
17	A drop of ink spreads on blotting paper.
18	Some liquids (e.g. H2O) form concave meniscus while certain liquids (e.g. Hg) form convex meniscus.
19	Water kept in earthen-ware jars keeps cooler.
20	Evaporation is a cooling process.
21	Spilled water evaporates more quickly than the same amount of water in the glass.
22	Glycerine is distilled at reduced pressure
23	Glycerin is more viscous than water
24	Honey is more viscous than water. OR It is easier to pour water than honey
25	A liquid is less viscous at high temperature.
26	At b.p. the temperature of a liquid remains constant in spite of continuous supply of heat
27	A liquid boils at different temperatures at sea level and at mountain./Boiling point of liquid decreases at high altitude
28	Pressure cooker is used for rapid cooking
29	Steam produces more severe burns than does the boiling water
30	Increasing pressure stops on boiling
31	Density of liquids (or gases) decreases with the rise of temperature.
32	Graphite is a good conductor of electricity whereas diamond is not.
33	NaF and MgO are isomorphous compounds
34	Diamond is hard while graphite is soft although both are allotropes of carbon
35	On heating substances like iodine and camphor directly change from solid to gas.
36	In electric or magnetic field, the deflection of b-rays is greater than a-rays.
37	Radioactivity is confined to heavy elements.
38	X-rays are considered as anode rays.
39	Elements give line spectrum
40	Atomic or ionic radii decrease along each period while increase down each group.
41	A cation is smaller in size than its parent atom while an anion is larger in size than its parent atom.
42	In iso-electric ions, the ionic radii decrease with the increase of positive charge or increase with the increase of negative charge.
43	Second I.P. is always greater than First I.P
44	I.P. of N is higher than oxygen, although O comes later than nitrogen in periodic table
45	I.P. increases along each period while decreases down each group
46	Electronegativity decreases down each group while increases along each period
47	K shell accommodates only two electrons
48	Fluorine has the highest electronegativity
49	The 2nd and higher electron affinities of atoms is always positive
50	Halogens have high electron affinity values

 

 

 

51	Electron Affinity of Fluorine is less than that of chlorine.
52	Cations are more electronegative while anions are less electronegative than their parent
53	Phosphorus and nitrogen both belong to same group, phosphorus forms PCl5 but nitrogen doesn’t
54	Phosphorous forms PCl3 and PCl5 but when it combines with iodine it only forms PI3
55	Lithium resembles with Mg (Beryllium resembles with aluminium) in properties
56	Li and Be differ from other members of their group
57	Electropositivity increases down the group
58	Aluminium is more metallic than Boron
59	Boron is considered as semi-conductor OR Boron generally forms covalent compounds
60	The elements of same group are chemically similar but their physical properties gradually change
61	Some ionic solids do not dissolve in water.
62	KCl is solid while HCl is gas.
63	HF is more polar than HCl.
64	A covalent bond b/w unlike atoms is partially ionic but not b/w like atoms.
65	NH3 and H2O can form co-ordinate bond with H+ but CH4 cannot do so.
66	Sigma bond is stronger than pi bond.
67	Carbon is tetravalent but not divalent
68	Water has a high B.P. than hydrogen fluoride although F is more electronegative than O.
69	Dipole moment of CO2 (CS2) is zero while H2O (SO2) has some dipole moment
70	In CO2 molecule each C – O bond is polar but the overall dipole moment of CO2 molecule is zero
71	The density of ice is less than that of water.
72	Water expands when cooled below 4oC.
73	The bond angle in H2O is lesser than in NH3.
74	At room temperature, ionic solids do not conduct electricity but metals conduct electricity. =
75	PV has dimension of energy
76	The dimensions of pressure are the same as that of energy per unit volume.
77	It is essential to mention physical states of reactants and products in thermochemical equations.
78	Dilute HCl should be added to get ppt. of basic radicals of group II before passing H2S.
79	Salt precipitates when product of its ionic concentration is greater than its Ksp.
80	A catalyst does not change the position of equilibrium but equilibrium position reaches earlier.
81	Powdered marble (CaCO3) gives more effervescence with HCl than a piece of marble.
82	Powdered zinc reacts more vigorously with water than the chunks of metallic zinc
83	Milk sours more rapidly in summer than in winter.
84	Unless ignited H2 gas does not react with O2 gas of air but a stream of H2 gas passed over platinum gauze bursts into flame
85	Food kept in refrigerators can be stored for a longer time
86	Positive catalyst increases rate of reaction
87	Inhibitor decreases the rate of reaction
88	Why some reactions are fast and some reactions are slow?
89	Gasoline burns more rapidly in internal combustion engine than in open air
90	Hydration energy decreases down each group
91	An Aqueous solution of NH4Cl is acidic
92	An Aqueous solution of Na2CO3 is basic
93	Conductance increases with the increase in dilution

 

 

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Scientific Reasons on Introduction to Chemistry

 

1. Stoichiometric calculations are only possible for irreversible reactions

The concept of limiting reactant is not applicable to the reversible reactions, because certain amounts of each reactant is left behind at the equilibrium state and thus any reactant is not consumed completely and cannot be considered as the limiting reactant of the reactions.

2. Why the actual yield is less than theoretical yield?

In most of chemical reactions the actual yield is less than theoretical yield because following reasons:

(i) By product obtained in the side reaction

(ii) Reaction may be reversible

(iii) Mechanical loss of product due to filtration purification crystallization etc.

(iv) Unskilled workers.

3. 23 g of sodium and 238 g of Uranium have equal numbers of atoms.

23 g of sodium and 238 g of uranium also equals to their mole and one mole of all substances contains 6.022 x 10²³ atoms. Therefore 23 g of sodium and 238 g uranium have equal numbers of atoms.

             

Scientific Reasons on Three States

             

4.         –273.15°C is the lowest possible temperature.

–273.15°C (or –459.7°F) is referred to as Absolute Zero, which is the lowest possible temperature. When the volume–temperature graph is extra plotted downward, it always intercepts the temperature axis at –273.15°C. This temperature represents the point at which gases (if they did not condense) would have zero volume which is against the law of conservation of mass. At this temperature all molecular motion ceases to exist and gases are condensed to liquid and then to solid. Hence no low temperature can be achieved than –273.15°C.

5.         Rates of diffusion of N₂ and CO are same

According to Graham’s law of diffusion, rates of diffusion of gases are inversely proportional to the square roots of their densities or molar masses. Gases having same molar masses will have same rates of diffusion. Both N₂ and CO have same molar mass of 28 amu, therefore both have same rates of diffusion.

6.         Lighter gases diffuse more rapidly than heavier gases

According to Graham’s law of diffusion, rates of diffusion of gases are inversely proportional to the square roots of their densities or molar masses. Thus lighter gases with less molar mass have greater rate of diffusion than heavier gases with high molar masses. 

