XI Chemistry Model Test Questions Test # 1 for Chapter # 3 (Chemical bond)

 XI Chemistry Model Test Questions Test # 1 for Chapter # 3 (Chemical bond) 


Q1. Draw dot and cross structures of the following molecules; O2, N2, , CH4,  CO2, CHCl3, C2H2 , C2H4     






Q2.  What is bond energy? Give applications What are the various parameters, which correlate bond energy with bond strength?


Q3. What is bond length? Give its importance. Describe factors which affect on bond length.


Q4.  What is Dipole moment? Give its unit. On what factors it depends?


Q5. Explain the ionic character of covalent bond.


Q6. Explain the effect of bonding on physical and chemical properties of compounds.


Q7.  Define any five of the following:

chemical bond, single bond, multiple bond and lone pair, bond length, bond energy, dipole moment, bond order, covalent radius


Q8. Which of the following molecules have dipole moment? In each case, give a reason for your answer

                 CO2, CHCl3, SCl2, H2O, CCl4


Q9.   What type of bonds are present in protein of hairs? On what factor hair curling depends upon? Why does hair becomes ruing in humid day?  


Q10.  Give brief answers with scientific reasons of the following:

(i)  HF has greater ionic character than HCl. (greater polarity due to greater ∆EN)

(ii)  HF forms stronger bond than HI.  (HF has BE due to strong ESF in the form polar bond)

(iii)  Bond energy of molecules possessing multiple bond is high. (Greater bond order)

(iv)  CO2 is non-polar while H2O is Polar molecule. (Zero DM of CO2 due to linear shape)

(v) Sigma bond is stronger than pi bond. (greater extent of overlapping)

(vi)  Oil is insoluble in water but soluble in hexane. Explain why? (their non-polar nature, London forces)

(vii) A polar bond is more stronger than a non-polar bond (additional ESF in the form partial ionic character)                

(viii)  The dipole moment of water is 1.84 D but CO2 has zero dipole moment why? (H2O is angular, CO2 is linear)

(ix)  CO2 has zero dipole moment while SO2 has some Dipole moment. (SO2 has bent structure giving some DM)

(x)  The boiling point of water is greater than that of HF even though F has greater electronegativity that O.

(xi)   s-s sigma bond is weaker than s-p or p-p sigma bond. (p-p overlap is greater due to its directional nature)     


Answers of XI Chemistry Model Test Questions Test # 1 for Chapter # 3 (Chemical bond)


Q2.  What is bond energy? Give applications. What are the various parameters, which correlate bond energy with bond strength?

General Definition

The energy change involved for the breaking or formation of 1 mole of particular type of bonds in the molecule is known as bond energy

 

Definition of Bond Energy in bond formation

The enthalpy change i.e. amount of energy released when one mole of (i.e. Avogadro’s number) of bonds are formed from the free constituent atoms in the gaseous state to form a gaseous molecule is known Bond making energy or bond formation energy. Bond formation energy is less accurate than bond dissociation energy. It is always negative

 

  2H(g)  ¾®   H–H(g)     DB.M.  =   – 435 kJ/mole

2O(g)     ¾®   O=O(g)     DB.M.  =   – 498 kJ/mole

2N(g)     ¾®  NºN(g)     DB.M.   =   – 946  kJ/mole

 

It is expressed in kJ/mole (i.e. energy changes per 6.02 x 1023 bonds).

Examples

 

Definition of Bond Energy in bond Breaking

The enthalpy change i.e. the amount of energy required to break one mole of bonds of a particular bond (in the gaseous phase) between two atoms in one mole of gaseous covalent molecule to form neutral gaseous atoms (or free radicals) is termed as Bond Dissociation Energy or simply bond energy designated by D (stands for dissociation) and is always a positive quantity. It is characteristic of a particular bond.

