Calcium carbonate, CaCO3(s), decomposes when heated, as shown. CaCO3(s) → CaO(s) + CO2(g) The enthalpy change of reaction, ∆Η, for the thermal decomposition of CaCO3(s) cannot be measured directly. Instead, a procedure involving two experiments is used. In each experiment, the enthalpy change of a different reaction is determined. The equation for the reaction in experiment 1 is shown. The enthalpy change for this reaction is ∆Η₁. experiment 1 CaCO3(s) + 2HCl (aq) → CaCl2(aq) + H2O(l) + CO2(g) The equation for the reaction in experiment 2 is shown. The enthalpy change for this reaction is ∆Η2. experiment 2 CaO(s) + 2HCl (aq) → CaCl2(aq) + H2O(l) Experiment 1 step 1 Weigh a 0.0500 mol sample of powdered CaCO3(s). step 2 Transfer 50.00cm³, an excess, of 2.00 moldm⁻³ hydrochloric acid, HCl(aq), into a small glass beaker. step 3 Start a timer and measure the temperature of the HCl(aq) in the beaker every 30 seconds for 2 minutes. step 4 After 3 minutes add the sample of CaCO3(s) to the HCl(aq) in the beaker. Continue measuring the temperature of the reaction mixture every 30 seconds for a further 5 minutes. Experiment 2 Repeat experiment 1 using calcium oxide, CaO(s), instead of CaCO3(s).

1. (a)Suggest why the enthalpy change of reaction for the thermal decomposition of calcium carbonate cannot be measured directly.[1]
2. (b)(i)Calculate the mass, in g, of CaCO3(s) to be weighed using a two-decimal-place balance in step 1. mass of CaCO3(s) = .............................................. g [1][1]
3. (b)(ii)Outline how a student should weigh by difference using a weighing boat in order to determine the exact mass of CaCO3(s) added to HCl(aq) in the beaker. Draw a results table, with appropriate headings, ready for the student to complete.[2]
4. (c)Identify which piece of apparatus should be used to measure the volume of HCl(aq) in step 2 and give a reason for your choice.[1]
5. (d)Without making any changes to the apparatus, suggest an instruction to be added to step 3 and step 4 to make the experiment more accurate.[1]
6. (e)(i)Plot a graph on the grid in [Figure 1.1] to show the relationship between temperature and time. Use a cross (x) to plot each data point. The points and line of best fit for the data before 3 minutes have been drawn for you. Draw a line of best fit for the data after 3 minutes that will enable you to determine the theoretical temperature increase at 3.0 minutes.[2]
7. (e)(ii)Use your graph to determine the theoretical temperature increase at 3.0 minutes. theoretical temperature increase at 3.0 minutes = .............................................. °C [1][1]
8. (f)Suggest why the temperature measured at 3.5 minutes is lower than the temperature measured at 4.0 minutes.[1]
9. (g)A student carries out experiment 2 and determines a temperature increase of 62.0°C. The heat released by the reaction, q, is given by: q = mc∆T where m is the mass of HCl(aq). Assume that 1.00 cm³ of HCl(aq) has a mass of 1.00g and that the specific heat capacity of the solution, c, is 4.18 J g⁻¹ K⁻¹. Calculate q, in J, for experiment 2 and hence determine ∆H₂ in kJ mol⁻¹. q = .............................................. J ∆H₂ = .............................................. kJ mol⁻¹ [2][2]
10. (h)Use the energy cycle below, your answer to (g) and the information given to determine ∆H, for the thermal decomposition of CaCO3. Enthalpy change for experiment 1, ∆H₁ = −84 kJ mol⁻¹. (If you were unable to calculate a final answer in (g), assume a value of 179kJ mol⁻¹. This is not the correct answer and the sign has been omitted.) CaCO3(s) + 2HCl(aq) → CaO(s) + 2HCl(aq) + CO2(g) \ / \ / \ / CaCl2(aq) + H2O(l) + CO2(g) ∆H = .............................................. kJ mol⁻¹ [1][1]
11. (i)Identify the main weakness of the experimental procedure and suggest one improvement to overcome this weakness. The main weakness is not the type of thermometer used. main weakness ........................................................................................................ improvement ........................................................................................................ [2][2]

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