7.         H₂ and He are ideal at room temperature and ordinary pressure while SO₂ is non-ideal.

Ideal gas is an imaginary gas which obeys gas laws at all conditions of temperature and pressure while the gases which do not obey the gas laws strictly and show deviation from ideal behaviour under all or certain conditions are called real or non-ideal gas. H₂ and He show more ideal behaviour due to their smaller size of molecules and weak forces of attraction (van der Waal Forces) between their molecules. Due to weak attractive forces, these gases behave more ideally and do not show deviation from gas laws at room temperature and ordinary pressure.

 

SO₂ is a polar molecule in which additional attractive force is present in the form of dipole-dipole interaction. Due to greater intermolecular forces SO₂ does not show ideal behaviour at room temperature and ordinary pressure.

8.  High pressure and low temperature makes the gas non-ideal/Real gases deviate from ideal behaviour at low temperature and high pressure

Ideal gas is an imaginary gas which obeys gas laws at all conditions of temperature and pressure while the gases which do not obey the gas laws strictly and show deviation from ideal behaviour under all or certain conditions are called real or non-ideal gas. At high pressure and low temperature, gas molecules come closer to each other and intermolecular attractive forces arise in them. These forces include:

 Dipole-dipole interaction; It is the force of attraction acting among polar molecules of gases.

van der Waal Forces; It is the force of attraction between gaseous molecules due to transit/temporary polarity.

at high pressure and low temperature, gases behave non-ideally i.e. they deviate from ideal behaviour and do not obey gas laws strictly due to decrease in intermolecular distance and increases in intermolecular attractive forces.

9.         Heat is evolved when a liquid (or gas) condenses.

Condensation is a process of conversion of vapours into liquid and is reverse of evaporation. Evaporation is an endothermic process and absorbed heat energy is used to overcome intermolecular attractive forces among liquid molecules to change them in vapour form. On the other hand condensation is an exothermic process, caused by cooling effect of evaporation. Due to cooling K.E. of vapours decrease and they condense back to liquid by giving out heat energy. Hence, heat is evolved when a liquid (gas) condenses.

10.  Different gases move with different velocities at the temperature although their K.E is same.

According to KMT, at same kelvin temperature, molecules of all gases have the same K.E:

It is clear from above relation that mass and velocity are inversely related i.e. smaller the mass of a molecule, greater will be its velocity. Thus gases having high velocities are lighter and have low relative molecular masses. Similarly gases having low velocities are heavier and have high relative molecular masses. Thus product of mass and square of velocity remains constant. Therefore different gases move with different velocities although they have same kinetic energy.

11. The surface of a liquid appears like a stretched sheet of rubber trying to contract.

The surface molecules of the liquid behave differently as compared to the internal molecules of the liquid. The intermolecular forces acting on a molecule which are situated within the body of liquid are balanced in all directions due to presence of neighbouring molecules and hence there is no net operative force acting on that molecule. On the other hand molecules which are situated at the surface of a liquid are attracted unequally and they experience more downward pull because there is no upward attractive force acting on the molecules. This downward pull is expressed in terms of surface tension, which tends to cause the surface to volume ratio of the liquid to be as small as possible. It is due to this force i.e. surface tension, the surface of the liquid behaves like a stretched sheet of rubber trying to contract in all directions. [This stretched sheet of surface molecules can also support the mass of lighter insects like mosquitoes].

12. A freely falling drop of a liquid is spherical.

The outer layer of a liquid behaves like a stretched sheet of rubber, trying to contract, because the surface molecules have a downward pull by interior molecules. This inward pull is expressed in terms of surface tension, which makes the surface area of liquid shrink to as small an area as possible. In other words, surface tension tends to reduce the surface to volume ratio of a liquid as small as possible. Since surface to volume ratio for given weight of a liquid is the least for a spherical shape (as compared to any other geometrical shapes of equal volume), therefore, a freely falling drop or a small quantity of a liquid tends to be spherical in shape.

Short Reason

Surface tension of liquids is responsible for formation of spherical droplets formation in liquids. sphere has least surface area to volume ratio so falling drop of any liquid acquires spherical shape.

13.        Mercury does not wet glass surface.

Liquids having low surface tension tend to spread over the solid surface readily and are called wetting or spreading liquids e.g. H₂O. Mercury being a non-wetting or non-spreading liquids, has high value of surface tension (471.6 ergs/cm²), therefore, it pushes away from glass surface and does not wet the it and forms a spherical drop.

14.        A liquid (water or organic liquid) rises in capillary tube.

The rise of a liquid in a capillary tube against the force of gravity is called Capillary Action which is the common property of all the liquids. This property is the result of tendency of liquid to reduce their surface area due to surface tension. The liquid (water) decreases its surface area by rising in the tube due to surface tension. (The liquid (water) will rise in the capillary tube until the upward driving force due to surface tension is just balanced by the downward gravitational pull. In wetting liquids such as water and organic liquids, surface tension is low and adhesive forces are stronger than cohesive forces. These liquids have tendency to climb up in the tube above the level of liquid in surrounding surface).

15.        Level of mercury falls in capillary tube.

In non-wetting liquids such as mercury, the surface tension is high. In such liquids, downward gravitational pull dominates over surface tension due to stronger cohesive forces i.e. downward gravitational pull > Upward surface tension. In other words, cohesive forces are stronger than adhesive forces. That is why, level of mercury falls in capillary tube.

16.        Liquid rises in a capillary tube.

The rise of liquid in a capillary tube is called capillary action which is due to surface tension. Due to surface tensions liquid’s surface tends to reduce the surface area to volume ratio. Since capillary tube has a small bore having smaller surface area to volume ratio, liquid tends to rise in a capillary tube to stronger adhesive forces.

 

17.        A drop of ink spreads on blotting paper.

A drop of ink spreads on blotting paper due to capillary action. The thin pores of blotting paper act like fine capillaries and due to capillary action ink rises in these fine capillaries to reduce the surface area and hence it spreads on the blotting paper.

 

18.  Some liquids (e.g. H₂O) form concave meniscus while certain liquids (e.g. Hg) form convex meniscus.

The shape of the surface of a liquid in a cylindrical container is called Meniscus (Greek word meaning small moon). The formation of meniscus depends upon the interaction between liquid and surface of the vessel.

 

Liquids like water which wet glass surface forms a concave meniscus which is a meniscus in which surface of the liquid curved downward. This is due to strong interaction between liquid (water) and the glass surface, by which unbalanced forces on the surface molecules due to surface tension are partially overcome and the liquid tends to climb a short distance up the walls of vessel and increase the surface area of contact with the vessel. This is because the forces of adhesion exceed the forces of cohesion. Thus a liquid with stronger adhesive forces than cohesive forces shows concave meniscus (downward curve).

Liquids like mercury which does not wet glass surface forms a convex meniscus which is the meniscus in which surface of the liquid curved upward. This is due to weak interaction between liquid (mercury) and the glass surface. Since Mercury is non-wetting liquid, the cohesive forces acting among its molecules are much stronger than the adhesive forces with glass. Thus there is repulsion between the surface molecules of mercury and the walls of container. Therefore, mercury is pushed away from the walls of glass at the point of contact. In such cases cohesion exceeds the adhesion. The non-wetting liquids form upward curvature (convex meniscus).

