2015 HCI H1 Chemistry Prelims P1 Questions

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2 SECTION A 1 A mixture of 10 cm3 of methane and 10 cm3 of ethane was sparked with an excess of oxygen. After cooling to room temperature, the residual gas was passed through aqueous potassium hydroxide. All gas volumes were measured at the same temperature and pressure. What volume of gas was absorbed by the alkali? A 15 cm 3 B 20 cm3 C 30 cm3 D 40 cm3 2 In which species are the numbers of protons, neutrons and electrons all different? 3 When iron reacts with aqueous iron(III) ions, iron(II) ions are formed as the only product. A final mixture, after the reaction has taken place, contains equal numbers of moles of Fe 2+(aq) and Fe3+(aq). Assuming the reaction has gone to completion, how many moles of Fe(s) and Fe3+(aq) were in the starting mixture? 4 The reaction pathway for a reversible reaction is shown below. Which statement is correct? A The activation energy of the reverse reaction is +80 kJ mol –1. B The enthalpy change for the forward reaction is +30 kJ mol–1. C The enthalpy change for the forward reaction is +50 kJ mol–1. D The enthalpy change for the reverse reaction is +30 kJ mol-1.

3 5 Sulfur dioxide gas is converted into sulfate ions when it is bubbled into a solution containing aqueous manganate(VII) ions. SO 2(g) + 2H2O(l) → SO4 2–(aq) + 4H+(aq) + 2e– MnO4 –(aq) + 8H+(aq) + 5e– → Mn2+(aq) + 4H2O(l) How will the pH of the reaction mixture change as sulfur dioxide is bubbled at constant rate into a well-stirred solution of manganate(VII) ions until its colour just fades? A a decrease B a decrease then an increase C an increase D an increase then a decrease 6 Enthalpy changes that are difficult to measure directly can often be determined using Hess’ Law to construct an enthalpy cycle. Which enthalpy change is indicated by X in the enthalpy cycle shown? A – 4 × the enthalpy of combustion of hydrogen B + 4 ×