(B410U20-1) 12 Examiner only 10. (a) A student was asked to calculate the enthalpy change, ΔrH, for the reaction of magnesium oxide and carbon dioxide to form magnesium carbonate. Since this is difficult to measure directly he decided to determine the enthalpy changes for the reactions of magnesium oxide and magnesium carbonate with excess dilute hydrochloric acid in two similar, separate experiments and apply Hess’s law to his results. (i) The first experiment was to find the molar enthalpy change, ΔH1, for the reaction MgO(s) + 2HCl(aq) MgCl2(aq) + H2O(l) ΔH1 was calculated to be –115 kJ mol–1. Give one assumption made when finding the value of ΔH1 from experimental results. [1] (ii) The second experiment was to find the molar enthalpy change, ΔH2, for the reaction MgCO3(s) + 2HCl(aq) MgCl2(aq) + H2O(l) + CO2(g) The following values were recorded during the experiment: Mass of magnesium carbonate 3.50 g Volume of hydrochloric acid 50.0 cm3 Initial temperature of hydrochloric acid 22.0 °C Final temperature of solution 30.8 °C I. Calculate the molar enthalpy change for this reaction, ΔH2, in kJ mol–1. Give your answer to an appropriate number of significant figures. [4] ΔH2 = ................................................. kJ mol–1 © WJEC CBAC Ltd. 13 (B410U20-1) Examiner only II. State and explain the effect, if any, on the calculated enthalpy change, ΔH2, if the experiment were repeated with the same mass of damp magnesium carbonate. [2] (iii) The hydrochloric acid used in these experiments could be measured using either a volumetric pipette or a measuring cylinder. Give one practical advantage of using each piece of apparatus. [2] Volumetric pipette Measuring cylinder (iv) Each reading on the thermometer is accurate to ± 0.1 °C. Calculate the percentage error in the temperature rise recorded. [1] Percentage error = ................................................. % © WJEC CBAC Ltd. Turn over. (B410U20-1) 14 Examiner only © WJEC CBAC Ltd. (v) A Hess cycle connecting ΔrH to ΔH1 and ΔH2 is shown below. ΔrH MgO(s) + CO2(g) MgCO3(s) 2HCl(aq) ΔH1 ΔH2 2HCl(aq) MgCl2(aq) + H2O(l) + CO2(g) Calculate the value of ΔrH in kJ mol–1. [2] ΔrH = ................................................. kJ mol–1 (B410U20-1) Turn over. © WJEC CBAC Ltd. (b) The equation for the reaction between hydrazine and nitrogen dioxide is as follows. 2N2H4(l) + 2NO2(g) 3N2(g) + 4H2O(l) ΔH = −1313 kJ mol–1 Using this value and the standard enthalpy changes of formation, ΔfHθ, given in the table below, calculate the standard enthalpy change of formation of NO2. [2] Substance ΔfHθ / kJ mol–1 N2H4(l) 50.4 N2(g) 0 H2O(l) −286 ΔfHθ = ................................................. kJ mol–1 14 15 Examiner only