Short Reason

Mercury forms convex meniscus due to stronger cohesive forces than adhesive forces which cause mercury to push away from the wall at the point of contact.

 

Water forms concave meniscus due to stronger adhesive forces than intermolecular cohesive forces which cause water to climb up along the glass thereby increasing its point of contact.

 

19.        Water kept in earthen-ware jars keeps cooler.

We know that earthenware jars are porous. Since evaporation takes place at all temperature, therefore liquid molecules manage to escape out of the pores of the earthenware jars. Only those molecules which have higher kinetic energy are able to overcome the intermolecular attractive forces and go off as vapours in the air. Due to escaping of higher energy molecules, the kinetic energy of remaining molecules get lowered. As average kinetic energy is the measure of temperature of the liquid i.e. kinetic energy varies directly with absolute temperature, the temperature of remaining liquid will also fall. That is why water kept in earthenware jars keeps cooler.

 

Short Reason

The water kept in an earthen pot remains cool even in summer because of evaporation. Earthen pot has a large number of tiny pores in its walls and some of the water molecules continuously keep seeping through these pores to outside the pot. This water evaporates continuously and takes the latent heat required for vaporization from the remaining water. In this way, the remaining water loses heat and gets cooled.

20.  Evaporation is a cooling process.

Evaporation is the spontaneous escaping of liquid molecules from its surface as vapours. Due to escaping of higher energy molecules from the liquid’s surface caused the average kinetic energy of the remaining molecules to decrease resulting in fall in temperature and thus cooling is produced.

21.Spilled water evaporates more quickly than the same amount of water in the glass.

Evaporation is the spontaneous escaping of liquid molecules from its surface as vapours. Rate of evaporation increases with increasing surface area. Spilled water has greater surface area than water contained in the glass and that is why evaporation takes place more rapidly in spilled water.

22.        Glycerine is distilled at reduced pressure.

The process of distillation that is carried out under reduced pressure or in vacuum is called Vacuum Distillation or Reduced Pressure Distillation. It is based on the fact that boiling point of liquid is decreased at lower vapour pressure by decreasing external pressure, so that they can be distilled before their decomposition. This technique is used for the distillation of high boiling points liquids, which decompose at their b.p. In order to avoid decomposition of such liquids, they are distilled out at lower temperature under reduced pressure before their decomposition temperature. For example; b.p. of glycerine is 290°C and it boils with decomposition. Therefore, b.p. of glycerine is lowered to 210°C by decreasing the external pressure to 50 mm Hg, so that it can be distilled before its decomposition.

 

Short Reason

Distillation under reduced pressure or vacuum distillation is used to purify liquids having very high boiling points and those, which decompose at or below their boiling point. Glycerol, at normal pressure, the b.p. of glycerol is 563K but it decomposes on this temperature. Therefore, under reduced pressure of 12mmHg, glycerol can be distilled at 453K without decomposition.

23.        Glycerin is more viscous than water.

Glycerin molecule has strong intermolecular forces in the form of strong hydrogen bonding owing to presence of three – OH groups and bigger size of its molecule than water. Hence viscosity of glycerin (1490 cp) is much greater than that of water (1 cp).

24.        Honey is more viscous than water. OR It is easier to pour water than honey.

Viscosity of a liquid is the measure of its internal resistance to flow due to intermolecular attractions. Viscosity depends upon intermolecular forces, molecular size and density of liquid. Viscosity varies directly with density of liquids. As the honey is more denser than water, therefore it is more viscous than water. The more viscous a liquid is, the less easily it flows. Thus honey, which is a thick liquid, flows slowly. Therefore, it is easier to pour water (thin liquid) than honey.

25.        A liquid is less viscous at high temperature.

Viscosity is the internal frictional resistance in the flow of liquid. It varies inversely with temperature. At high temperature, the kinetic energy of liquid molecules increases causing them move far away from each other thereby decreasing intermolecular attractive forces between them. This decreases the viscosity of liquids. Hence a liquid is less viscous and flow quickly at high temperature.

26. At b.p. the temperature of a liquid remains constant in spite of continuous supply of heat.

Boiling point is the temperature at which vapour pressure of liquid becomes equal to atmospheric pressure. Initially, the heat supplied to a liquid is mostly consumed in increasing the average kinetic energy of the molecules or simply the temperature of the liquid, which leads to increase the rate of evaporation till the liquid starts boiling. At B.P. the molecules of liquid attain maximum average K.E. and any further increase in heat does not affect the temperature. Instead of raising the temperature of the liquid, the heat supplied is used to overcome the intermolecular attractive forces and excessive energy is carried away by the liquid molecules into their vapour state. Heat is taken away by the escaping molecules (vapours) in the form of Latent Heat of Vaporization (which is the amount of heat required to vaporize one gram of liquid at its B.P. without change in temperature). Thus the average K.E. of liquid molecules or the temperature of liquid at B.P. remains constant at its maximum. Therefore, the B.P. of liquid remains constant, although heat is continuously supplied to it at its B.P.

Short Reason

The temperature remains constant during boiling of water even though heat is supplied constantly because all the heat energy provided is used up in changing the state of water from liquid to gaseous water vapour. The extra heat supplied to the boiling water is used in the vaporization of a liquid, which is known as the latent heat of vaporization. 

27.  A liquid boils at different temperatures at sea level and at mountain./Boiling point of liquid decreases  at high altitude. 

Boiling point of a liquid is defined as the temperature at which the vapour pressure of the liquid becomes equal to atmospheric pressure. It is apparent that B.P. is directly proportional to atmospheric pressure i.e. if atmospheric pressure increases, B.P. will also increase and vice versa. At sea level atmospheric pressure is higher than at mountains. At sea level the vapour pressure of the liquid becomes equal to the atmospheric pressure at certain higher temperature. This results, an increase in B.P.

At high altitude mountains, the atmospheric pressure decreases and the vapour pressure of liquid becomes readily equal to the external pressure and B.P. of the liquid decreases, hence liquid boils at a lower temperature than its normal boiling point. That is why water boils at temperature lower than 100°C at higher altitudes. At sea level, water boils at 100°C at 1 atmosphere while at Murree Hills where atmospheric pressure is lowered (about 0.921 atm) water boils at about 98°C.

28.        Pressure cooker is used for rapid cooking.

Pressure cooker is a closed container where vapours are not allowed to escape which are accumulated over the surface of the liquid exerting more vapour pressure. The basis of working pressure cooker is that the b.p. of a liquid increases with external pressure. If external pressure is increased, the vapour pressure of liquid become equal to increased external pressure at a certain higher temperature and b.p. of a liquid increases. Therefore in Pressure cooker where pressure is higher than atmospheric pressure, the b.p. of a liquid (e.g. water) is increased and energy supplied is conserved in it which cooks the meat and vegetables quickly.

