3.2

Question 1

3.2 Determination of a enthalpy change of reaction by Hess’s Law

Answer 1

You will determine the enthalpy change of reaction by Hess’s Law based on experimental evidence. This will involve measuring the temperature changes in the reactions of both K2CO3 and KHCO3 with hydrochloric acid and applying your knowledge of Hess’s Law to calculate the enthalpy change of thermal decomposition of KHCO3.
You are given instructions for the practical procedure. It is your responsibility to work safely and to organise your time efficiently. You should try to work in such a way as to make your results as accurate as possible.
• To determine the enthalpy change of thermal decomposition for KHCO3 by carrying out two reactions of K2CO3 and KHCO3 with hydrochloric acid by accurate measurement of temperature. You will use Hess’s Law calculations to determine the enthalpy changes.

Question 2

hazards 2.00 mol dm3 hydrochloric acid, HCl(aq)

Answer 2

Irritant to skin and eyes

Question 3

hazards K2CO3 (s)

Answer 3

Irritant to skin and eyes

Question 4

hazards KHCO3 (s)

Answer 4

Irritant to skins, eyes and respiratory

Question 5

Equipment

Answer 5

• Access to balance that records to two decimal places
• polystyrene cup
• beaker (250 cm3)
• thermometer (10° –110 °C) graduated in divisions of 0.5 °C or less
• measuring cylinders (50 cm3)
• weighing bottles
• stirring rod

Question 6

Reaction 1

Answer 6

1. Weigh an empty polystyrene beaker which you will use for the reaction.
2. Measure 30 cm3 (an excess) of approximately 2 mol dm–3 hydrochloric acid into a polystyrene beaker.
3. Weigh accurately a weighing bottle containing between 2.5 g and 3.0 g of potassium carbonate, K2CO3.
4. Take the temperature of the acid and record this value.
5. Add the weighed portion of K2CO3 to the acid carefully. Stir the mixture, take the highest or lowest temperature and record this value.
6. Reweigh the weighing bottle and the polystyrene beaker and record this value.
7. From your results, calculate the heat absorbed or evolved by the solution. (specific heat capacity of solution = 4.18 J g–1 K–1)
8. Calculate the amount, in mol, of K2CO3 that reacted and determine the enthalpy change of reaction, ∆H1, for the equation above.

Question 7

Reaction 2

Answer 7

KHCO3(s) + HCl(aq)  KCl(aq) + CO2(g) + H2O(l) ∆H2

1. Repeat the first experiment using between 3.2 g and 3.7 g of KHCO3.
2. Calculate the amount, in mol, of KHCO3 that reacted and determine the enthalpy change of reaction, ∆H2, for the equation above.
   Using Hess’s Law
3. The equation for the thermal decomposition of KHCO3 is:
   2KHCO3(s)  K2CO3(s) + CO2(g) + H2O(l)
   Construct an energy cycle to link this reaction with the two reactions of HCl (aq) with K2CO3 and KHCO3.
4. Use your energy cycle to help you to calculate the enthalpy change for the thermal decomposition of potassium hydrogen carbonate.

Question 8

1. Explain why you use hydrochloric acid in excess.

Answer 8

1. Hydrochloric acid is added in excess to ensure that all of the solid reactant has reacted.

Question 9

2. The literature value for the enthalpy of reaction for this reaction is +95 kJ mol–1. Compare your answer with this and suggest one or more reasons for the difference.

Answer 9

2. Learners should recognise that the temperature difference that they measured is too small – a larger difference would have resulted in a greater enthalpy change, closer to the literature value.
   The major source of error in this experiment is heat loss to the surroundings. Learners should be able to suggest some improvements to minimise this – for example adding a lid to the polystyrene cup.
   If there is time, it is instructive to allow students to attempt to improve their procedure. In addition to improving the insulation, students may wish to e.g. increase the concentrations of the acid and alkali to see if this makes a difference. Can students achieve a smaller error? If not, can they explain this?