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Chemical bonding and molecular shape
8 MDCAT Chemistry questions with answers and explanations.
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What is the shape of the ammonia (NH3) molecule?
- 1 Linear
- 2 Trigonal planar
- 3 Tetrahedral
- 4 Trigonal pyramidal
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Correct answer: Trigonal pyramidal
Nitrogen has three bond pairs and one lone pair. The lone pair pushes the bonds into a pyramid shape.
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Which molecule is linear?
- 1 Water (H2O)
- 2 Carbon dioxide (CO2)
- 3 Ammonia (NH3)
- 4 Methane (CH4)
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Correct answer: Carbon dioxide (CO2)
CO2 has two double bonds and no lone pairs on carbon, so the molecule is a straight line (180 degrees).
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What is the hybridisation of each carbon atom in ethene (C2H4)?
- 1 sp
- 2 sp2
- 3 sp3
- 4 sp3d
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Correct answer: sp2
Each carbon forms three sigma bonds and one pi bond, which uses sp2 hybrid orbitals.
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The unusually high boiling point of water is mainly due to:
- 1 Ionic bonding
- 2 Hydrogen bonding
- 3 Metallic bonding
- 4 Covalent bonds in water being weak
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Correct answer: Hydrogen bonding
Hydrogen bonds between water molecules need a lot of energy to break, so water boils at a much higher temperature than similar small molecules.
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Which of these is an ionic compound?
- 1 HCl gas
- 2 Sodium chloride
- 3 Methane
- 4 Carbon dioxide
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Correct answer: Sodium chloride
Sodium chloride is formed by transfer of an electron from sodium to chlorine. The others are covalent.
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How many sigma and pi bonds are there in ethyne (C2H2)?
- 1 2 sigma and 3 pi
- 2 3 sigma and 2 pi
- 3 5 sigma and no pi
- 4 4 sigma and 1 pi
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Correct answer: 3 sigma and 2 pi
The C-C triple bond has one sigma and two pi bonds. Add two C-H sigma bonds and the total is 3 sigma and 2 pi.
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Which of these molecules is polar?
- 1 CO2
- 2 CH4
- 3 H2O
- 4 BF3
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Correct answer: H2O
Water is bent, so the bond dipoles do not cancel. CO2, CH4 and BF3 are symmetrical and non-polar.
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The bond angle in a water molecule is about:
- 1 90 degrees
- 2 104.5 degrees
- 3 109.5 degrees
- 4 120 degrees
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Correct answer: 104.5 degrees
Two lone pairs on oxygen squeeze the H-O-H angle to about 104.5 degrees, smaller than the tetrahedral 109.5.