20 7 The enthalpy change for the decomposition of sodium hydrogencarbonate can be determined indirectly using Hess’s Law. 2NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g) A student carried out two experiments. (a) Experiment 1 involved the reaction between sodium hydrogencarbonate and hydrochloric acid. The student used the following procedure: • use a measuring cylinder to measure 50 cm3 of 2.00 mol dm−3 hydrochloric acid and pour it into a polystyrene cup • measure the initial temperature of the acid • weigh the test tube containing sodium hydrogencarbonate • tip the sodium hydrogencarbonate into the hydrochloric acid in the polystyrene cup, stir the mixture and record the lowest temperature reached • weigh the empty test tube. Results Measurement Value Mass of test tube + NaHCO3 / g 21.23 Mass of empty test tube / g 15.61 Mass of NaHCO3 used / g Initial temperature / °C 21.0 Final temperature / °C 14.4 Temperature fall / °C (i) Complete the table. (1) (ii) Show, by calculation, that the hydrochloric acid is in excess. You must show your working. NaHCO3(s) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(g) (2) ∆rH Turn over 21 (iii) Calculate the enthalpy change for the reaction between sodium hydrogencarbonate and hydrochloric acid, using the results of the experiment. Include a sign and units in your answer. f p Assume: mass of reaction mixture = 50.0 g specific heat capacity of the reaction mixture = 4.18 J g−1 °C−1 (3) 22 (b) Experiment 2 involved the reaction between sodium carbonate and hydrochloric acid. The student repeated the procedure for Experiment 1 but used sodium carbonate instead of sodium hydrogencarbonate and measured the maximum temperature rise. Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g) The student calculated the enthalpy change for this reaction as −29.4 kJ mol−1. (i) Complete the Hess cycle with appropriate formulae and labelled arrows. (2) 2NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g) (ii) Calculate the enthalpy change for the decomposition of sodium hydrogencarbonate. Include a sign and units in your answer. 2NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g) (3) ∆rH ∆rH Turn over 23 (c) Another student carried out the same two experiments and obtained a value for the enthalpy change of decomposition of sodium hydrogencarbonate of +74 kJ mol−1. The data book value for this enthalpy change is +90 kJ mol−1. (i) Calculate the percentage error in this student’s value. (1) (ii) Calculate the percentage uncertainties in measuring 50 cm3 of hydrochloric acid using a burette and using a measuring cylinder. (1) Apparatus Uncertainty Percentage uncertainty Measuring cylinder ±0.5 cm3 for each volume measured Burette ±0.05 cm3 for each reading (iii) Give a reason why using a burette rather than a measuring cylinder will not improve the accuracy of the experiment. 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Your description should involve the use of a clock and plotting a graph. 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(Total for Question 7 = 19 marks) Turn over 25 BLANK PAGE