2022 TMJC H2 Chem Prelim P3 (Ans)

2022 H2 Chem Prelim P3 (Ans) 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] Acids are hydrogen-containing substances that produce H+ in aqueous solution while bases are hydroxide-containing substances that produce OH− in aqueous solution. [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 b oth of these solutions. Explain these observations using the Arrhenius theory. Write equations for all the reactions described. [3] Both Na2O and MgO produced OH– when dissolved in water. Na2O + H2O → 2NaOH OR Na2O + H2O → 2Na+ + 2OH– MgO + H2O Mg(OH)2 OR MgO + H2O Mg2+ + 2OH− [1] for both equations However, NaOH is much more soluble in water than Mg(OH)2 so the [OH−] is higher and thus pH value is higher. [1] OR NaOH dissociates fully while Mg(OH)2 dissociates partially so the [OH−] is higher and thus pH value is higher. SO3 dissolves in water to produce H+, an acidic solution. Hence, its pH value is lower than the other two oxides. SO3 + H2O → HSO4− + H+ Or SO3 + H2O → SO42− + 2H+ [1] for equation and explanation Markers’ Comments • Some candidates confuse the Arrhenius theory with the Bronsted Lowry and Lewis Theory of acids and bases. Candidates are reminded to do their bookwork and learn the definitions well. Markers’ Comments • Candidates need to make clear reference to the Arrhenius theory in answering this part to link to the H+ and OH− produced in the solution. • Candidates need to take note that the higher [OH−] and hence higher pH of the solution formed from dissolving Na2O as compared to that of MgO is due to full dissociation of the NaOH and partial dissociation of MgOH)2 respectively.

2 2022 H2 Chem Prelim P3 (Ans) Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry (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] Max volume of N2 gas collected = 60 cm3 (100%) The two half-lives are determined from graph at volume = 30 cm3 (50%) & 45 cm3 (75%) 1st t1/2 is at a volume of ½ × 60 = 30 cm3 2nd t1/2 is at a vo