Enthalpy – Thermometric titration

Introduction:

Thermometric titration is a method used to determine the unknown concentration of a solution by measuring the heat produced or absorbed during a chemical reaction. The process involves titrating the solution of unknown concentration with a standard solution of a known concentration and monitoring the temperature change during the reaction.

The purpose of this experiment is to determine the concentration of two acids, hydrochloric acid (HCl) and ethanoic acid (CH₃CO₂H), by thermometric titration. The enthalpy change of neutralization for each reaction will also be calculated.

Procedure:

To perform the thermometric titration, a 50.0 cm³ of standard sodium hydroxide solution is transferred into a polystyrene cup using a pipette and filler. The temperature of the solution is recorded before adding 5.0 cm³ of HCl solution from a burette. The mixture is stirred with a thermometer, and the temperature is recorded. The process is repeated for successive 5.0 cm³ portions of HCl solution until a total of 50.0 cm³ of HCl solution is added. The same procedure is followed for titrating ethanoic acid with standard sodium hydroxide solution.

Results:

The results are recorded in a table, with the volumes of the acid added and the corresponding temperatures recorded after each addition.

Results table for the titration of HCl.

Volume, cm3	Temperature, °C
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0

Results table for the titration of ethanoic acid.

Volume, cm3	Temperature, °C
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0

Calculation:

The temperature (y-axis) is plotted against the volume of acid added (x-axis) for each acid on the same graph. The line is extrapolated in both the positive and negative gradients of the graph, and the point where these two lines intersect is used to determine the maximum temperature achieved during the neutralization. This process is repeated for both HCl and ethanoic acid. From the maximum temperature rise, the quantity of energy released in each titration is determined. Assuming the specific heat capacity of the solutions is the same as that for water (4.18 J g^-1 K^-1) and that the heat capacity of the cup is zero, the standard enthalpy change of neutralization for each reaction is calculated.

Questions:

1. The enthalpy change of neutralization for a very dilute strong acid reacting with a very dilute strong base is constant at -57.6 kJ mol^-1 (where mol^-1 refers to one mole of water produced) because the heat of neutralization is the same for any acid and base combination that produces one mole of water.
2. Two reasons for the experimental results for HCl being a little less negative than -57.6 kJ mol^-1 could be due to: Imperfect insulation of the calorimeter resulting in heat loss to the surroundings. Incomplete dissociation of the acid and the base in the solution resulting in less energy being released during the reaction.
3. The heats of neutralization for reactions involving weak acids and/or weak bases are always less negative than for strong acids and bases because weak acids and bases are only partially ionized in solution. As a result, the number of ions produced during the reaction is smaller than that produced by strong acids and bases. The production of fewer ions means that less energy is released during the reaction, resulting in a less negative enthalpy change of neutralization.