BTYSS 2024 FYE 4E CHEM MS

Answers Paper 1 1 2 3 4 5 6 7 8 9 10 D C B B D C B C B A 11 12 13 14 15 16 17 18 19 20 B C B B D A B C D C 21 22 23 24 25 26 27 28 29 30 D A D D D D C D A B 31 32 33 34 35 36 37 38 39 40 C D B C C B C A D A Paper 2 Section A Question no Answers Marks 1 (a) PbO 1 (b) KI 1 (c) SO2 1 (d) CuCO3 1 (e) PbBr2 1 Total:5 Remarks/ common mistakes: - If students wrote 2 answers, only the first one will be marked. - For (b), many students wrote PbBr2 which is incorrect as it is insoluble - For (e), many students wrote KI and it is incorrect as KI is a soluble salt and does not prepared using the precipitation method. 2(a) 76/100 x 35 + 24/100 x 37 = 35.5 Accept: 35.48. No mark is penalised for wrong units. 1 (b)(i) Simple (covalent) molecular structure./Simple covalent molecule/ simple covalent structure with small discrete molecules. It has a low melting and boiling point as it is made up of weak intermolecular forces of attraction between the molecules where little energy is required to overcome the forces of attraction. Reject: simple covalent bond/ simple covalent structure. Remarks: Most students unable to state the structure present. Some students confused with ionic bonding or giant covalent structure. 1 1

(b)(ii) 2 Remarks: Some students draw ionic bonding instead. Students did badly for this question, Total:5 3(a)(i) It indicates that chlorine has an oxidation state of +5. Badly done. Reject: Charge of chlorine is +5, valence electrons are 5, just state the (V) represent oxidation state but did not indicate value. 1 (a)(ii) The statement is true. As the oxidation state of chlorine in NaClO3 is decreased from +5 to −1 in NaCl, chlorine is being reduced. As the oxidation state of oxygen in NaClO3 is increased from -2 to 0 in O2, oxygen is being oxidised. As both reduction and oxidation is happening simultaneously, it is a redox reaction. Remarks: Many students compared the oxidation state of sodium instead of chlorine. Stated the oxidation state of oxygen as -6 instead of -2. 1 1 (a)(iii) Use a magnet to attract the iron catalyst from the mixture and a pure and dry sample of sodium chloride will be obtained. Accept: using magnetic attraction /magnetic separation to separate iron form sodium chloride. 1 (b) Sodium chlorate(V) will decompose before its boiling point can be measured. Reject: sodium chlorate (V) sublimes/temperature is too high to be measured. 1 (c) Number of moles of O2 = 450 /24000 = 0.01875mol From equation, 3 mol O2 : 2 mol NaClO3 0.01875 mol O2 : 0.0125 mol NaClO3 1 T T Cl Cl

Mr of NaClO3 = 106.5 Mass of NaClO3 = 106.5 x 0.0125 = 1.33g Percentage purity = (1.33/5) *100 = 26.6% (R: if round off to 1dp, A: 26.625%) Accept: No of moles of NaClO3 = 5/106.5 = 0.0469mol Volume of O2 (theoretical) = 0.0469 /2 x3 x24 = 1.69dm3 Percentage purity = 450/1690 x100 = 26.6% 1 1 Total: 8 4(a) Petrol is not a pure substance/mixture/ contains impurities OR it consists of a mixture of hydrocarbons. Hence, there is no fixed enthalpy change of combustion. 1 (b)(i) CH4 + 2O2 → CO2 + 2H2O 1 (ii) As methane has a higher percentage of carbon than methanol, More energy is released in forming carbon dioxide and water vapour than energy absorbed in breaking the bonds in methane and oxygen 1 1 (iii) 1 m for labelled reactant and product 1m for labelling of activation energy and ∆H 1m for correct energy diagram 3 (c) Complete combustion of hydrogen produces only water which is a non-pollutant/no negative impact to the environment. However, combustion of methane produces carbon dioxide which is a greenhouse gas that leads to global warming. 1 1 (d) The carbon monoxide produced is toxic as it binds to haemoglobin / prevents the blood to transport oxygen around the body and leads to breathlessness and eventually death. Or The methanol used is toxic as it can lead to headache, dizziness or loss of consciousness. 1 Total: 10 progress of reaction energy EA ∆H = -891 kJ/mol CH4 + 2O2 CO2 + H2O

5(a) To ensure the piece of magnesium is fully reacted/ all Mg is fully used up. Reject: To ensure Mg is fully dissolved 1 (b) Increase temperature – particles move faster and have energy equal to or greater than the activation energy / particles move faster, which increases the frequency of effective collisions. Higher concentration, more particles present per unit volume. Increased frequency of effective collision, higher rate of reaction. Note:3rd mark is only allocated if students wrote increased frequency of collision leads to increase in frequency of effective collisions for both factors 1 1 1* (c) Hydrogen gas produced pushes / carried up the metal to the surface from reacting acid and metal / The presence of effervescene / bubbles pushes / carried up the metal to the surface Remarks: A lot of students mentioned that magnesium is less dense than water which is incorrect. 1 (d)(i) Copper does not react with hydrochloric acid and no gas is produced. / zinc and iron react with acid to produce hydrogen gas. R: Should not just mention it is unreactive. Need to link the the experiment which is the reaction with acids. 1 (d)(ii) The experiment with copper. Copper is acting as a catalyst in the experiment. Note: Copper is a transition metal which act as a catalyst in the reaction. 1 (d)(iii) The gradient of the graph will be gentler / rate of reaction is slow than both graphs but with the same amount of hydrogen gas produced. 1 Total:8 6(a) Electrode 2 is a cathode as (zinc loses electrons) as it is the more reactive metal and the electrons flow to electrode 2/ flow from negative to positive terminal. 2H+(aq) + 2e- → H2(g) R: If no state symbols are seen in half equation./ Move from more reactive to less reactive electrode. 1 1 (b) Effervescence will be seen./bubbles will be seen/ greenish yellow gas is seen. 1

(c) The colour of the electrolyte (universal indicator) changes from green to purple/blue. H+ and Cl- are being discharged, leaving behind K+ and OH-. This results in the formation of potassium hydroxide which is an alkali which turns universal indicator purple/blue. Accept: Concentration of hydrogen ions is less than concentration of hydroxide, thus universal indicator turns purple/blue R: The concentration of hydroxide ions will increase. 1 1 (d) Cu2+(aq) + 2e- → Cu(s) 1 (e) Replace zinc with a metal, magnesium which is further apart in the reactivity series than zinc and copper. (Do not accept reactive metals such as sodium, potassium and calcium) R: If no mentioned of the metal to be replace. 1 (f) Bubbles of colourless gas would be observed at electrode 1 instead of 2 OR Pale greenish-yellow gas would be observed at electrode 2 instead of 1. 1 Total:8 7(a) Both emission of unburnt hydrocarbons and carbon mon oxde has similar trends. As the air: fuel increases , incomplete combustion of carbon - containing fuels decreases, this leads to lower emission of unburnt hydrocarbons. The amount of oxygen increases indicating lesser amount of carbon monoxide produced and more carbon dioxide produced. Remarks: If oxygen is not mentioned in the answer, max 1 for this Q. 1 1 (b) High in air : fuel ratio results in complete combustion which will then result in large amount of energy to be released. More energy is given out/ higher temperature results in more nitrogen to react with the oxygen to form nitrogen oxides. 1 1 (c) At air to fuel ratio of 13.8:1, although the emission of carbon monoxide and nitrogen oxide are the lowest, the emission of unburnt hydrocarbons is still high. 1 (c)(i) 2CO + 2NO