Topic 7.4
Hess’s law
Energetics
Aims
 To calculate standard enthalpy changes for a reaction from experimental data.
 To construct a thermochemical cycle and calculate an enthalpy change indirectly.
 To consider the experimental errors and limitations associated with an energetics
experiment.
Teaching notes
This activity draws together the work on energetics from Chapter 7 of the student book. It is
important that students are confident in all areas of this work before attempting this sheet, as
they need to apply many of the ideas in this activity.
Answers
1 2KOH(aq) + CO2(g) → K2CO3(aq) + H2O(l)
2 2KOH(aq) + H2SO4(aq) → K2SO4(aq) + 2H2O(l)
3 q = 75 × 4.18 × 9.5 = 2978.25 J
moles of alkali =
1.0  50
= 0.05 mol
1000
2978.25
= 59 565 J or 59.565 kJ
0.05
So one mole would give
H = −59.6 kJ mol−1
(negative because heat is given out)
4 K2CO3(aq) + H2SO4(aq) → K2SO4(aq) + H2O(l) + CO2(g)
5 q = 50 × 4.18 × 3.0 =627 J
moles of alkali =
1.0  25
= 0.025 mol
1000
So one mole would give
H = −25.1 kJ mol−1
627
= 25 080 J or 25.08 kJ
0.025
(negative because heat is given out)
AQA Chemistry AS Level Teacher Notes © Nelson Thornes Ltd 2008
1
Topic 7.4
Hess’s law
6
[2(−59.6) − (−25.0)] = −144.3 kJ mol−1
7
a) potassium hydroxide experiment: pipette used three times
error = 3 ×
0.06
× 100 = 0.72%
25
potassium carbonate experiment: pipette used twice
error = 2 ×
0.06
× 100 = 0.48%
25
1.0
× 100 = 10.5%
9.5
1.0
potassium carbonate experiment: thermometer error =
× 100 = 33.3%
3.0
b) potassium hydroxide experiment: thermometer error =
c) the potassium carbonate experiment had the greater equipment experimental error
due to the low temperature reading.
8
Heat loss to surroundings; heat loss to polystyrene cup; solution not stirred properly.
AQA Chemistry AS Level Teacher Notes © Nelson Thornes Ltd 2008
2