O=O(g)  →  2O(g)                          DH = +495 kJ/mol  

H–H(g)  →  2H(g)                          DH = +435 kJ/mol  

NºN(g)  →  2N(g)                          DH = +946 kJ/mol

 

 

Unit

It is reported in kJ/mol (i.e. energy change per 6.02 x 1023 bonds).

 

 

Applications/Significance of Bond Energy

1. Bond energy shows strength of bond/ Measuring Bond’s strength

Bond energy provides some idea of relative strength of different bonds in diatomic molecules. Greater the bond energy, the greater will be the strength of bond.

(e.g. The bond energy of Cl – Cl and H – H bond is 244 kJ/mole and 435 kJ/mole respectively. Hence it is obvious that Cl – Cl bond is much weaker than H–H bond).

 

2.Measuring Chemical Reactivity/Bond energy helps in understanding chemical reaction

Bond energy helps in understanding chemical reactivity of different substances. In general greater the bond energy, lesser will be the reactivity of molecules.

 

(For instance, bond energies of H2 (435 kJ mol–1) and O2 (498 kJ mol–1) are sufficiently higher than for most diatomic molecules and due to this reason when their molecules collide with each other at room temperature, they simply rebound back and no chemical reaction occurs).

 

Parameters or Factors affecting bond energy    

1. Bond Length or bond distance (inverse proportional)

2.Polarity of molecule/Partial Ionic Character/ E.N. Difference         (Directly proportional)

3.Multiple Bonds or Bond order or Bond multiplicity (Directly proportional)

 

1.    Polarity of Molecule/Ionic character of Polar Molecules

The bond energy increases with the increase in polarity of the bond. Thus bond energy of a polar molecule is always greater than non-polar molecule. It is due to the additional attraction between the partially charged atoms.

 

The bond energy of hetero diatomic molecule is relatively higher since atoms lie closer due to the presence of ionic character or opposite charges on the bonded atoms

 

For example; HCl has higher bond energy than Cl2 molecule due to polarity in the molecules. Cl2 molecule is non-polar and hence lacks polarity. This can be evident from their atomization processes:

 HCl(g) → H(g)  + Cl(g)                          DH = +431 kJ/mol  

Cl2(g)  → Cl(g)  + Cl(g)                         DH = +242  kJ/mol  

 

2.    Size of the bonded Atoms and Bond length

The value bond energy depends upon the size or radii of the bonded atoms. Two atoms of smaller size are bonded tightly due to short inter-nuclear distance thus they can be broken with difficulty and required high energy.

 

Bond energy is inversely proportional to bond length. The shorter the bond length, the stronger will be chemical bond, and greater is the value of bond energy i.e. more energy is required to break bond.

For example

Both H2 and Cl2 are non-polar molecules but bond energy of H2 is much higher than that of Cl2 due to smaller size hydrogen atoms or short bond of H2. Here polarity plays no role showing that bond energy is also affected by the size of atoms or bond length.

 

H–H(g)    →  H(g)  +     H(g)    DH = + 436 kJ/mol  

Cl–Cl(g)  →  Cl(g)   +   Cl(g)    DH = + 242 kJ/mol  

 

2.    Presence of Multiple Bonds

Generally molecules with multiple bonds require high bond energy for dissociation. As the multiple bonds are shorter, they have greater bond energies than in the case of single bonds.

For example:

Consider the bond energies involve in the carbon-carbon bond breaking. The C–C single bond has the least bond energy due to lesser number of bonds while CºC triple bond has the highest bond energy due to greater number of bods. The greater the number of bonds between two atoms, the higher the bond energy. This is due to shortening of bond or bond lengths. C–C single bond has the largest bond length while CºC triple bond has least bond length. Bond energy is inversely proportional to bond length.

