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 cell is being replaced with aqueous magnesium chloride. Using relevant data from the Data Booklet, state and explain the reactions taking place at both electrodes when this change is made. [2] iron electrode inert gas out gas produced molten magnesium chloride inert gas in molten magnesium graphite electrode to external circuit

5 2022 H2 Chem Prelim P3 (QP) [Turn over Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry [Total: 17]

6 2022 H2 Chem Prelim P3 (QP) 2 Chromium is a steely -grey, hard and dense transition element. Its name came from the Greek word ‘chrδma’, which means colour, because many of its compounds are coloured. (a) Define transition element. [1] (b) Transition elements have significantly higher density and melting point compared to main group metals. (i) Briefly explain why transition elements exhibit higher density. [1] (ii) Explain why the melting point of chromium is significantly higher than the melting point of calcium. [1]

7 2022 H2 Chem Prelim P3 (QP) [Turn over Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry (c) Anhydrous chromium(III) chloride may be prepared by the chlorination of chromium metal. Cr(s) + 3 2 Cl2(g) → CrCl3(s) The thermodynamic data at 298 K are given in the table below. (i) Explain the significance of the sign of Sfo. [1] (ii) Using the information above, calculate Go for the formation of CrCl3 (s). [1] (iii) Using your answer from (c)(ii), suggest whether the ratio of [product] / [reactants] at equilibrium for the formation of CrC l3 at 298 K will be less than, equal to or greater than 1. Give a reason for your answer. [1] (iv) Comment on the effect of increasing temperature on the spontaneity of the reaction. [2] Substance Sfo / J mol–1 K–1 Hfo / kJ mol–1 CrCl3 –236.0 –556.6

8 2022 H2 Chem Prelim P3 (QP) (d) The following sequence of reactions in Fig. 2.1 involves chromium. chromium metal blue solution formed that slowly turns green filter to remove unreacted Cr metal excess NaOH(aq) X dark green solution H2SO4 (aq) grey green solid effervescence NaOH H2O2 green solution W Na2CO3 H + CrO4 2− yellow solution Y orange solution Fig. 2.1 (i) Using relevant Eo values from the Data Booklet, explain why the blue solution slowly turns green in air. [2] (ii) State the type of reaction and w rite a balanced equation to account for the observation when a small amount of aqueous sodium hydroxide was added to a solution W. [2] (iii) Suggest the formula of the chromium containing species in X and Y. [2] (iv) State, with reasoning, the role of hydrogen peroxide in Fig. 2.1. [1]

9 2022 H2 Chem Prelim P3 (QP) [Turn over Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry

10 2022 H2 Chem Prelim P3 (QP) (e) Chromium(III) picolinate is a chemical compound with the formula Cr(C5H4N(CO2))3, commonly abbreviated as CrPic3. It is sold as a nutritional supplement to treat type-2 diabetes and promote weight loss. The structure of the bidentate ligand picolinate is shown below. Pic Draw the structure of chromium(III) picolinate. [1] (f) Aluminium hydroxide, Al(OH)3, was used as white pigment for paints. A l(OH)3 has a solubility of 2.90 × 10–9 mol dm–3 in pure water. (i) Write the expression for the solubility product, Ksp, of Al(OH)3 and calculate its solubility product in pure water. [3] (ii) How would you expect the solubility of Al(OH)3 in excess NaOH(aq) to compare with that in pure water? Briefly explain your answer with an equation with state symbols. [2]