VS 2024 O Level Pure Chemistry P1+P2 Answer Scheme

Victoria School 2024 Chemistry O Levels / 6092 Suggested Answer Key PAPER 1 Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 Q11 Q12 Q13 Q14 Q15 Q16 Q17 Q18 Q19 Q20 B C D D A B A A C B D D C B C B D C D A Q21 Q22 Q23 Q24 Q25 Q26 Q27 Q28 Q29 Q30 Q31 Q32 Q33 Q34 Q35 Q36 Q37 Q38 Q39 Q40 C A B D D B B D D A A A C B A A A B C C PAPER 2 (SECTION A) Question Answer Mark 1 ai A and E 1 aii A and D 1 aiii A and F 1 b colourless; purple 1 c 2C 3 H 6 (g) + 9O 2 (g) → 6CO 2 (g) + 6H 2 O(g) Since 2C 3 H 6 9O 2 ≡ Hence, volume of O 2 gas at r.t.p = x 9 = 450 cm 3 100 2 Volume of air required = x 450 = 2142 cm 3 2140 cm 3 (to 3 sig. fig) 100 21 ≈ 3 2024 Answer 1

2 a Iron, chromium and nickel are elements found in the Periodic Table. They are pure substances that cannot be split up into two or more simpler substances by chemical processes or by electricity . Iron( III ) oxide (Fe 2 O 3 ) is compound that is made up of two or more elements chemically combined in a fixed ratio of iron and oxygen. Steel is a mixture formed when two or more substances are added together without chemical bonds being formed/physically combined . In this steel, the percentage by mass of the 3 constituents, namely iron, nickel and chromium are different with no fixed ratios / proportions. 1 1 1 b To justify this statement, the amounts / number of atoms of the respective metals need to be calculated. Assume 100 g of steel, Amount of Cr = = 0.300 mol 15 . 6 52 Amount of Ni = = 0.151 mol 8 . 9 59 Since the mole ratio of Cr to Ni is approximately 2:1, the statement made by the student is correct. 1 1 c ● The atoms of the different metals should have different sizes/atomic radii but the figure shows particles of the same size . ● There are three different types of atoms in this steel but the figure only shows two different types . ● There are different and varying amounts of constituents in the steel but the ratio of the different particles is almost equal due to the alternate arrangement of the two different constituents shown in the figure. ● The arrangement of particles / atoms in alloys should be disorderly but the figure shows orderly arrangemen t of the particles in steel. (Any three of the above answers are sufficient to score full marks.) 3 2024 Answer 2

d Steel is harder, stronger , less malleable and more brittle than iron. When different elements are added to the pure metal, the atoms of the different elements have different atomic radii. The atoms of different atomic radii disrupt the orderly arrangement of the layers of iron atoms within the metal lattice. Hence, it is more difficult for the layers of iron atoms to slide over one another without breaking any metallic bonds when a force is applied. In pure iron, the atoms are of the same size and arranged in orderly layers . When a force is applied, the layers slide past each other easily . 1 1 3 a silver nitrate and lead : grey coating forms on surface of metal zinc nitrate and lead : no reaction magnesium nitrate and lead : no reaction 2 b Zn (s) + 2Ag + (aq) → Zn 2+ (aq) + 2Ag (s) [ss not required] 1 c Zinc and magnesium are more reactive than iron and undergoes sacrificial protection of iron. Zinc and magnesium atoms lose their valence electrons/oxidised more easily than iron atoms to corrode in place of iron . 1 1 d least stable: silver carbonate lead( II ) carbonate most stable: magnesium carbonate 1 4 a Nitrogen is obtained from fractional distillation of liquified air . Hydrogen is obtained from catalytic cracking of crude oil or steam reforming. 1 1 b From the graph, the best operating conditions are 350 °C and 400 atm. Below 450 °C, the percentage yield is high but the reaction is slow . Above 450 °C, the percentage yield is low . 1 1 2024 Answer 3

Below 200 atm, the percentage yield is low . Above 200 atm, the percentage yield is high and there is high risk of explosion . Hence, Haber process operates at 450 °C and 200 atm which gives the maximum yield and faster rate of reaction at minimum cost . 1 1 c Ca(OH) 2 + (NH 4 ) 2 SO 4 CaSO 4 + 2NH 3 + 2H 2 O Calcium hydroxide can be added to soil to neutralise excess acidity in the soil. However, it should not be added to the soil at the same time with ammonium sulfate because ammonium sulfate will react with calcium hydroxide to form ammonia gas which will escape into the atmosphere. Hence, causing nitrogen content in the fertiliser will be lost . 1 1 5 a Manganate( VII ) and Vanadate( V ) Both oxyanions consists of manganese and vanadium , which are transition metals . Hence, will form coloured transition metal compound. 1 1 bi A l (C l O 2 ) 3 1 bii Chromate ( VI ) 1 biii SO 3 2- 1 c 3 6 a PbC l 4 has a simple covalent structure . Hence, a low amount of energy is needed to overcome the weak intermolecular forces of attraction s, resulting in low melting point . 1 2024 Answer 4

PbC l 2 has a giant ionic structure . Hence, a high amount of energy is needed to overcome the strong ionic bonds , resulting in high melting point . 1 b Carbon has a giant covalent structure. Each carbon atom can form strong covalent bonds with 3 other carbon atoms (in graphite) or 4 other carbon atoms (in diamond) by sharing three or all the valence electrons . Lead has a giant metallic structure. The lead atoms lose their valence electrons to form cations and a sea of mobile valence electrons . There are strong metallic bonds between the cations and the sea of delocalised valence electrons . 1 1 c SnC l 4 will be a liquid at 25 °C while SnC l 2 will be a solid at 25 °C. Tin is a metal and in Group 14, like lead. Hence, it will have similar chemical properties as lead, forming chlorides of tin that have the same simple covalent structure. Hence, a low amount of energy is needed to overcome the weak intermolecular forces of attraction s, resulting in the low melting and boiling points that are similar to that of lead chlorides. 1 1 d PbC l 4 (l) PbC l 2 (s) + C l 2 (g) 1 e Green solution turns red . Water is neutral with pH of 7. Hence, upon the addition of UI, it will form green solution. When silicon chloride is added, one of the products, i.e. HC l , will dissociate completely to form a high concentration of H + ions and C l - . Hence, causing the green solution to turn red as it is strong acid. White precipitate of silicon dioxide will also be formed. 1 1 Question Answer Mark 7 ai Alkanes. Since R = C n H 2n+1 , thus the formula of R-H is C n H 2n+2 which similar to the general formula of alkanes 1 1 aii Hydrogen/H 2 The oxidation state