C–C (1.54°A)  → 2C                         DH  =  348  kJ/mol

C=C (1.34°A)  → 2C                        DH  =  614  kJ/mol

CºC (1.20°A)  → 2C                        DH  =  839  kJ/mol

It is important to note that the bond energy of C=C is not exactly double to that of C – C and similarly the bond energy of CºC is not exactly thrice of CC bond. The reason is that double bond between two atoms express one sigma and one pi bond. Since the pi bond is relatively weak and required less energy for the breaking. The same reason is true for CºC.

 

Q3. What is bond length? Give its importance. Describe factors which affect on bond length.

Definition of Bond Length

The distance between the centres of the nuclei of two combining atoms in a molecule is called Bond Length. Since the bonded atoms are always in vibrational motion due to their kinetic energy, precise bond length cannot be determined and hence we take the average of the bond length.

 

Mathematical Definition of Bond Length (not in book)

The sum of covalent radius of two bonded atoms in a molecule is called Bond Length. For example; the covalent radius of chlorine is 0.99Å , hence bond length in Cl2 molecule would be 1.98Å.

                                Bond Length  =  r1  +  r2

 

Importance of Bond Length (Bond Length and bond strength)

Bond length is a characteristic of bond, which describes the strength of bond. The shorter the bond length the stronger is the bond.

 

Factors affecting bond length

(i)   Bond Polarity

The bond length between two different atoms is generally higher than similar atoms and its value depends upon the extent of polarity.

For example;

bond length of H–Cl (1.27 Å) is shorter than Cl–Cl (1.99 Å) due to polar nature of HCl.

 

(ii)  Bond order

The greater the number of bonds between the two atoms, the shorter the bond length.

For example;

the bond length in CºC is shortest than C=C and CC due to highest bond order of 3 of triple bond.

 

Q4. What is Dipole moment? Give its unit. On what factors it depends?

Definition of Dipole Moment

 The extent or tendency of a polar molecule to turn or orient in an electric field is called Dipole Moment (m). The moment of a particular bond is called bond moment. The sum of all bond moments in a molecule is called dipole moment.

 

Mathematically it is the product of magnitude of charge at each pole and the inter-nuclear distance (bond distance) between two opposite charges.

 

Dipole moment (m) =   Charge x distance between two opposite charge  

or              

m = e x d

                                                                                                                                                                               

Unit of Dipole Moment

1.  The S.I. unit for dipole moment is coulomb-meter (C-m). In c.g.s. system, it is expressed in electrostatic unit-centimeter (esu-cm).

                                                                                                           

2.   Dipole moment is commonly expressed in debye (D).

One debye corresponds to the dipole moment which would be produced by a negative charge of an electron (4.8 x 10–10 e.s.u or 1.6 x10–19 C) separated by 10–8 cm (10–10 m) from an equal but opposite charge.

 

Therefore, dipole moment is of the order of 10–18 e.s.u-cm or 10–20 e.s.u-m (≈10–30 C-m) which is known as debye.

1 D        =   3.34 x 10–30 C.m        (1C = 2.99 x 109 e.s.u.)

 

Factors affecting on Dipole Moment

(a)        Polarity of a molecule

(b)       Geometry of molecule

 

(a) Polarity of a molecule

The dipole moment depends upon the polarity of a molecule. For this reason, polar molecules usually possess some dipole moment. In general greater the difference of electronegativity (DE.N.) between the atoms, greater will be polarity and higher will be the dipole moment. e.g.

        DE.N.     α    Polarity α     Dipole moment


Thus HF with DE.N. of 1.9 is more polar showing dipole moment of 1.90 D than HCl with DE.N. of 0.9 showing dipole moment of 1.03 D.

 

(b) Geometry of molecule

The dipole moment of a polar triatomic or polyatomic molecule depends on both the polarity of the bonds and molecular geometry. In case of polyatomic molecule, the dipole moment of the entire molecule is made of two or more bond moments (i.e. individual dipole moments). The measured dipole moment is equal to the vector sum of the bond moments.