29.        Steam produces more severe burns than does the boiling water. 

Both the boiling water and steam may be at same temperature but the amount of heat energy contained in one gram of steam is much greater than in one gram of water. 1 gram of water absorbs 2.26 x 10³ joules of energy at B.P. in the form of Latent Heat of Vaporization and changes to steam. This heat is stored in steam. This means that each gram of steam has 2.26 x 10³ joules more energy than boiling water. Hence burning effect of steam will be more severe than does the boiling water.

30.        Increasing pressure stops on boiling

At boiling point vapour pressure of liquid becomes equal to atmospheric pressure. At boiling point, heat supplied is stored in the form of latent heat for rapid state change from liquid to gas state due to which no change in is observed in the vapour pressure. Thus at boiling point, the vapour pressure of any liquid becomes constant.

31.        Density of liquids (or gases) decreases with the rise of temperature.

The mass per unit volume of a substance is called its density. On heating, the liquids (or gases) expand and the number of molecules per unit area decreases thereby increasing its volume. Since density is inversely proportional to volume, therefore, density of liquids (or gases) decreases with the rise of temperature.

32.        Graphite is a good conductor of electricity whereas diamond is not.

In diamond, each carbon is sp³-hybridized and is covalently bonded to four other carbon atoms by forming 4 sigma bonds consuming all of its 4 electrons at an angle of 109.5° to give crystal lattice of diamond. Due to unavailability of free electron, diamond is bad conductor of electricity.

In graphite, each carbon is sp²-hybridized and is covalently bonded to three other carbon atoms by forming 3 sigma bond leaving behind one free electron on each carbon at an angle of 120°, forming layers of hexagons. Due to presence of unpaired electron in the form of free electron, graphite conducts electricity.

33.        NaF and MgO are isomorphous compounds

Isomorphous compounds have crystal structure and same empirical formula. Both compounds have same empirical formula or atomic ratio of 1:1 giving same structural features. Hence both NaF and MgO are isomorphous (cubic).

34.        Diamond is hard while graphite is soft although both are allotropes of carbon.

In diamond, each carbon atom is bonded strongly to four other carbon atoms at an angle of 109.5^o to form basic tetrahedral units that are united with one another to give cubic unit cell of diamond. Thus atoms occupy fixed positions and it is difficult for the atoms to slide pass over the other. Due to closely packed tetrahedral structure, diamond is hard.

In graphite, each carbon atom is bonded to three other carbon atoms at an angle of 120^o to form basic trigonal units that are united with one another to give layers of hexagons. The interplanner distance between layers of graphite is very large (3.35°A) and these hexagonal layers of carbon are held loosely by weak van der Waal’s Forces. Hence these layers can slide easily over one another. That is why, graphite is soft and slippery to touch having low density, hence used as a lubricant.

35. On heating substances like iodine and camphor directly change from solid to gas.

Sublimation is the process of direct conversion of some solid called sublime solids into vapour on heating. Dry ice or solid CO₂, naphthalene, ammonium chloride, Iodine and camphor are well-known sublime solids which change directly to vapour on heating without passing through intermediate liquid phase. This is because, in sublime solids, strength of intermolecular forces is less than ordinary solids. Hence high energy molecules at solid surface of such substances overcome the attractive forces holding them and directly pass into vapours.

Scientific Reasons on Atomic Structure

 36.        In electric or magnetic field, the deflection of b-rays is greater than a-rays.

In electric or magnetic field, the deflection of massive particles is less than lighter particles. Since b-rays which are actually electrons, are lighter than a-rays which are actually heavy double positively charged helium nuclei, therefore deflection of b-rays is greater than a-rays in electric or magnetic field.

37.        Radioactivity is confined to heavy elements.

Radioactivity is a nuclear phenomenon in which nuclei of certain unstable heavy elements with atomic number greater than 83 change into nuclei of other stable element along with the liberation of particles and nuclear energy (in the form of nuclear radiation). Heavy elements with atomic number greater than 83 are naturally unstable because their nuclei have neutron to proton ratio are more than 1.5. In order to gain stability, they undergo spontaneous nuclear decay by continuously emitting different types of particles or electromagnetic radiations until a stable atom of lead (₈₂Pb) is formed. On the contrary, n/p ratio in nuclei of lighter elements is usually less than 1.5, that is why atoms of lighter elements being stable do not emit nuclear radiations or particles.

38.        X-rays are considered as anode rays.

X-rays are originated from the atoms of anode when high energy cathode rays strike them. Since energy of X-rays depend upon the metal used as anode that is why they are considered as anode rays.

39.        Elements give line spectrum

According to Bohr’s theory, electrons in shells of elements are present in specified energy level and can move between these stationary energy state by absorbing or emitting energy in the form photon of definite wavelength resulting in spectral lines of coloured line in the form of line spectrum.

 

40.        Atomic radii decrease along each period while increase down each group. OR Ionic radii decrease along each period while increase down each group.

On moving from left to right in a period, nuclear charge increases without increasing number of shells thereby increasing nuclear force of attraction on valence electrons, which results in decreasing the atomic (ionic) size. Thus atomic (ionic) radii decrease along each period.

On moving from top to bottom in a group, number of shells increase producing greater shielding effect which reduces nuclear attractive forces on valence electrons thereby increasing atomic (ionic) radii. Thus atomic (ionic) radii increases down each group.

 

41. A cation is smaller in size than its parent atom while an anion is larger in size than its parent atom.

A cation is formed when an atom looses electron. Thus in a cation, the number of electrons is always less than the number of protons i.e. positive charge. The effective nuclear charge increases due to loss of electron. Thus the nucleus attracts lesser number of electrons with greater force thereby reducing size of atom. In certain cations, the loss of electron leads to disintegration of an orbit while nuclear charge remains the same in both cases. Thus nucleus attracts lesser number of orbits more strongly, thereby reducing atomic size. For example:

An anion is formed when an atom gains electron. Thus the number of electrons in anion is always greater than number of protons i.e. positive charge. The effective nuclear charge decreases due to gain of electron. (Due to the decrease in the force of attraction on the outer electron and the increased repulsive forces due to added electron(s), the size of anion gets increased.                                                                                    

42. In iso-electric ions, the ionic radii decrease with the increase of positive charge or increase with the increase of negative charge.

Those ions that have same electronic configuration are called Iso-Electric ions. E.g. Na⁺, Mg²⁺ and Al³⁺ are considered to be iso-electric ions because they possess same electronic configuration i.e. 1s², 2s² 2p⁶, although their atomic numbers are different i.e. 11,12 and 13.

 

In Iso-electronic Ions, the ionic radii decrease with the increase in nuclear charge. The decreased ionic radii in iso-electronic ions is obviously due to increased nuclear charge i.e. the nucleus with increasing nuclear charge attracts the same number of electrons more strongly, ultimately reduction in radii occur e.g. Na⁺, Mg²⁺ and Al³⁺ ions are iso-electric and their ionic radius are 0.95°A, 0.65°A and 0.50°A respectively.

