NCHS Pure Chem 2023 TYS Suggested Answers

2023 Chemistry O level (Suggested Answers) Paper 2 Qn / part Suggested answer and comment Section A A1(a) Y: 2; Li+ Z: VI (Group 16), Se2- Y is an ion and not an atom, therefore Y cannot be Helium. (b) X/F-. It has 10 neutrons and 9 protons, so it has a relative atomic mass of 19, same as that in the Periodic Table. Must show how information in the table leads to the conclusion that X has a relative atomic mass of 19. (c) Helium (d) Aqueous bromine remains red-brown. Bromine is less reactive than fluorine and is unable to displace fluorine from its halide solution. Halogen and halide ions are not the same. It is fluorine (not fluoride) that is more reactive than bromine (not bromide). Br2(aq)brown + F-(aq) → no reaction remains brown F2(aq) + 2Br-(aq)colourless → 2F-(aq) + Br2(aq)brown A2(a) product at negative electrode: Ag/silver product at positive electrode: O2/oxygen (b) When electricity passes through the platinum electrodes, the electric current is conducted by the free-moving “sea of delocalised electrons”. When electricity passes through the solution, the electric current is conducted by the free-moving silver, Ag+ and nitrate, NO3- ions. Question is referring to electrical conductivity through different structure (metal vs ionic compound in solution). Students mistake question and explain which ions were discharged instead which is already asked in (a) Delocalised does not describe free-moving. (c) compound: HNO3/nitric acid explanation: OH- and Ag+ ions are preferentially discharged at the anode and cathode, respectively, leaving behind H+ and NO3- ions, which combine to form HNO3 which turns the Universal Indicator red. Hydrogen ion is not a compound.

Explain why UI turns red – presence of acid – state where H+ ions formed – explain why H+ remains in the electrolyte (must mention what was discharged from the electrolyte. (d)(i) hydrogen (d)(ii) H+ ions are discharged at the cathode and reduced to form hydrogen gas. At the start, Ag+ ions from the electrolyte are preferentially discharged over H+ at the cathode / negative electrode to form silver metal as silver is less reactive than hydrogen. Wrong phrasing: Silver ions are less reactive than hydrogen ions. Correct phrasing: Silver that is less reactive than hydrogen, which results in Ag+ ions being preferentially discharged over H+. A3(a)(i) 4CuFeS2 + 11 O2 → 4Cu + 2Fe2O3 + 8SO2 Check balancing and know your formulae well! Common carelessness include mistaking iron(II) oxide (a)(ii) 1 tonne = 1000 kg = 1000000 g Mr of CuFeS2 = 64 + 56 + 32 x 2 = 184 Composition by mass = 64 184 x 1000000 g = 348000 g or 348 kg or 0.348 tonnes (3 s.f.) (b)(i) acid: SiO2/silicon dioxide/silicon(IV) oxide base: Fe2O3/iron(III) oxide (b)(ii) Extraction of copper: SiO2 is added to the furnace to react with iron(III) oxide, which is the impurity, to form slag. Extraction of iron: CaCO3 is added to the blast furnace, which is decomposed by heat to form CaO and CO2. CaO then reacts with SiO2, the impurity, to form slag. Blast furnace is not in new syllabus. However students must know that this is an example of acid-base reaction (c) 4CuO + CH4 → 4Cu + CO2 + 2H2O 3 CuO + CH4 → 3Cu + CO + 2H2O carbon monoxide/CO, carbon dioxide/CO2, water/H2O (any 2) Question says ‘other’ so copper is not accepted

A4(a) (a) The ratio of atoms of metal to atoms of oxygen decreases from 2:1 in Li2O to 1:1 in Na2O2 to 1:2 in KO2. Some students did not simplify the 2:2 ratio to 1:1. Many failed to describe the trend as required and merely stated the number of metal and oxygen atoms in each formulae. (b) Common mistakes: - did not show the two additional electrons (one gained by each atom) that resulted in the 2- charge. - drawing separate ions instead of a polyatomic ion (c)(i) O2- (c)(ii) One of the oxygen atoms only has 7 valence electrons instead of a fully filled valence shell of 8 electrons. Oxygen normally forms a molecule with 8 electrons in the outermost shell after sharing a double bond. However, when the pair shared a single bond, it has 7 electrons in the outermost shell. (hence this dows not follow normal bonding rules) The molecule must gain 2 electrons to form a superoxide ion with a -2 charge. Some students thought that an ‘odd number of electrons are shared’ in the superoxide ion, which is incorrect. (d)(i) Metal oxides react with water to produce metal hydroxides (specifically Group 1 metal hydroxides) which are soluble in water and dissolve in water to form alkalis, which turn red litmus blue. Both metal peroxide and superoxides react with water to form hydrogen peroxide, which bleaches the litmus indicator to turn colourless. When metal superoxide reacts with water, oxygen gas is also formed, which gives rises to the formation of bubbles/effervescence. Some students did not consider the information that hydrogen peroxide is bleach carefully, which lead them to think that hydrogen peroxide neutralizes the solution, which is incorrect. Some students only explained one or two observations, instead of all three.

(d)(ii) 2 KO2 + 2H2O → 2KOH + H2O2 + O2 A5(a) Photosynthesis (b) A (produce oxygen) (c) B. CO2(g) dissolves in water to form CO2(aq), and vice versa or there is a forward and backward arrow. Avoid reusing the term ‘reversible’ in the answer. This process is not a chemical reaction. (d) CO2(g) dissolves in water to form CO2(aq). CO2(aq) evaporates to form CO2(g). Some students only gave one change in one direction, instead of both directions. (e) A: endothermic C: exothermic D: exothermic (f) Process A (photosynthesis) takes in CO2 from the air whereas process B produces CO2(g) from CO2(aq) through ocean uptake and evaporation. The decrease in the concentration of CO2 in the air due to process A is offset by process B, resulting in the stabilisation of the concentration of carbon dioxide in the air. Many students failed to refer to processes A and B in their answers, or included other processes such as combustion of fossil fuels or respiration. Do not repeat the statement in the question. A6(a) (b) Similarity: Dicarboxylic acid and another monomer is used to make both polymers undergo a condensation reaction to form the polymer product with the removal of water. Difference: The monomers used to make this polymer are a dicarboxylic acid and a diol, whereas the monomers used to make the polymer in (a) are a dicarboxylic acid and a diamine. Some students compared the polymers and their linkages, rather than the monomers. (c)(i) A macromolecule consists of many covalent molecules joined together to form a long molecular chain. (c)(ii) Condensation polymerisation involves the removal of small molecules such as water, whereas addition polymerisation does not involve the removal of small molecules.

Addition polymerisation involves monomers with a C=C bond whereas condensation polymerisation involves monomers with –COO and –OH/-NH2 groups. Addition polymerisation produces a polymer with the same empirical formula as the monomers, whereas condensation polymer does not produce a polymer with the same empirical formula as the monomers. Comment: Some students mentioned about the ‘elimination of atoms’, which is incorrect. Section B B7(a) The impurity ethanoic acid is produced from the oxidation of ethanol by bacteria in the air, which is the product from the fermentation of sugar from sugarcane. The impurity ethyl ethanoate is produced from the esterification reaction between ethanol, which is the product from the fermentation of sugar from sugarcane, and ethanoic acid, which is the impurity produced from the oxidation of ethanol. Comment: Some students only described the formation of one, but not both, of the impurities. Some did not clearly identify