2021 P1 Q6 - Deduce Shape and Polarity of Molecules
Here's 2021 A Levels H2 Chemistry Paper 1 Question 6.
We are required to determine which molecule has the correct molecular shape and polarity.
For each option, we need to:
- consider the dot and cross diagram to determine the number of bond pairs and lone pairs with respect to the central atom
- deduce the shape using VSEPR theory
- determine if the molecule is polar or non-polar
Let's take a look at each of these molecules.
A. Boron trichloride, BCl3
Central atom B has 3 bond pairs and no lone pair.
Shape is trigonal planar which is correct.
B-Cl bond is polar but the dipole moments from these 3 B-Cl bonds will cancel out exactly due to the high symmetry of trigonal planar shape.
Hence BCl3 is non-polar and option A is not correct.
B. Nitrogen trichloride, NCl3
Central atom N has 3 bond pairs and 1 lone pair.
Shape is trigonal pyramidal which is correct.
N-Cl bond is polar and the dipole moments from these 3 N-Cl bonds will have a net dipole moment pointing upwards.
Hence NCl3 is polar and option B is not correct.
C. Sulfur dioxide, SO2
Central atom S has 2 bond pairs and 1 lone pair.
Shape is bent and hence option C is not correct.
D. Trichloromethane, CHCl3
Central atom C has 4 bond pairs and no lone pair.
Shape is tetrahedral which is correct.
C-Cl bond is polar and C-H bond is not polar. The dipole moments from the 3 C-Cl bonds will have a net dipole moment pointing downwards.
Hence CHCl3 is polar and option D is correct.
Therefore the answer to this question is option D.
Topic: Chemical Bonding, Physical Chemistry, A Level Chemistry, Singapore
Back to list of questions for 2021 A Level H2 Chemistry Paper 1.
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