chemistry : class11 Symmetrical Bond Polarity : Polar character of covalent bond : Chemical Bonding and Molecular Structure MCQ Questions and Answers
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Symmetrical Bond Polarity
MCQ questions and answers.
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MCQ Questions
1. Which molecule has polar bonds but zero resultant dipole moment because its bond dipoles cancel symmetrically?
A.
H2O
B.
NH3
C.
CO2
D.
SO2
Correct Answer:
CO2
Explanation:
Each C=O bond in CO2 is polar, but the molecule is linear and symmetrical. The two equal bond dipoles act in opposite directions and cancel, giving zero resultant dipole moment.
2. In which species do three identical bond dipoles cancel because they are directed symmetrically at 120° to one another in one plane?
A.
BF3
B.
NH3
C.
PCl3
D.
SO2
Correct Answer:
BF3
Explanation:
BF3 is trigonal planar and symmetrical. Its three identical B–F bond dipoles are separated by 120°, so their vector sum is zero despite the polarity of each B–F bond.
3. Which statement correctly distinguishes bond polarity from molecular polarity in a symmetrical molecule?
A.
A molecule is always polar if at least one of its bonds is polar.
B.
Polar bonds can produce a nonpolar molecule when their dipole moments cancel vectorially.
C.
A molecule with zero dipole moment must contain only nonpolar bonds.
D.
Symmetry increases the resultant dipole moment of identical polar bonds.
Correct Answer:
Polar bonds can produce a nonpolar molecule when their dipole moments cancel vectorially.
Explanation:
Bond polarity depends on electronegativity difference, whereas molecular polarity depends on the vector sum of all bond dipoles. In a sufficiently symmetrical arrangement, polar bond dipoles can cancel completely.
4. Which molecule has a zero dipole moment specifically because four identical bond dipoles are directed toward the corners of a regular tetrahedron?
A.
CHCl3
B.
CCl4
C.
CH2Cl2
D.
SiHCl3
Correct Answer:
CCl4
Explanation:
CCl4 has four identical C–Cl bonds arranged tetrahedrally around carbon. The high symmetry causes the four equal bond-dipole vectors to cancel, resulting in zero molecular dipole moment.
5. Which pair contains molecules in which the central atom is bonded to identical surrounding atoms, but only one molecule has zero resultant dipole moment because of its molecular symmetry?
A.
CO2 and H2O
B.
NH3 and BF3
C.
BF3 and NH3
D.
SO2 and CO2
Correct Answer:
BF3 and NH3
Explanation:
BF3 has three identical B–F bonds arranged trigonal-planarly, so its bond dipoles cancel. NH3 has a trigonal pyramidal shape due to its lone pair, so the N–H bond dipoles do not cancel completely.
6. A molecule has six identical polar bonds arranged symmetrically along the six directions of an octahedron around the central atom. What is expected for its resultant dipole moment?
A.
It is equal to the dipole moment of one bond.
B.
It is twice the dipole moment of one bond.
C.
It is zero because opposite and equivalent bond dipoles cancel.
D.
It is nonzero because six polar bonds reinforce one another.
Correct Answer:
It is zero because opposite and equivalent bond dipoles cancel.
Explanation:
In an octahedral arrangement such as that of SF6, each bond has an opposite equivalent bond. The six identical bond-dipole vectors cancel pairwise, producing zero resultant dipole moment.
7. Which structural change would destroy the zero dipole moment of a perfectly symmetrical tetrahedral molecule EX4 while retaining the tetrahedral arrangement?
A.
Replacing one X atom by a different atom Y
B.
Rotating the entire molecule in space
C.
Changing the direction from which the molecule is viewed
D.
Reversing the labeling of two identical X atoms
Correct Answer:
Replacing one X atom by a different atom Y
Explanation:
The zero dipole moment of EX4 depends on four identical E–X bond dipoles arranged symmetrically. Replacing one X by Y makes the bond dipoles unequal, so complete cancellation is lost.
8. Which molecule is nonpolar overall even though its central atom is connected to four electronegative atoms through polar bonds?
A.
CH2Cl2
B.
CCl4
C.
CHCl3
D.
CF2Cl2
Correct Answer:
CCl4
Explanation:
The four C–Cl bonds in CCl4 are identical and symmetrically arranged in a tetrahedral geometry. Their dipole moments cancel, whereas the mixed substituents in the other molecules destroy this symmetry.
9. For a linear symmetrical molecule X–A–X, where both A–X bonds have equal bond-dipole magnitudes, which conclusion is correct?
A.
The molecule must have a nonzero dipole moment because both bonds are polar.
B.
The two bond dipoles add in the same direction along the molecular axis.
C.
The two bond dipoles are equal and opposite, giving zero resultant dipole moment.
D.
The molecular dipole moment is independent of the linear arrangement.
Correct Answer:
The two bond dipoles are equal and opposite, giving zero resultant dipole moment.
Explanation:
In a linear symmetrical X–A–X molecule such as CO2, the two identical bond dipoles point in opposite directions along the same line. Their vector sum is therefore zero.
10. Which comparison correctly explains why XeF4 has zero dipole moment while SF4 has a nonzero dipole moment?
A.
XeF4 has nonpolar bonds, whereas SF4 has polar bonds.
B.
XeF4 has a symmetrical square-planar arrangement, whereas SF4 has a distorted seesaw arrangement.
C.
XeF4 has a linear structure, whereas SF4 has a trigonal-planar structure.
D.
XeF4 contains no lone pairs, whereas SF4 contains no lone pairs.
Correct Answer:
XeF4 has a symmetrical square-planar arrangement, whereas SF4 has a distorted seesaw arrangement.
Explanation:
The Xe–F bonds in XeF4 are polar, but the square-planar symmetry makes their dipoles cancel. SF4 has a seesaw geometry with unequal spatial distribution of bonds and lone-pair influence, so cancellation is incomplete.
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