RI 2017 A-Level H2 Chem Solution

This document is copyrighted, please do not reproduce it without permission © Raffles Institution 2017 A-Level H2 Chemistry Paper 1 Suggested Solutions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 C B D C D B B B C B A A D D A 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 B A D A C A A D C D A A B C A Q1 (C) Isotope Abundance / % 28Si 92.23 29Si x 30Si (100 – 92.23 – x) = (7.77 – x) 28(92.23) + 29(x) + 30(7.77 – x) = 28.10(100) x = 5.54% Q2 ( B) 1 Correct. E.g. 9 4Be has 5 neutrons and 4 protons. 2 Correct. E.g. 16 8 Ohas 8 neutrons and 8 protons. 3 Incorrect. There are no elements from Li to Mg which have more protons than neutrons. Q3 (D ) H C H O 3 regions of electron density around C atom of methanal ⇒ trigonal planar shape and bond angle of 120° Q4 (C) 1 Yes. The hydrogen bonds between water molecules are stronger than the id-id interactions between methane, requiring more energy to overcome, resulting in the higher boiling point of water. 2 No. O–H bonds are stronger than C–H bonds. However, boiling does not involve breaking the O–H and C–H bonds. It involves overcoming their intermolecular forces of attraction. 3 No. Water (10 e–) contains more 2 electrons than methane (8 e–). With a slightly larger electron cloud, water has slightly stronger id-id interactions. However, this does not account for the much higher boiling point of water, which can only be attributed to hydrogen bonding between water molecules. Q5 (D) A Incorrect. A change in pressure has no effect on temperature of a gas. B Incorrect. The kinetic energy of gas molecules depends on the temperature, not the pressure. C Incorrect. Pressure has no effect on the size of the molecules. D Correct. Increasing pressure moves the molecules closer, increasing the intermolecular forces of attraction so significantly that the gas becomes liquid. Q6 (B ) Recall: Lewis base is an electron pair donor. Bronsted-Lowry acid is a proton donor. A Incorrect. H+ does not have electron pairs and cannot be a Lewis base. B Correct. Electron pairs on O atom of H2O allow water to act as a Lewis base. H 2O can donate a H + (H2O → OH– + H+) ans acts as a Bronsted-Lowry acid. C Incorrect. O2– is unable to donate H+. D Incorrect. Electron pairs on O atom of OH – allow it to act as a Lewis base. OH – can donate a H + (OH– → O2– + H+) and acts as a Bronsted-Lowry acid. Q7 (B ) Mg, A l, Si and P are consecutive elements in Period 3. From the data booklet, the following information about their atomic radii can be obtained: • a tomic radius of Mg > Al > Si > P. • atomic radius of Si is closer to that of P than of Al. Since atomic radius is the x -axis, going from left to right, the elements should be in the order P, Si, Al, Mg, and Si is closer to P than Al (i.e. more like option B than D). Electronegativity increases across the period: Mg < Al < Si < P. Since electronegativi