Determining either the enthalpy change of neutralisation, or the enthalpy change for a classic displacement reaction (they are pretty much the same experiment) is also a required practical.
If you are not familiar with the practical, then you should watch this demonstration.
The standard enthalpy change of neutralisation, ΔnH⦵, is the enthalpy change when aqueous solutions of an acid and an alkali react together in their standard states under standard conditions (1 mol dm-3,1 atm, 298K) to give 1 mole of water
HCl(aq) + NaOH(aq) ⇾ NaCl(aq) + H2O(l)
- mass of solutions is 100g since water has a density of 1.0 g cm-3
- assume that all of the heat from the reaction is transferred to the solution
- assume that the heat capacity of the solution is the same as for water, 4.18 JK-1g-1
We can calculate the heat transferred to the solution as
q = 2772 J
no. of mol of H+ (or OH–) = concentration x volume = 1.0 mol dm-3 x 0.05 dm3 = 0.05 mol
ΔnH⦵ = (1 / 0.05) x 2772 = – 55440 J = – 55.4 kJ mol-1 (remember this is an exothermic reaction as heat is produced, the temperature rises)
The recognised value for ΔnH⦵ for the reaction between a strong acid and a strong alkali is – 56.9 kJ mol-1. It doesn’t matter what strong acid or strong alkali we use as the enthalpy change of neutralisation is for the formation of 1 mole of water (see definition), so the value should always be the same.
But if we are considering neutralisation reactions involving weak acids or weak alkalis, ΔnH⦵ will be less exothermic as these substances are only partially dissociated in solution, so as the neutralisation reaction proceeds some of the energy released is used to further dissociation of the acid or alkali until completion.
Clearly assuming that all the heat from the reaction is transferred to the solution is the least reliable assumption we made, and we can compensate for heat loss in the way that we measure the temperature rise and analyse the results.
If we repeat the experiment above but measure the temperature of the acid every minute for five minutes before adding the alkali, and then continue to measure and record the temperature of the solution until it is reliably cooling, we can plot a cooling curve.
Practice questions
Assume that the specific heat capacity of water is 4.18 J g-1 K-1 and the density of water is 1.0 g cm-3 for all questions.
- An experiment was carried out to find the ΔrH⦵ for the reaction
Pb(NO3)2(aq) + 2KI(aq) ⇾ PbI2(s) + 2KNO3(aq)
The method involved measuring out 50.0cm3 of 1.30 mol dm-3 Pb(NO3)2(aq) into a polystyrene cup and taking the temperature of this solution (17.0°C). 50.0cm3 KI(aq) was added, the solutions stirred and the maximum temperature reached was recorded (28.5°C).
Assuming that both the specific heat capacity and the density of the two solutions are the same as that of water, calculate ΔrH⦵ for the reaction.
- When 50.0cm3 of 1.00 mol dm-3 HCl is neutralised by 50.0cm3 of 1.00. mol dm-3 NaOH, the temperature rises by 6.0°C. What will be the temperature rise if the experiment is repeated with 25.0cm3 of each solution?
- An experiment was carried out to measure the enthalpy change for the reaction between ethanoic acid and sodium hydrogen carbonate. Ethanoic acid was poured into polystyrene cup and the temperature measured every minute for three minutes. Sodium hydrogen carbonate was added then added at minute four. The results are shown below:
| Time / min | Temperature of solution / °C |
| 1 | 17.2 |
| 2 | 17.2 |
| 3 | 17.2 |
| 4 | sodium hydrogencarbonate added |
| 5 | 13.1 |
| 6 | 12.4 |
| 7 | 12.8 |
| 8 | 13.0 |
| 9 | 13.2 |
| 10 | 13.4 |
| 11 | 13.5 |
| 12 | 13.6 |
Plot the data on a graph and use it to find the maximum temperature change. Show your working on the graph.
Answers
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