A titration is usually done in the presence of an indicator such as phenolphthalein or methyl orange, but it is possible to do them in other ways such as the measuring the heat change or the electrical conductivity of the solution during the titration. In the case of sodium hydroxide and hydrochloric acid such techniques aren’t necessary; the reaction is simply used as an illustration of the method. However with a weak acid and a weak base – say ethanoic acid and ammonia – a direct titration method cannot be used. The pH does not change rapidly enough at the endpoint to give a clear colour change with any indicator.
In this experiment you measure the temperature change as you titrate, and use the data to find
(i) the concentration of the hydrochloric acid,
(ii) the heat change per mole of water formed for the reaction, i.e. the enthalpy of neutralisation.
Method:
1 Pipette 25.0 cm3 of an approximately 1 mol dm-3 solution of hydrochloric acid into a polystyrene cup, and measure its temperature as accurately as possible using a 0 – 50oC thermometer.
2 From a burette run in 2.0 cm3 (it need not be exactly 2.00 cm3, as long as you know what it is) of a standard 1.00 mol dm-3 solution of sodium hydroxide to the hydrochloric acid whilst stirring with the thermometer. Measure the temperature, and as soon as possible
3 run in a further 2.0 cm3 of sodium hydroxide and measure the temperature.
4 Continue this process with as little pause between additions as possible until a total of 30.0 cm3 of the sodium hydroxide solution has been run in.
Results:
| Vol NaOH/cm3 | 0.00 | |||||||
| Temp/oC: |
| Vol NaOH/cm3 | ||||||||
| Temp/oC: |
Calculation:
1 Plot a graph of temperature (y-axis) vs volume of NaOH added (x-axis), and use this to find the volume of sodium hydroxide that is equivalent to your solution of hydrochloric acid.
2 Find the heat change during the reaction, assuming that the heat capacity of the mixture is the same as that of water, 4.2 J K-1 g-1 and that the density of the solution is 1.0 g cm-3.
3 Find the heat change per mole of water formed in the neutralisation.