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