 

43.        Second I.P. is always greater than First I.P.

The energy required to remove second valence electron from the valence shell of gaseous monopositive cation is called second Ionization Potential. I.P. varies directly with nuclear charge. Greater the nuclear charge, greater will be the I.P. Monovalent cations have greater nuclear charge and smaller radii than its parent atom. Thus in Monovalent cations, remaining valence electrons are more firmly held and the subsequent removal of further electron is exceedingly difficult. That is why 2^nd I.P. is always greater than 1^st I.P. e.g. 2^nd I.P. of Mg is greater than its 1^st I.P.

44. I.P. of N is higher than oxygen, although O comes later than nitrogen in periodic table.

 Or I.P. of N is higher than oxygen, although O is more electronegtive than nitrogen.

I.P. increases along each period in periodic table due to increase in nuclear charge. However atoms with stable electronic configuration have relatively higher I.P. than expected from their position in periodic table.

 

The I.P. of Nitrogen (14.7 e.V) is higher than O (13.6 e.V) due to more stable half-filled p-orbitals of N. Thus removal of electron will be difficult and I.P. will become high. Whereas, oxygen has one extra electron than half filled, which require less energy for its removal.

45.        I.P. increases along each period while decreases down each group.

I.P. increases with the increase in nuclear charge (atomic number) in same period from left to right. The reason is that in a period, nuclear charge increases without increasing the number of shells, which reduces atomic size. Since I.P. is inversely proportional to atomic size, therefore, I.P. values become high gradually along each period.

I.P. decreases with the increase in nuclear charge (atomic number) in the same group from top to bottom. The reason is that in a group, increase in number of shells produces greater shielding effect of intervening orbits which reduces nuclear attractive force on valence electrons. Since I.P. is inversely proportional to shielding effect, therefore, I.P. values become less gradually down each group.

 

46. Electronegativity decreases down each group while increases along each period.

In group, E.N. values decrease from top to bottom. It is due to the fact that size of atoms increases down each group, so there will be lesser force of attraction for electrons. So E.N. values decrease down each group. For example; In VII A group, F, Cl, Br and I have E.N. 4.0, 3.0, 2.8 and 2.5 respectively.

In period, E.N. values increase from left to right. It is because that size of atom decrease along a period (due to increased nuclear charge without increasing number of shells) so there will be greater force of attraction for electrons. So E.N. values increase along each period.

47.        K shell accommodates only two electrons.

K shell has only one sub-energy level i.e. 1s which has only one value of magnetic quantum number i.e. 0 signifying that K shell has only orbital i.e.1s. According ot Pauli’s exclusion principle, an orbital can have only two electrons with opposite spins that is why K shell can accommodate only two electrons.

48.        Fluorine has the highest electronegativity

Small sized atoms having strong nuclear charge with almost full valence shell have high electronegativity. F being the smallest atom (after H and He) with greatest effective nuclear charge having 7 valence electrons has the strongest powered to attract a shared electron pair in a molecule and hence it is the most electronegative element.

49.        The 2^nd and higher electron affinities of atoms is always positive.

The 2nd and higher electron affinities of atoms are always positive i.e. the formation of bivalent (bi-negative) anion from a univalent anion is endothermic and positive sign represents energy released. The bivalent ion formation is endothermic because addition of second electron to univalent anion must overcome the repulsion of the negative charge already present on univalent anion. Thus univalent anion exerts an electrostatic repulsive force on an additional incoming electron. e.g.:

 

50.        Halogens have high electron affinity values.

Halogens (and other non-metals) have high values of electron affinity i.e. F, Cl, Br and I have electron affinities of   –333, –348, –340 and –297 kJ/mole respectively.

The high E.A. values of halogens are due to following reasons:

(a)  Their small atomic radii and great attraction for electrons.

(b) Moreover, addition of an electron produces the stable inert gas configuration (i.e. Octet)

 

51.        Electron Affinity of Fluorine is less than that of chlorine.

Electron affinity of Fluorine is less (– 333 kJ/mole) than that of Chlorine (– 348 kJ/mole). The anomalous E.A. value of Fluorine is probably due to following reasons:

(a)  Fluorine atom is very small sized.

(b)  The electrons already present repel incoming electron.

 

52.Cations are more electronegative while anions are less electronegative than their parent atom.

Cations are formed when neutral atom loses electron. So in cation the number of electrons is always less than the number of protons i.e. positive charge. Thus effective nuclear charge on remaining electrons increases. Therefore size of atom decreases due to greater charge. As small sized atoms are more electronegative so, cations are more electronegative than parent atom. For example; electronegativity of Li⁺ is 2.5 while that of Li atom is 1.

Anions are formed by the gain of electrons by a neutral atom. So in anion the number of electrons is always greater than the number of protons i.e. positive charge. Thus effective nuclear charge on added electron decreases. Therefore, size of atom increases due to lesser nuclear charge. As bigger sized atoms are less electronegative, so anions are less electronegative than parent atom. For example; electronegativity of F^- is 0.78 while that of F atom is 4.

53.  Phosphorus and nitrogen both belong to same group, phosphorus forms PCl₅ but nitrogen doesn’t.

The electronic configuration of nitrogen is [He], 2s² 2p³ showing that it lacks d-orbital. Hence nitrogen cannot increases is covalency beyond 4 due to absence of d-orbital. NCl₅ ​cannot be formed because nitrogen has no vacant d-orbitals. The electronic configuration of nitrogen is [Ne], 3s² 3p³ 3d⁰. phosphorous has vacant d-orbital it can extend its covalency to 5. Thus PCl₅​  can be formed.

54. Phosphorous forms PCl₃ and PCl₅ but when it combines with iodine it only forms PI₃.

The atomic radius of iodine is much larger than that of chlorine. Phosphorus atm can hold 5 small sized Cl atoms forming PCl₅ but it cannot accommodate 5 large sized iodine atoms and hence it cannot form PI₅.

 

55.        Lithium resembles with Mg (Beryllium resembles with aluminium) in properties

Li and Be has a number of common properties, such as their carbonates, fluorides, sulphates and phosphates are only slightly soluble in water. Lithium resembles more with Mg while Beryllium matches more with Al. This is called Diagonal Relationship. The element of the second period belonging to the IA or IIA group is similar in behaviour to the element of IIA or IIIA group in the third period. This close resemblance between the properties of an element of 2^nd period with those of the element of 3^rd period belonging to next group is called Diagonal Relationship. This is due to approximately equal sizes of Li⁺ (0.060 nm) and Mg²⁺ (0.065 nm). Similarly Be²⁺ (0.031 nm) and Al³⁺ (0.25 nm) ions are of equal sizes. Even their atoms are of nearly equal sizes.

 

 

56.        Li and Be differ from other members of their group

Lithium and Beryllium, the first member of groups IA and IIA differ markedly from other members of their respective groups. This is because of their small atomic sizes, which results in high charge densities on Li⁺ and Be²⁺ ions, which produces high heat of hydration. Thus Li and Be have a more tendency to form covalent compounds.

