Question 5
This question is about enthalpy changes.
(a) State the meaning of the term enthalpy change as applied to a chemical reaction.
(1)
(b) A student determines the enthalpy change for the reaction between calcium carbonate and hydrochloric acid.
CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The student follows this method:
- measure out 50 cm3 of 1.00 mol dm–3 aqueous hydrochloric acid using a measuring cylinder and pour the acid into a 100 cm3 glass beaker
- weigh out 2.50 g of solid calcium carbonate on a watch glass and tip the solid into the acid
- stir the mixture with a thermometer
- record the maximum temperature reached.
The student uses the data to determine a value for the enthalpy change.
Explain how the experimental method and use of apparatus can be improved to provide more accurate data.
Describe how this data from the improved method can be used to determine an accurate value for the temperature change.
(6)
(c) In a different experiment 50.0 cm3 of 0.500 mol dm–3 aqueous hydrochloric acid are reacted with 50.0 cm3 of 0.500 mol dm–3 aqueous sodium hydroxide.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) ΔH = –57.1 kJ mol–1
The initial temperature of each solution is 18.5 °C
Calculate the maximum final temperature of the reaction mixture.
Assume that the specific heat capacity of the reaction mixture, c = 4.18 J K–1 g–1
Assume that the density of the reaction mixture = 1.00 g cm–3
(5)
(d) Suggest how, without changing the apparatus, the experiment in part (c) could be improved to reduce the percentage uncertainty in the temperature change.
(1)
(Total 13 marks)