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3 marksEnergetics

CAPE Chemistry Unit 1 · 2007 · Paper 2 · Question 1(a)

Hess's law states that the total energy change in a reaction is independent of the route from reactants to products if the initial and final conditions of temperature and pressure are identical. Given the following data: Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) ΔH = -511.1 kJ; MgO(s) + 2H+(aq) → Mg2+(aq) + H2O(g) ΔH = -151.1 kJ; 2H2(g) + O2(g) → 2H2O(g) ΔH = -483.6 kJ.

Use Hess's law to calculate the enthalpy change for the following reaction: 2Mg (s) + O2(g) → 2MgO(s)

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  1. 1(b)(i)Explain the reason for stirring the mixture at regular intervals.[1 mark]
  2. 1(b)(ii)Given that the initial and final temperatures are 28°C and 57°C respectively, sketch a typical graph for the results of the experiment to illustrate how the…[2 marks]
  3. 1(b)(iii)Give the main source of error for the experiment.[1 mark]
  4. 1(b)(iv)Calculate the enthalpy change, in kJ mol⁻¹ of Mg for the reaction at constant pressure, given ΔT = 29.0°C, C = 4.20 Jg⁻¹ °C⁻¹. The density of dilute HCl is…[3 marks]
  5. 1(c)(i)Use the results of your calculation in (b)(iv) above to state whether the reaction is endothermic or exothermic.[1 mark]
  6. 1(c)(ii)Draw a clearly labelled energy profile diagram for the reaction between magnesium and hydrochloric acid. Include on your diagram the enthalpy change for the…[4 marks]

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