 

 

Q5. Explain the ionic character of covalent bond.

ionic character of covalent bond

In case of Cl2, H2, O2 and N2 molecules where the atoms are identical, the shared pair of electrons is equally attracted due to identical electronegativities. Electron density due to bonding electron-pair is symmetrically (equally) distributed over the two atoms in a molecule i.e. the two atoms do not differ in their ability to attract shared electron pair. Such molecules are called non-polar molecules.

 

But in case of H2O and HF molecules where the atoms are different, the shared pair of electrons is not equally attracted due to different electronegativities. Electron density due to bonding electron-pair is unsymmetrically (unequally) distributed over the two atoms in a molecule i.e. the two atoms differ in their ability to attract shared electron pair. Thus making the ends of molecules partial positive d+ and partial negative d. Such molecules are called polar molecules.

 

For example, the H and F atoms in HF have an electronegativity difference of 1.9, and the N and H atoms in NH3 a difference of 0.9, yet both of these compounds form bonds that are considered polar covalent.


Polar Covalent bond and Partial ionic Character

 

1. Since two atoms of different elements do not have exactly the same attraction for electrons in a bond, all bonds between unlike atoms are polar to some extent. Polar covalent bond is not a pure covalent bond but it has partial ionic character.

 

2.    The amount of ionic character in polar bond indicates the extent of shifting the electron pair (i.e. extent of charge separation).

 

3.   The amount of polarity of a bond is determined by the difference of electronegativity of the two bonded atoms. The greater the difference of electronegativity between two atoms, greater is the polarity. The extent of ionic character of a covalent bond depends on the difference of electronegativity of the two bonded atoms. Greater is the electronegativity difference between the bonded atoms, greater will be the magnitude of ionic character in the covalent bond.

 

For example;

Experimentally, the H – F bond is found to be 43% ionic while H–Cl bond is found to be 17% ionic. HF is the most polar molecule and HI is the least so.


Q6. Explain the effect of bonding on physical and chemical properties of compounds.

The nature and strength of bond affect on various physical and chemical properties of a substance such as solubility, melting point, boiling point, rate of reaction etc.

 

1. Solubility of Polar and Non Polar Covalent Compounds

The solubility of covalent compounds depends upon chemical nature of solvent and the solute.

 

(a)  Solubility of polar substances in polar solvent

when polar compound is added into a polar solvent, a dipole-dipole interaction is developed among their opposite ends, which makes it soluble.

 

For example; sugar is easily dissolved in water. This is due to several polar OH bond in sugar molecule which interact with positive and negative poles of water molecule to form hydrogen bonds. Due to this interaction sugar is soluble in water.

 

(b) Solubility of non-polar substance in polar solvent

Non-polar covalent are insoluble in polar solvent. For example when benzene put into water, it does not dissolve and form two layers. This reason is that benzene is non-polar whereas water is polar in nature. Thus the positive and negative poles of water repel benzene molecule and hence does not allow dissolving it.

 

(c) Solubility of non-polar substance into non-polar solvent

Non-polar covalent compounds are soluble in non-polar solvent on the basic principle that like dissolves like due to London forces. e.g. oil is dissolved in hexane because both are non-polar. Since interaction among the particles of both oil and hexane are weak and comparable thus on dissolution, the attractive forces of solvent are enough for overcoming the attractive forces among oil molecules hence it dissolves.

 

2.  Reactions of Polar and Non-Polar Covalent Compounds

Chemical reactions take place by the breaking of existing bonds and the formation of some new bonds. Breaking of bonds requires energy. The stronger the bond in the reactant molecule, the higher the energy needed for their breaking. Since the atoms in the polar covalent molecule are held more firmly, so high bond dissociation energy is required to proceed the reaction. The feasibility of a chemical reaction is determined by having the information of bond energy required to break the total number of reactant molecule and the bond energy release in bond formation in total number of product molecules.

 

 

Q7.        Define any five of the following:

chemical bond

“The attractive force which binds atoms together in the form stable molecule or a crystal (comprising of formula units) of a compound is called Chemical Bond.”

