2022 NTSS Chemistry P3 (Answers)

1 NEW TOWN SECONDARY SCHOOL Preliminary Examination Secondary 4 Express / 5 Normal (Academic) NAME CLASS INDEX NUMBER Science (Chemistry) Paper 3 Chemistry 5076, 5078 1 hour 15 minutes ANSWERS This document consists of 8 printed pages. Setter: Ms Caley Ng

2 Paper 3 Section A (45 marks) 1 description substance contributes to acid rain nitrogen dioxide used to control pH in soils calcium hydroxide reacts with both acids and bases zinc oxide is used to fill tungsten bulbs argon 4 2 (a) Isotopes are two or more atoms of the same element, with the same number of protons, but different number of neutrons. 1 (b) number of protons neutrons electrons a 24Mg atom 12 12 12 a 25Mg ion 12 13 10 4 (c) C: The chemical formula for magnesium chloride will always be MgCl2. E: Since the number of electrons for all isotopes of Mg atom is 12, the electronic configuration of all isotopes of Mg is 2,8,2. R: This means that they will all need to lose 2 valence electrons to chlorine atoms, hence forming the chemical formula MgCl2. 1 1 3 (a) Beryllium is reduced because the oxidation state of beryllium decreases from +2 in BeCl2 to 0 in Be; Potassium is oxidised because the oxidation state of potassium increases from 0 in K to +1 in KCl; 1 1

3 (b) Potassium reacts explosively with water and is dangerous / used up before it can react with beryllium chloride. Reject if merely mention “reacts with water”. 1 4 (a) Carbon monoxide combines with haemoglobin / binds irreversibly with haemoglobin in blood (to form carboxyhaemoglobin), which reduces the ability of haemoglobin to transport oxygen to the rest of the body, causing headache/fatigue/breathing difficulties/death in human. 1 1 (b) No. of moles of CO = 0.14 g / (12+16) = 0.005 mol 2 moles of CO : 1 mole of O2 Volume of O2 used = (0.005/2) mol x 24 dm3 = 0.06 dm3 or 60 cm3 1 1 1 (c) 1m for correct number of electrons for oxygen and carbon atoms. 1m for correct number of shared electrons. 2 (d) Carbon dioxide has a simple molecular structure with weak intermolecular forces of attraction between the molecules. Hence, little / not much energy is required to overcome these forces. All 3 points for 2m 2 points for 1m 1 1

4 5 students’ mistakes corrections to mistake Copper powder Copper(II) oxide/carbonate powder Heat the filtrate until most of the solvent has evaporated Heat the solution until saturation. Rinse the crystals with large amounts of cold water Rinse the crystals with a little cold distilled water. Both mistake + correction to be correct to obtain 1m. 3 6 (a) 2 (b) As temperature increases, kinetic energy of particles increases / particles move faster. [1] More particles have energy higher than the activation energy [1] of the reaction. Hence, frequency of effective collisions will increase [1], resulting in hig