chemistry : class11 Ionic Character of Covalent Bonds : Polar character of covalent bond : Chemical Bonding and Molecular Structure MCQ Questions and Answers
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Ionic Character of Covalent Bonds
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MCQ Questions
1. Using Pauling’s relation, % ionic character = [1 − exp(−0.25(Δχ)^2)] × 100, what is the approximate ionic character of a covalent bond for which Δχ = 1.50?
A.
22.1%
B.
30.2%
C.
43.0%
D.
63.2%
Correct Answer:
43.0%
Explanation:
Substituting Δχ = 1.50 gives [1 − exp(−0.25 × 1.50²)] × 100 = [1 − exp(−0.5625)] × 100 ≈ 43.0%. Thus, the bond has approximately 43% ionic character.
2. A diatomic molecule has a bond length of 1.00 Å and an observed bond dipole moment of 1.85 D. Taking the dipole moment for complete ionic separation at this distance as 4.80 D, what is the percentage ionic character of the bond?
A.
24.0%
B.
38.5%
C.
52.0%
D.
77.0%
Correct Answer:
38.5%
Explanation:
Percentage ionic character = (observed dipole moment/full ionic dipole moment) × 100 = (1.85/4.80) × 100 ≈ 38.5%. This compares the actual charge separation with complete transfer of one electronic charge.
3. Which sequence correctly represents the increasing ionic character of the hydrogen halide bonds, assuming the bond polarity is estimated primarily from electronegativity difference?
A.
HF < HCl < HBr < HI
B.
HI < HBr < HCl < HF
C.
HCl < HF < HI < HBr
D.
HBr < HI < HF < HCl
Correct Answer:
HI < HBr < HCl < HF
Explanation:
The electronegativity of the halogen decreases from F to I. Therefore, the electronegativity difference from hydrogen, and hence the ionic character of the H–X bond, decreases in the order HF > HCl > HBr > HI.
4. A covalent bond has a measured dipole moment of 2.40 D and a bond length of 1.20 Å. Using 1 eÅ = 4.80 D, what fraction of an electronic charge is effectively separated across the bond?
A.
0.20e
B.
0.42e
C.
0.58e
D.
0.96e
Correct Answer:
0.42e
Explanation:
The effective charge separation is q/e = μ/(4.80r) = 2.40/(4.80 × 1.20) = 0.4167, approximately 0.42e. This indicates partial, rather than complete, ionic character.
5. Two bonds have the same electronegativity difference and therefore the same estimated percentage ionic character. Bond X is 1.00 Å long, whereas bond Y is 1.50 Å long. Which statement about their bond dipole moments is most appropriate?
A.
Bond X must have the larger dipole moment because it is shorter
B.
Bond Y must have the larger dipole moment because the same fractional charge is separated over a greater distance
C.
Both must have zero dipole moment because their ionic characters are equal
D.
Their dipole moments cannot be compared because ionic character is unrelated to charge separation
Correct Answer:
Bond Y must have the larger dipole moment because the same fractional charge is separated over a greater distance
Explanation:
Bond dipole moment is μ = q × r. If both bonds have the same fractional charge q, the longer bond Y has the larger dipole moment because its charge separation distance is greater.
6. Carbon dioxide has polar C=O bonds, yet its net molecular dipole moment is zero. What does this most directly demonstrate regarding ionic character?
A.
Each C=O bond has zero ionic character
B.
Ionic character exists only in molecules with a nonzero net dipole moment
C.
Individual bonds can possess ionic character even when their bond dipoles cancel vectorially
D.
The electronegativity difference between carbon and oxygen is zero in carbon dioxide
Correct Answer:
Individual bonds can possess ionic character even when their bond dipoles cancel vectorially
Explanation:
Each C=O bond has unequal electron sharing because oxygen is more electronegative than carbon, giving the bond partial ionic character. In linear CO2, the two equal bond dipoles point in opposite directions and cancel.
7. According to Pauling’s empirical expression for ionic character, which bond would have the greatest ionic character among bonds with electronegativity differences of 0.8, 1.2, 1.6, and 2.0?
A.
The bond with Δχ = 0.8
B.
The bond with Δχ = 1.2
C.
The bond with Δχ = 1.6
D.
The bond with Δχ = 2.0
Correct Answer:
The bond with Δχ = 2.0
Explanation:
The Pauling expression increases monotonically with the square of Δχ. Therefore, the bond with Δχ = 2.0 has the greatest predicted ionic character, approximately 63.2%.
8. A bond has a dipole moment of 1.44 D and a bond length of 1.00 Å. Taking 4.80 D as the dipole moment corresponding to complete ionic separation over 1.00 Å, what is the approximate ionic character of the bond?
A.
15%
B.
30%
C.
44%
D.
69%
Correct Answer:
30%
Explanation:
Percentage ionic character = (1.44/4.80) × 100 = 30%. The observed dipole corresponds to an effective separation of 0.30 electronic charge.
9. Which statement best explains why a covalent bond may be described as having partial ionic character rather than being classified as completely ionic?
A.
The bonded electrons are completely transferred to the more electronegative atom
B.
The bonded electrons are shared unequally, producing partial charges rather than full unit charges
C.
The two atoms must have identical electronegativities
D.
The bond must have no measurable dipole moment
Correct Answer:
The bonded electrons are shared unequally, producing partial charges rather than full unit charges
Explanation:
In a polar covalent bond, the electron pair remains shared but is displaced toward the more electronegative atom. This produces partial positive and negative charges, giving the bond some ionic character without complete electron transfer.
10. Two covalent bonds have identical bond lengths of 1.25 Å. Bond P has a dipole moment of 1.50 D and bond Q has a dipole moment of 2.40 D. Which conclusion follows from their dipole moments?
A.
Bond P has greater ionic character because its dipole moment is smaller
B.
Bond Q has greater ionic character because the same bond length supports greater effective charge separation
C.
Both bonds have identical ionic character because their lengths are equal
D.
Neither bond can have ionic character unless its dipole moment is 4.80 D
Correct Answer:
Bond Q has greater ionic character because the same bond length supports greater effective charge separation
Explanation:
Since μ = q × r and both bond lengths are equal, the larger dipole moment of bond Q indicates a larger effective partial charge separation. Therefore, bond Q has greater ionic character.
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