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)
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) DH°B.M. = – 435 kJ/mole
2O(g) ¾® O=O(g) DH°B.M. = – 498 kJ/mole
2N(g) ¾® NºN(g) DH°B.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 C–C 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 C–C 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 O–H 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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