 

lone pair

a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bond and is sometimes called an unshared pair or non-bonding pair. 


bond length

The average optimum distance between the centers of the nuclei of two covalently bonded atoms in a molecule in an equilibrium position is called Bond Length.

OR

The sum of covalent radius of two bonded atoms (having very small energy difference) is called Bond Length. For example; the covalent radius of chlorine is    0.99 A°, hence bond length in Cl2 molecule would be 1.98 A°.

 

Bond Energy

Definition of Bond Energy in bond formation

The enthalpy change i.e. amount of energy released when one mole of (i.e. Avogadro’s number) of bonds are formed from the free constituent atoms in the gaseous state to form a gaseous molecule is known Bond making energy or bond formation energy. Bond formation energy is less accurate than bond dissociation energy. It is always negative

 

Definition of Bond Energy in bond Breaking

The enthalpy change i.e. the amount of energy required to break one mole of bonds of a particular bond (in the gaseous phase) between two atoms in one mole of gaseous covalent molecule to form neutral gaseous atoms (or free radicals) is termed as Bond Dissociation Energy or simply bond and is always a positive quantity.

 

dipole moment

The extent or tendency of a polar molecule to turn or orient in an electric field is called Dipole Moment (m). The moment of a particular bond is called bond moment. The sum of all bond moments in a molecule is called dipole moment. It is the measure of orientation (to rotate) of a polar molecule in an electric filed.

 

Mathematically it is the product of magnitude of charge at each pole and the inter-nuclear distance (bond distance) between two opposite charges.

 

bond Order

the bond order is the number of bonds that forms in between the two atoms in a molecule.

H – H Bond order = 1

O = O Bond order = 2

N ≡ N Bond order = 3

C ≡ O Bond order = 3

 

single bond

The covalent bond which involve sharing of one electron pair bonding atoms denoted by single short line is called single bond. It is equivalent to one sigma bond.

 

multiple Bond

The covalent bond which involve sharing of two or three electron pairs bonding atoms denoted by double or triple line is called multiple bond. In fact double and triple bonds are collectedly called multiple bonds. It is equivalent to one or two pi bond.

 

covalent Radius

A covalent radius is the half of the bond length between the nuclei of two identical atoms which are bonded through single covalent bond. 

 

 

Q8.  Which of the following molecules have dipole moment? In each case, give a reason for your answer

CO2, CHCl3, SCl2, H2O, CCl4

Answer 

CO2, has zero dipole moment due to linear geometry as vector sum of bond moments is zero.


CHCl3, has some dipole moment due to unsymmetrical tetrahedral geometry as vector sum of bond moments is not zero.


SCl2, has some dipole moment due to angular geometry as vector sum of bond moments is not zero.


H2O, has some dipole moment due to angular geometry as vector sum of bond moments is not zero.


CCl4 has some zero dipole moment due to symmetrical tetrahedral geometry as vector sum of bond moments is zero

 

Q9.  What type of bonds are present in protein of hairs? On what factor hair curling depends upon? Why does hair becomes ruing in humid day? 

Hairs contains protein called keratin composed of many amino acids (cysteine) link to each other by permanent disulphide bonds. The distance between sulphur atoms cause hairs to curl. The greater the distance among sulphur atoms, the more keratin molecule bend and more our hair curls.

 

Another type of connection among neighbors’ amino acid chains is hydrogen bond, which is much weaker and temporary. This hydrogen bond is affected by water. In humid day, moisture of atmosphere breaks hydrogen bond of hair and makes the hair ruin.