57.        Electropositivity increases down the group.

The tendency of an atom to lose electrons and to change to positive ions is called Electropositivity. This tendency decides the metallic character. The greater the electropositivitiy, the more is the metallic nature of an element. The metallic character increases down the group in all s and p-block elements. This is because of following reasons:-

(i)    Decrease in ionization potential (inversely proportional to E.P)

(ii)   Electron population of outermost shell (inversely proportional to E.P)

58.        Aluminium is more metallic than Boron.

Aluminium in group IIIA is more metallic than boron. This is because of low I.P. and low electron population ratio of Al. Electron population ratio is defined as the ratio of number of electrons in valence shell and total capacity of valence shell.

For example

Electronic configuration of 5B       = 1s2, 2s2 2p1 Total valence electrons    of 5B     = 3 Total electron capacity in 2nd orbit = 8 Electron population ratio              = 3/8

Electronic configuration of 13Al      = 1s2, 2s2 2p6, 3s2 3p1 Total valence electrons of 13Al      = 3 Total electron capacity in 3rd orbit = 18 Electron population ratio               = 3/18

Electron population ratio in Al is less than B, so Al is more metallic than B.

 

59. Boron is considered as semi-conductor OR Boron generally forms covalent compounds.

The electropositivity i.e. tendency to give out electrons is inversely proportional to ionization potential and directly proportional to atomic size.

 

Being a first member, Boron has least atomic size and highest I.P. in group IIIA. That is why its metallic character is least as compared to other members of its group. Therefore it is regarded as a non-metal. Its non-metallic character is proved by its non-conductivity at room temperature. However, its conductivity increases 100 times at 600°C. Therefore, it is considered to be semi-conductor and semi-metal. This is also the reason, why boron having 3 valence electrons, does not form tri-positive B³⁺ ion, and in most of its compounds, boron is covalently bonded.

 

60.        The elements of same group are chemically similar but their physical properties gradually change down the group.

The chemical properties of elements depend upon the number and arrangement of electrons in valence shells. Atoms with same number of valence electrons would be expected to be chemically similar. Since elements of same group have same valence shell electronic configuration, therefore, they have same chemical properties

The regular variation in physical properties such as atomic size, I.P, E.N, etc. down a group is called Group Trend. Group trend in physical properties such as atomic size, ionic size, metallic character is that they increase down the group whereas E.N., I.P. and E.A. decrease down the group. The physical properties of elements depend upon atomic mass and the total number of electrons in inner shells. As going down a group, a new shell is added in each period, hence there is a regular change in physical properties.

 For Example

In group IIIA elements, the number of total valence electrons are 3 (ns² np¹) but number of inner electrons are different. e.g. B, Al and Ga have 2, 10 and 28 inner electrons respectively, due to this reason, regular change in physical properties is observed down a group.

₅B         =  5e    =  1s², 2s² 2p¹

₁₃Al       =  13e  =  1s², 2s² 2p⁶, 3s² 3p¹

₃₁Ga     =  31e  =  1s², 2s² 2p⁶, 3s² 3p⁶, 3d¹⁰, 4s², 4p¹

Scientific Reasons on Chemical Bonding 

 

61.        Some ionic solids do not dissolve in water.

ionic compounds containing highly polarizing (small and highly charged) ions will usually not dissolve in water. Ionic solids with high lattice energy are insoluble in water because insufficient hydration energy is available from the ions to overcome the high lattice energy for these compounds to be soluble.

 

62.        KCl is solid while HCl is gas.

The physical state of a substance depends upon the strength of intermolecular force of attraction. In KCl, positively charged potassium (K⁺) ion and negatively charged chloride (Cl^‒) ions are strongly bonded by electrostatic forces of attraction forming giant ionic crystal, thus KCl is solid due to strong electrostatic forces. HCl is polar covalent molecule with weak intermolecular forces of attraction (hydrogen bonding), so HCl is a gas due to weak intermolecular forces.

 

63.        HF is more polar than HCl.

 The polarity of bond increases with the increase in the electronegativity difference between the bonded atoms. The electronegativity difference between H and F is 1.9 (4-2.1) while in HCl is it just 0.9 (3-2.1). Hence H – F bond is strongly polar with strong partial negative charge on F atom as compared to H – Cl bond. Hence HF is molar than HCl.

Alternate Description

The extent of the ionic character of a covalent bond depends on polarity of molecule which in turn is proportional to the difference of electronegativity b/w the bonded atoms. The DE.N in HF is 4-2.1=1.9 while DE.N in HCl is 3-2.1 = 0.9. Since HF has greater DE.N. than HCl, therefore, HF is more polar than HCl. It is also proved by their ionic character since HF is 43% ionic and HCl is 17% ionic.

 

64.        A covalent bond b/w unlike atoms is partially ionic but not b/w like atoms. OR A covalent bond  between dissimilar atoms is stronger than that of identical atoms. OR A polar bond is stronger than non-polar bond.

A polar covalent bond is formed b/w two dissimilar atoms having different values of electronegativity, which has partial negative and partial positive charges at ends or poles b/c of unequal distribution of electrons. Polar bond, thus have partial ionic character. Thus polar bond has two types of interatomic binding forces namely Joint Inter-nuclear Controlling Force (Covalent Bond) and Electrostatic Force b/w dipoles (Partial Ionic Bond). The additional electrostatic forces b/w dipoles of polar bond shorten the bond length and elevate bond energy. On the other hand, a non-polar covalent bond is formed b/w two identical atoms having no or very small difference of electronegativity lacking poles due to equal electronic distribution. A non-polar covalent bond is a pure covalent bond devoid of ionic character. Thus non-polar molecules have less bond energy than polar molecules. Thus a polar bond is always stronger than non-polar bond. E.g. A non-polar bond b/w H-H or Cl - Cl is weaker than H-Cl or H-F bond.

65.        NH₃ and H₂O can form co-ordinate bond with H⁺ but CH₄ cannot do so.

In order to form co-ordinate covalent bond, one of the two atoms should have a lone pair of electrons. Since NH₃ and H₂O have one and two lone pair of electrons on N and O atoms respectively, therefore they can form co-ordinate covalent bond with H⁺ (which is deficient of electrons). In CH₄ there is no lone pair of electrons on C, therefore it cannot form co-ordinate bond with H⁺ (proton).

66.        Sigma bond is stronger than pi bond.

A sigma bond results from end to end overlap of atomic orbitals, resulting a maximum overlap. The extent of overlapping in sigma bond is sufficient. Thus, the electron density is maximum in the region b/w two nuclei. This results in strong bond b/w two atoms. Hence bond formed has higher bond energy and is strong i.e. less reactive and more stable.

A Pi-bond results from side-ways (lateral) overlap of two atomic (p) orbitals at the two sides of the lobe, resulting in less extent of overlapping. Thus, electron density is maximum above and below the line joining the two nuclei. So Pi-bond has less bond energy and is weaker than sigma bond i.e. more reactive and less stable.

