The enthalpy change of reaction, ∆H, for the decomposition of sodium hydrogencarbonate, NaHCO3(s), cannot be measured directly. 2NaHCO3(s) → Na2CO3(s) + H2O(I) + CO2(g) A student must carry out two separate experiments and use the results of these experiments to determine the enthalpy change of reaction for the decomposition of sodium hydrogencarbonate. In both experiments the student used a weighing boat. A weighing boat is a small vessel used to contain solid samples when they are weighed. Experiment 1 Reaction between sodium carbonate, Na2CO3(s), and dilute hydrochloric acid, HCl(aq) step 1 The student added approximately 3g of Na2CO3(s) to a weighing boat and accurately measured the combined mass of the weighing boat and Na2CO3(s). This mass was recorded in Table 1.1. step 2 The student used a measuring cylinder to measure 50 cm³ of 2 moldm-3 HCl(aq). step 3 The experiment was carried out and the results were recorded in Table 1.2. step 4 The student reweighed the empty weighing boat and recorded the mass in Table 1.1. [Table 1.1 mass results from Experiment 1] [Table 1.2 temperature results from Experiment 1] Experiment 2 Reaction between sodium hydrogencarbonate, NaHCO3(s), and dilute hydrochloric acid, HCl(aq) step 1 The student weighed an empty weighing boat and recorded the mass in Table 1.3. step 2 The student added exactly 4.20g of NaHCO3(s) to the weighing boat and recorded the mass in Table 1.3. step 3 The student carried out the same experimental procedure as in steps 2 and 3 of Experiment 1. [Table 1.3 mass results from Experiment 2]

1. (a)Suggest why the enthalpy change of reaction, ΔΗ, for the decomposition of sodium hydrogencarbonate cannot be measured directly.[1]
2. (b)Outline how the student carried out step 3 of the experiment. You may find it helpful to write your answer as a series of smaller steps. Draw a labelled diagram of the apparatus.[3]
3. (c)Use the graph to determine the theoretical temperature increase at 4 minutes. theoretical temperature increase = ................................. °C[1]
4. (d)Use Table 1.1 on page 2 to determine the mass of Na2CO3(s) which was added to the HCl(aq). Use this value and your answer to (c) to determine the enthalpy change, ∆Η₁, for the reaction shown. Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g) Give your answer to three significant figures. [Assume that the specific heat capacity of the solution is 4.18Jg-1 K-1.] [A: Na, 23.0; C, 12.0; O, 16.0] ΔΗ₁ = ................................. kJ mol-¹[3]
5. (e)(i)Explain why the student did not add the Na2CO3(s) to the HCl(aq) at 0 minutes.[1]
6. (e)(ii)Suggest why the temperature measured at 5 minutes was lower than the temperature measured at 6 minutes.[1]
7. (f)Explain why the method of determining the mass of solid added in Experiment 2 is less accurate than the method of determining the mass of solid added in Experiment 1.[1]
8. (g)(i)In Experiment 2 a 50 cm³ measuring cylinder was used to measure the 50 cm³ of HCl(aq). The 50 cm³ measuring cylinder had 1 cm³ graduations. Calculate the maximum percentage error in measuring 50 cm³ of HCl(aq) with this 50 cm³ measuring cylinder. maximum percentage error = ................................. %[1]
9. (g)(ii)Explain why measuring the concentration of the 2 mol dm-3 HCl more precisely would not affect the result of the experiment.[1]
10. (g)(iii)Suggest what the student should change to reduce the percentage error associated with the temperature readings without changing the apparatus.[1]
11. (h)The student used the results from Experiment 2 and correctly determined the enthalpy change for the reaction between NaHCO3(s) and HCl(aq), ΔH₂, to be +24.2 kJ mol-1. NaHCO3(s) + HCl(aq) → NaCl(aq) + H2O(I) + CO2(g) ΔH₂ = +24.2 kJ mol-1 Use the axes to draw a sketch graph of the expected results of Experiment 2. [Figure: Graph axes][2]
12. (i)Use ∆Η₁ from (d) and ∆H₂ from (h) to determine the enthalpy change of reaction, ∆H, for the decomposition of NaHCO3(s). 2NaHCO3(s) → Na2CO3(s) + H₂O(I) + CO2(g) An energy cycle has been drawn for you. [Figure: Energy cycle diagram] If you were unable to calculate ∆H₁ in (d), assume ∆H₁ = −26.7 kJ mol-1. This is not the correct value of AH₁. ∆H = ................................. kJ mol-1[2]

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