2022 TMJC H2 Chem Prelim P3 (QP)

1 2022 H2 Chem Prelim P3 (QP) [Turn over Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry Section A Answer all the questions from this section. 1(a) Samples of three different oxides were added to water separately. (i) State the Arrhenius theory of acids and bases. [1] (ii) The pH value of the solution formed when sodium oxide is shaken with water is greater than the pH value of the solution formed when magnesium oxide is shaken with water. The pH of the solution formed when sulfur trioxide is shaken with water is less than both of these solutions. Explain these observations using the Arrhenius theory. Write equations for all the reactions described. [3]

2 2022 H2 Chem Prelim P3 (QP) (b) Diazomethane, CH2N2, reacts with water to give methanol and nitrogen gas. CH2N2 + H2O → CH3OH + N2 When 2.50 × 10–3 mol of CH2N2 was added into water , the volume of nitrogen gas evolved at various time intervals after the start of the reaction were measured. At the end of the reaction, 60 cm3 of nitrogen gas was collected. The experiment results are plotted in Fig. 1.1 below. Fig. 1.1 (i) Use Fig. 1.1 and the information given to show that the order of reaction with respect to [CH2N2] is 1. [2] 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 0 10 20 30 40 50 60 70 Volume of N2 (g) / cm3 time / min

3 2022 H2 Chem Prelim P3 (QP) [Turn over Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry The reaction of CH2N2 with water takes place in the presence of an acid. Two experiments were conducted at different pH values to investigate the kinetics of this reaction. T he results are shown in Table 1.1. Table 1.1 Experiment [CH2N2] / mol dm−3 pH Relative rate 1 1.00 × 10−3 1.00 1 2 4.00 × 10−3 1.30 2 (ii) Calculate the concentration of H+(aq) in experiment 1 and 2. [1] (iii) Use the answer in (b)(ii) and data provided to determine the order of reaction with respect to [H+], and hence write the rate equation for the reaction. [2]

4 2022 H2 Chem Prelim P3 (QP) (c) Pure magnesium needed for making alloys can be obtained by the electrolysis of molten magnesium chloride as shown in Fig. 1.2. Fig. 1.2 (i) Write the half -equations, including state symbols , for the reactions occurring at the graphite and iron electrodes. Label your equations clearly to indicate the reaction occurring at the graphite and iron electrode respectively. [2] (ii) Draw and label the direction of electron flow in the cell on Fig. 1.2. [1] (iii) Calculate the mass of magnesium obtained if a current of 3.00 A is supplied for 10.0 h. [2] (iv) A gas is continuously passed over the molten magnesium in the electrolytic cell to provide an inert environment. Suggest a gas that can be used for this. [1] (v) Molten magnesium chloride in the c