 

67.        Carbon is tetravalent but not divalent

The number of unpaired electrons or half-filled orbitals in an atom is called its valency. The ground state electronic configuration of carbon is 1s², 2s² 2p_x¹ 2p_y¹ 2p_z⁰. From its E.C., carbon appears to be divalent due to two unpaired electrons but it exhibits tetravalency in most of its compounds (organic compounds). To explain this discrepancy, it is assumed that one of the 2s electron is promoted to an empty 2p_z orbital making its configuration 1s², 2s¹ 2p_x¹ 2p_y¹ 2p_z¹. Energy needed for promotion and unpairing the 2s electron is assumed to be obtained from energy released during covalent bond formation. Thus according to excited state E.C., carbon atom has four unpaired electrons, therefore, it accounts for tetravalency of carbon.

68.        Water has a high B.P. than hydrogen fluoride although F is more electronegative than O.

Boiling Point of a liquid is the measure of strength of intermolecular forces. Both water and hydrogen fluoride are polar covalent molecules. The polarity of a molecule depends on difference in electronegativity of bonded atoms. Since F is more electronegative than O, therefore HF is more polar than H₂O. In addition, both water and HF can form hydrogen bond.

HF has strongest hydrogen bonding than water. But in HF the net attractive forces are less due to the fact water has two polar hydrogen (H^d+) enabling three dimensional hydrogen bonding of greater chain length where as in HF there is only one polar hydrogen (H^d+) permitting only two dimensional hydrogen bonding giving chain of limited length. That is why water has higher B.P. than HF.

 

 

69.        Dipole moment of CO₂ (CS₂) is zero while H₂O (SO₂) has some dipole moment

Dipole moment is the extent of a polar molecule to turn or orient in an electric field. Dipole moment is a vector quantity. If the vector sum of bond moments is zero, the dipole moment will be zero and the molecule will be non-polar. If the bond moments do not cancel, then the dipole moment will be greater than zero and the molecule will be polar.

In polyatomic molecules, dipole moment depends upon geometry of molecules. Linear Polyatomic molecules have zero dipole moment as vector sum of bond moments is zero. CO₂ has linear structure in which dipole moment of one side is cancelled by the dipole moment of other side. Thus CO₂ has zero dipole moment and CO₂ molecule, as a whole, is non-polar.

Angular Polyatomic molecules have m > 0 as vector sum of bond moments is not zero. H₂O has angular or bent structure due to which bond dipoles do not cancel out. Thus H₂O show m = 1.84 D and H₂O molecule as a whole is polar.

 

70.   In CO₂ molecule each C – O bond is polar but the overall dipole moment of CO₂ molecule is zero

Each C – O bond in CO₂ is polar due to high electronegativity of oxygen. But due to linear geometry of CO₂ molecule, the bond moments of both C – O bond cancel each other giving the overall dipole moment of CO₂ zero. Hence CO₂ molecule as a whole is non-polar

 

71.        The density of ice is less than that of water.

Density of ice is less than that of water because of open porous structure of ice due to hydrogen bonding due to which volume of ice more than liquid water. Ice is less denser than water because in ice, the molecules arrange themselves in a rigid tetrahedral structure due to which cage like spaces remain in their bonding but water molecules remain in linear bonding form. As the volume of ice becomes greater, it is less denser. 

72.        Water expands when cooled below 4^oC.

When liquid water is cooled, it contracts like one would expect until a temperature of approximately 4^oC is reached. After that, it expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%. This is known as anomalous behaviour of water. At 4^oC, water has the highest density (1 g/cm³). Hence cooling or heating water along 4^oC, water expands.

When water cooled below 4^oC, the molecules move further apart due to breaking down of intermolecular hydrogen bonding producing a lot of empty space between them. This makes the water takes up more space and expands as it cools down or freezes

73.        The bond angle in H₂O is lesser than in NH₃.

According VESPER theory, the presence of lone pairs on the central atom reduces bond angle due to interelectronic repulsion. The order of interelectronic repulsions is

Lone pair-lone pair repulsion > lone pair-bond pair repulsion > Bond pair-bond pair repulsion

In water there are 2 lone pairs on central O atom and thus repulsion is greater between two lone pairs which compress the bond pairs making the bond angle decreases giving bent structure to water. In NH₃ there is only one lone pair on central N atom thus repulsion is less between lone pair and bond pair.

74.        At room temperature, ionic solids do not conduct electricity but metals conduct electricity.

Free electrons or free ions are responsible for conduction. Metals are conductor of electricity at room temperature due to presence of free flowing mobile valence electrons. Ionic solids although have ions but they are tightly packed ad held together by strong electrostatic forces at room temperature, so they do not conduct electricity due to immobility of free ions.

             Scientific Reasons on Thermodynamics 

75.        PV has dimension of energy

PV represents pressure-volume work. PV has the same dimension to that energy.

Pressure           x    volume        =    energy

  N/m²                 x    m³            =    joule

 N                      x    m               =    joule

 N –  m                                      =    joule

                           joule            =    joule    ( since N-m =J )

 

76.        The dimensions of pressure are the same as that of energy per unit volume.

77.        It is essential to mention physical states of reactants and products in thermochemical equations.

The heat of reaction (heat of formation) depends upon the physical states of the reactants and products. The heat of reaction of a same compound may be different in different physical states. This is because heat contents of same reactants and products are different for different physical states. For example; heat of formation of liquid water and steam are different i.e. –285.8 kJ/mole and –241.8 kJ/mole although same compound (H₂O) is formed. This difference of heat of formation is due to difference of physical states of H₂O. Thus it is necessary to mention the physical states of reactants and products in thermochemical equations.

Scientific Reasons on Chemical Equilibrium 

 

78.        Dilute HCl should be added to get ppt. of basic radicals of group II before passing H₂S.

The sulphides of basic radicals of group II (Cu²⁺, Hg²⁺, Pb²⁺, Sn²⁺, Cd²⁺, Bi³⁺, Sb³⁺, As³⁺, Sn⁴⁺) are precipitated as sulphides by passing H₂S gas through their solution in presence of dilute HCl. The K_sp of sulphides of group II is very low (10^–28). H₂S, which is a precipitating agent, is a weak electrolyte and partially dissociates to give H⁺ and S^2– ions. Inspite of partial dissociation, the concentration of S^2- ions is high enough to exceed the Ksp of group II as well as cation of higher groups too. To encounter this effect, HCl is added before passing H₂S gas.

HCl is a good ionizer, produces common ion effect by providing H⁺ ions, which combines with S^2– ions to form unionized H₂S thereby lowering the concentration of S^2– ions. HCl suppresses the ionization of H₂S to such an extent at which S^2– ion concentration is just sufficient to precipitate the sulphides of group II only. In this low concentration of S^2– ions, the sulphides of other higher groups’ cations are not precipitated due to their higher K_sp.