 

 

Q10.  Give brief answers with scientific reasons of the following:

(i)           HF has greater ionic character than HCl. (greater polarity due to greater ∆EN)

(ii)         HF forms stronger bond than HI.  (HF has BE due to strong ESF in the form polar bond)

(iii)        Bond energy of molecules possessing multiple bond is high. (Greater bond order)

(iv)        CO2 is non-polar while H2O is Polar molecule. (Zero DM of CO2 due to linear shape)

(v)         Sigma bond is stronger than pi bond. (greater extent of overlapping)

(vi)        Oil is insoluble in water but soluble in hexane. Explain why? (their non-polar nature, London forces)

(vii)       A polar bond is more stronger than a non-polar bond (additional ESF in the form partial ionic character)                

(viii)     The dipole moment of water is 1.84 D but CO2 has zero dipole moment why? (H2O is angular, CO2 is linear)

(ix)        CO2 has zero dipole moment while SO2 has some Dipole moment. (SO2 has bent structure giving some DM)

(x)          The boiling point of water is greater than that of HF even though F has greater electronegativity that O.

(xi)        s-s sigma bond is weaker than s-p or p-p sigma bond. (p-p overlap is greater due to its directional nature)     

 

(i) HF has greater ionic character than HCl. (greater polarity due to greater ∆EN)

Ionic character of covalent bond increases with the increase in electronegativity difference between the two bonded atoms. Since HF has highest electronegativity difference (1.9) than HCl (0.9), it has greater polarity which in turn increases its extent of ionic character. Hence HF is 43% ionic while HCl is 17% ionic.

 

(ii)HF forms stronger bond than HI.  (HF has BE due to strong ESF in the form polar bond)

Fluorine being the most electronegative element and due to its very small size is able to form strong polar bond with hydrogen. In addition, the strength of hydrogen bonding in HF is the strongest. HI bond is almost non-polar due to small electronegativity of iodine.

   

(iii) Bond energy of molecules possessing multiple bond is high. (Greater bond order)

As the multiple bonds are shorter, they have greater bond energies than in the case of single bonds due to greater bond order.

For example:

The C–C single bond has the least bond energy due to lesser number of bonds or lesser bond order while CºC triple bond has the highest bond energy due to greater number of bonds or bond order.

                              


(iv)   CO2 is non-polar while H2O is Polar molecule. (Zero DM of CO2 due to linear shape)

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.


 

(v) Sigma bond is stronger than pi bond. (greater extent of overlapping)

The relative strength of a sigma bond is related to the extent of overlap of the atomic orbitals. In sigma bonds, there is sufficient overlapping between atomic orbitals as compared to pi bond where there is partial or less extent overlapping. Thus sigma bond is stronger due to greater overlapping than pi bond.

 

(vii) A polar bond is more stronger than a non-polar bond (additional ESF in the form partial ionic character)                

Polar bond has partial ionic character due to difference in electronegativity. 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. 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 polar bond.

 


(ix)  CO2 has zero dipole moment while SO2 has some Dipole moment. (SO2 has bent structure giving some DM)

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. SO2 has angular or bent structure due to which bond dipoles do not cancel out. Thus SO2  show m = 1.84 D and SO2 molecule as a whole is polar.



(x)  The boiling point of water is greater than that of HF even though F has greater electronegativity that 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 (Hd+) enabling three dimensional hydrogen bonding of greater chain length where as in HF there is only one polar hydrogen (Hd+) permitting only two dimensional hydrogen bonding giving chain of limited length. That is why water has higher B.P. than HF.


(xi)  s-s sigma bond is weaker than s-p or p-p sigma bond. (p-p overlap is greater due to its directional nature)     

The relative strength of a sigma bond is related to the extent of overlap of the atomic orbitals. This is known as the 'principle of maximum overlap'. Strength of sigma bond is also directly proportional to directional character.

Due to spherical charge distribution and non-directional nature of s-orbital s-s overlapping is not so effective as s-p and p-p overlapping.

Whereas p-orbitals have directional charge distribution and longer lobes which cause more effective overlapping. Thus s-s sigma bond is relatively weak. So order for strength of sigma bond should be:

p-p > s-p > s-s



 

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