9 C 10 D 11 C The CH 3 C(CH 3 ) 2 CH 3 molecule is spherical and compact. The molecular surface area available for coming into contact with neighbouring molecules is the smallest. Thus, the van der Waals’ forces between CH 3 C(CH 3 ) 2 CH 3 molecules are weakest. 12 D 13 D Molecules of Y and Z can form hydrogen bonds with water molecules while those of X cannot. Thus, the water solubilities of Y and Z are higher than that of X. Each molecule of Z has two – OH groups while each molecule of Y has only one. Molecules of Z can form more hydrogen bonds with water molecules. Thus, the water solubility of Z is higher than that of Y. The order of increasing water solubility is: X < Y < Z 14 B (2) The electronegativity of Group I elements decreases with increasing atomic number. 15 A (1) Both alcohols have a – OH group. Their boiling point difference is due to the difference in the strength of van der Waals’ forces between molecules.
New 21st Century Chemistry (2nd Edition) Suggested answers to in-text activities and unit-end exercises 5 © Jing Kung. All rights reserved . Topic 6 Unit 24 Molecule of X has a linear shape while that of Y has a more spherical and compact shape. The molecular shape of X allows greater surface contact between molecules. The van der Waals’ forces in X are stronger. Thus, the boiling po int of X is higher than that of Y. (2) The van der Waals’ forces in X are stronger, pulling the molecules close together. Thus, the density of X is higher than that of Y. 16 a) b) i) Octahedral ii) Molecule of BF 3 Molecule of NH 3 ii) BF 3 has a symmetrical shape. The three identical bond dipole moments cancel one another out exactly. 17 a) i) ii) A hydrogen sulphide molecule has two lone pairs and two bond pairs of electrons in the outermost shell of the sulphur atom. These electron pairs repel to get as far apart as possible. So, the four electron pairs will adopt a tetrahedral arrangement. The shape of a molecule is determined only by the arrangement of atoms. Thus, the hydrogen sulphide molecule is V-shape or bent shape.