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Enthalpy changes

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Question 2

A student plans to determine the enthalpy change of reaction 1 shown below:

Li2O(s)+2HCl(aq)→2LiCl(aq)+H2O(l)reaction 1 \text{Li}_2\text{O(s)} + 2\text{HCl(aq)} \rightarrow 2\text{LiCl(aq)} + \text{H}_2\text{O(l)} \quad \text{\textbf{reaction 1}} Li2​O(s)+2HCl(aq)→2LiCl(aq)+H2​O(l)reaction 1

This enthalpy change can be determined indirectly using Hess' Law from the enthalpy changes of reaction 2 and reaction 3 shown below:

Li2O(s)+H2O(l)→2LiOH(aq)reaction 2 \text{Li}_2\text{O(s)} + \text{H}_2\text{O(l)} \rightarrow 2\text{LiOH(aq)} \quad \text{\textbf{reaction 2}} Li2​O(s)+H2​O(l)→2LiOH(aq)reaction 2 HCl(aq)+LiOH(aq)→LiCl(aq)+H2O(l)ΔrH=−57.3 kJ mol−1reaction 3 \text{HCl(aq)} + \text{LiOH(aq)} \rightarrow \text{LiCl(aq)} + \text{H}_2\text{O(l)} \quad \Delta_r H = -57.3 \text{ kJ mol}^{-1} \quad \text{\textbf{reaction 3}} HCl(aq)+LiOH(aq)→LiCl(aq)+H2​O(l)Δr​H=−57.3 kJ mol−1reaction 3

The student determines the enthalpy change of reaction 2 using a polystyrene cup calorimeter as outlined below:

  • Weigh a bottle containing Li2O(s)\text{Li}_2\text{O(s)}Li2​O(s) and weigh a polystyrene cup.
  • Add about 40 cm340\text{ cm}^340 cm3 of water to the polystyrene cup and measure its temperature.
  • Add the Li2O(s)\text{Li}_2\text{O(s)}Li2​O(s), stir the mixture, and measure the maximum temperature reached.
  • Weigh the empty bottle and weigh the polystyrene cup with the final solution.

Mass readings

  • Mass of bottle + Li2O(s)\text{Li}_2\text{O(s)}Li2​O(s) = 15.34 g15.34\text{ g}15.34 g
  • Mass of empty bottle = 13.85 g13.85\text{ g}13.85 g
  • Mass of empty polystyrene cup = 24.20 g24.20\text{ g}24.20 g
  • Mass of polystyrene cup + final solution = 65.69 g65.69\text{ g}65.69 g

Temperature readings

  • Initial temperature of water = 19.5 ∘C19.5\text{ }^{\circ}\text{C}19.5 ∘C
  • Maximum temperature of final solution = 83.5 ∘C83.5\text{ }^{\circ}\text{C}83.5 ∘C

The density and specific heat capacity, ccc, of the solution are the same as for water (c=4.18 J g−1 K−1c = 4.18\text{ J g}^{-1}\text{ K}^{-1}c=4.18 J g−1 K−1). Use the relative atomic masses: Li=6.9\text{Li} = 6.9Li=6.9, O=16.0\text{O} = 16.0O=16.0.

a.

Calculate the enthalpy change of reaction 2 (in kJ mol−1\text{kJ mol}^{-1}kJ mol−1) and the enthalpy change of reaction 1 (in kJ mol−1\text{kJ mol}^{-1}kJ mol−1). Show your working.

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b.

The uncertainty in each temperature reading is ±0.1 ∘C\pm 0.1\text{ }^{\circ}\text{C}±0.1 ∘C. The uncertainty in each mass reading is ±0.01 g\pm 0.01\text{ g}±0.01 g. Determine whether the mass of Li2O\text{Li}_2\text{O}Li2​O or the temperature change has the greater percentage uncertainty. Show your working.

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c.

Suggest a modification to this experiment, using the same apparatus, which would reduce the percentage errors in the measurements. Explain your reasoning.

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d.

Lithium oxide, Li2O\text{Li}_2\text{O}Li2​O, can be prepared by the redox reaction of LiNO3\text{LiNO}_3LiNO3​ and lithium metal. Nitrogen gas is also formed. (i) What is the systematic name for LiNO3\text{LiNO}_3LiNO3​? (ii) Using oxidation numbers, with signs, show the element that is oxidised and the element that is reduced in this reaction. (iii) Construct the balanced chemical equation for this reaction.

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Markscheme

Enthalpy changes Questions

  1. A Level
  2. /Chemistry
  3. /Enthalpy changes

31 exam-style questions on OCR A A Level Chemistry Enthalpy changes. Each one has a worked solution and a mark scheme showing where the marks go.

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