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 higher speed of reaction. 3 (c) Test the gas with a lighted splint. In the presence of hydrogen, it will extinguish with a ‘pop’ sound / ‘pops’ with a lighted splint. 1 1 (d) Rate/speed of reaction is slower (zinc is less reactive than magnesium). Must show comparison. 1 Graph (a) Gentle gradient + longer time for complete reaction [1] Same final mass [1]

5 7 (a) K – iron(II) sulfate / FeSO4 L – barium sulfate / BaSO4 M – iron(II) hydroxide / Fe(OH)2 N – iron(III) hydroxide / Fe(OH)3 1 1 1 1 (b) Fe(NO3)2 + 2NaOH Fe(OH)2 + 2NaNO3 OR Ba(NO3)2 + FeSO4 Fe(NO3)2 + BaSO4 1m correct formulas 1m balancing of equations 2 8 (a) Gradual change in physical properties as number of C atoms increases Same functional group Same general formula The molecular formula of each member differs from the next by a unit of –CH2 Similar chemical properties Any 2 2 (b) Circle the –COOH functional group and label ‘carboxylic acid’ 1 (c) 1 (d) Nickel 1

6 Paper 3 Section B (20 marks) 9 (a) (i) Colour gets darker down the group. Melting / boiling point increases down the group. Density increases down the group. Any 1 1 (ii) Black solid remains in colourless solution. Accept: no visible change. Astatine is less reactive than iodine, hence, it is unable to displace iodide ions from potassium iodide. 1 1 (b) (i) Note: - correct number of valence electrons for both ions - correct charge written for both ions - correct number of ions (2 for Cl -) / can accept ‘2’ written in front of 1 Cl - diagram 1 or 2 points, 1m All 3 points to get 2m 2 (ii) C: It cannot conduct electricity [1]. E: It is in the solid state at room temperature. R: The ions are in a fixed crystal lattice and cannot move around freely [1]. 2 (c) 1) Add the silver nitrate solution to a beaker containing sodium chloride (or other soluble chloride / hydrochloric acid) solution. [1] 2) Filter the mixture to obtain silver chloride as residue. [1] 3) Wash the residue with cold distilled water and dry residue between pieces of filter paper. [1] 3 Ca

7 10 (a) (i) Silicon dioxide. 1 (ii) Limestone [1] and coke / carbon [1]. 2 (iii) The limestone decomposes in the furnace to produce calcium oxide and carbon dioxide. [1] CaCO3 (s) CaO (s) + CO2 (g) The calcium oxide reacts with sand / silicon dioxide to produce molten slag, which is removed from the blast furnace. [1] CaO (s) + SiO2 (s) → CaSiO3 (l) Explanation need not be accompanied by an equation. 2 (b) (i) No. of moles of Fe2O3 = = 0.0125 mol 1 (ii) No. of moles of KOH = 0.02 dm3 x 4.00 mol/dm3 = 0.08 mol 1 (iii) 1 mole of Fe2O3 : 10 moles of KOH 0.0125 moles of Fe2O3 : 0.125 moles of KOH Since 0.125 mol > 0.08 mol, KOH is the limiting reactant. Working can be shown the other way round as well (using KOH as the starting point). Working is not required to obtain the answer. 1 (iv) 10 moles KOH : 2 moles K2FeO4 0.08 moles KOH : x 0.08 = 0.016 moles K2FeO4 [1] Mass of K2FeO4 = 0.016 x (39 x 2 + 56 + 16 x 4) = 3.17 g (to 3 s.f.) [1] 2

8 11 (a) (i) 600⁰C / high temperature Aluminium oxide or silicon dioxide as catalyst Both conditions to obtain 1m. 1 (ii) C3H8 Reject “propane” as question asks for molecular formula. 1 (iii) Test: Add aqueous bromine to each sample. Observation: If reddish-brown bromine is decolourised, that sample is ethene. If reddish-brown bromine remains, that sample is X. Both positive and negative results must be mentioned to obtain 1m. 1 1 (b) Reject if O-H is drawn as “OH” without the bond. 1 (c) (i) At high temperature, the enzyme in yeast will be denatured and the reaction stops. Accept “yeast will be denatured” without mentioning enzyme. 1 (ii) The blue litmus paper will turn red [1] and (the red litmus paper remains red). This is due to the presence of ethanoic acid formed from the oxidation of ethanol. [1] 2 (iii) C2H5OH + 3O2 → 2CO2 + 3H2O 1m correct formulas 1m balancing of equations Ignore state symbols. 2