 

79.        Salt precipitates when product of its ionic concentration is greater than its K_sp.

When ionic product of dissolved salt exceeds its K_sp, the ionization equilibrium of salt is shifted to left and hence ions of salt associate to form unionized salt which precipitate out from solution.

 

80.        A catalyst does not change the position of equilibrium but equilibrium position reaches earlier.

Catalyst is a substance, which increases the rate of a chemical reaction without itself being consumed in the reaction. A catalyst speeds up the rates of both forward and reverse reactions to the same extent by decreasing the energy of activation. Thus catalyst enables equilibrium to be reached more quickly thereby increasing the rate at which equilibrium is attained. However, a catalyst does not change the equilibrium concentration (and also K_c) and the yield of a reaction. Thus a catalyst does not change the position of equilibrium but simply help in the early approach of equilibrium position by decreasing activation energy.

 

Scientific Reasons on Chemical Kinetics

 

81.        Powdered marble (CaCO₃) gives more effervescence with HCl than a piece of marble.

The chemical equation for the reaction b/w marble and HCl is given below:

                        CaCO_3(s)     +         2HCl_(aq)   ¾¾®    CO_2(g)    +  CaCl_2(aq)    +  H₂O_(l)

The above reaction is a heterogeneous reaction in which reactants are present in different phases. In a heterogeneous reaction, rate of reaction is proportional to the surface area of the solid reactant i.e. greater the surface area of solid reactant, higher will be the rate of reaction. Powdered marble gives more effervescence of CO₂ with HCl than a piece of marble b/c powdered marble offers greater surface area to reacting HCl molecules. So the chances of actual contact i.e. effective collision is considerably increased when powdered marble is used. Hence, rate of reaction increases and formation of effervescence of CO₂ becomes rapid.

Short Reason

In heterogeneous reactions, increasing the surface of the solid reactant increases the rate of reaction by increasing the number of effective collisions. Powdered marble offers greater surface area to the reacting molecules that is why the reaction rate of powdered marble is greater than its chunks.

82.        Powdered zinc reacts more vigorously with water than the chunks of metallic zinc.

The chemical equation for the reaction b/w zinc and water is given below:

                        Zn_(s)      +          H₂O_(g)  ¾¾¾¾¾¾®   ZnO_(s)     +      H_2(g)

The above reaction is a heterogeneous reaction in which reactants are present in different phases. In a heterogeneous reaction, rate of reaction is proportional to the surface area of the solid reactant i.e. greater the surface area of solid reactant, higher will be the rate of reaction. Powdered zinc reacts more rapidly with water than the metallic pieces of zinc b/c powdered zinc provides greater surface area to the reacting water molecules. So, the chances of actual contact i.e. effective collisions is considerably increased when finely divided zinc is used, consequently rate of reaction enhances.

Short Reason

In heterogeneous reactions, increasing the surface of the solid reactant increases the rate of reaction by increasing the number of effective collisions. Powdered zinc offers greater surface area to the reacting molecules that is why the reaction rate of powdered zinc is greater than its chunks.

 

83.        Milk sours more rapidly in summer than in winter.

Souring of milk is a biochemical process called Fermentation. The process occurs due to the activity of enzymes present in milk. Fermentation is accelerated at high temperature. In summer, temperature is high as compared to winter, so process of fermentation occurs more rapidly in summer due to which milk sours more rapidly.

Short Reason

Reaction rate is influenced by temperature. The increase in temperature increases the kinetic energy of the reacting molecules thereby increasing the reaction rate. The sour taste of milk is due to fermentation process of milk which proceeds much more rapidly in summer due to greater environmental temperature than in winter.

84.  Unless ignited H₂ gas does not react with O₂ gas of air but a stream of H₂ gas passed over platinum   gauze bursts into flame.

Reaction between H₂ and O₂ to form H₂O is an example of slow molecular reaction. It is due to very high energy of activation of H₂ and O₂. On ignition, given heat is used to overcome high activation energy and H₂ and O₂ can react to form water rapidly. When H₂ is passed over O₂ in the presence of catalyst, platinum in the form gauze, the activation energy for the reaction decreases. So reaction occurs at low temperature so vigorously that H₂ gas bursts into flame.

 

85.        Food kept in refrigerators can be stored for a longer time.

The rate of reaction is directly proportional to temperature. The rate of decomposition of food slows down in refrigerators due to low temperature. So the food kept in refrigerators can be stored for longer time.

 

86.        Positive catalyst increases rate of reaction

Positive catalyst decreases activation energy of reacting molecules by providing an alternate lower energy path to them for their collisions to be effective. On account of low energy, a greater fraction of molecules acquire transition state easily thereby converting into product quickly making the rate of reaction fast

87.        Inhibitor decreases the rate of reaction.

Inhibitor or negative catalyst increases activation energy by providing an alternate higher energy path to them for their collisions to be effective. On account of high energy, a lesser fraction of molecules acquire transition state easily thereby converting into product slowly making the rate of reaction slow.

88.        Why some reactions are fast and some reactions are slow?

Activation energy controls the rate of reaction. The rate of reaction is inversely proportional to activation energy. The reactions which have lower activation energy are fast reaction. The reactions which have higher activation energy are slow reaction.

89.        Gasoline burns more rapidly in internal combustion engine than in open air

The temperature of the internal combustion engine is high as compared to open air. Due to high temperature, the kinetic energy of the gasoline molecules increases which produces greater number of effective collisions, hence rate of reaction increases. Thus gasoline burns more rapidly in internal combustion engine than in open air due to high temperature.

 

Scientific Reasons on Solution 

90.        Hydration energy decreases down each group

Hydration energy decreases down each group because down the group the size of ion increases which consequently decreases the charge density (which is inversely proportional to size of ion) which ultimately decreases the hydration energy. For example:

Hydration energy of Li⁺  =  –499 kJ/mole

Hydration energy of Na⁺=  –390 kJ/mole

Hydration energy of K⁺  =  –306 kJ/mole

 

91.        An Aqueous solution of NH₄Cl is acidic

When salts of strong acid and weak base like NH₄Cl, CuSO₄ etc. are dissolved in water, they produce respective strong acid and weak base. A higher concentration H⁺ ions make the solution acidic and turns the pH below 7.

  

92.        An Aqueous solution of Na₂CO₃ is basic

When salts of weak acid and strong base like Na₂CO₃, CH₃COONa etc. are dissolved in water, they produce respective weak acid and strong base. A higher concentration OH^– ions make the solution alkaline and turns the pH above 7.

 

93.        Conductance increases with the increase in dilution.

Conductance is defined for the number of ions per unit volume (mL). Conductivity changes with the concentration of the electrolyte. Conductance in a solution is due to presence of free ions. When solution is diluted, the total number of ions increases due to increase in the degree of dissociation. Hence, conductance increases with dilution.

As dilution increases, the concentration (molarity) of the solution decreases. The oppositely charged ions move further away from each other and due to this there is less interactions between the oppositely charged ions of the electrolyte. Due to this the ions are free from any interionic interaction (interaction between the particles of electrolyte) behaving as independent particle and hence conduct more.