Enthalpy – Thermometric Titration

Aim

The purpose of this experiment is to determine the concentrations of two acids, hydrochloric acid, HCI, and ethanoic acid, CH₃CO₂H, by thermometric titration; and having done that, to calculate the enthalpy change for each reaction – the enthalpy change of neutralization.

Introduction

You titrate both hydrochloric acid and ethanoic acid in turn with a standardised solution of sodium hydroxide and record the temperatures of the mixtures during the titrations. In each case a plot of temperature against time will enable you to determine the maximum temperature rise, from which you calculate both the concentration of the acid and the enthalpy change of neutralization.

Requirements

• safety glasses
• pipette, 50.0 cm³
• pipette filler
• polystyrene cup
• sodium hydroxide solution, 1 M NaOH (CORROSIVE)
• thermometer or data logger capable of measuring ±0.1°C
• burette, 50.0 cm³
• filter funnel, small
• hydrochloric acid, 2.0 M HCI (IRRITANT )
• ethanoic acid, 2.0 M CH₃CO₂H (IRRITANT)

Procedure

Titration of hydrochloric acid with standard sodium hydroxide solution

1. LUsing a pipette and filler, transfer 50.0 cm³ of NaOH solution into the polystyrene cup. Allow to stand for a few minutes.
2. Record the temperature of the solution.
3. From a burette, add 5.0 cm³ of HCI solution to the cup.
4. Stir the mixture with the thermometer and record its temperature.
5. Add successive 5.0 cm³ portions of HCI solution stirring the mixture and record the temperature after each addition, until you have added a total of 50cm³ of HCl.

Titration of ethanoic acid with standard sodium hydroxide solution

7. Follow the same procedure as you did for the titration of HCI, except that you use ethanoic acid in the burette.
8. Record your results in a copy of Results Table

Results table for the titration of HCl.

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

Results table for the titration of ethanoic acid.

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

Calculation

1. Plot temperature (y-axis ) against volume of acid added (x-axis) for each acid on the same graph.
2. Extrapolate the line in both the positive and negative gradients of the graph and where these two points intersect use this to work out the maximum temperature achieved during the neutralisation.
3. Repeat steps 1 and 2 for the titration with ethanoic acid.
4. From the maximum temperature rise, determine the quantity of energy released in each titration. Assume that 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.
5. Calculate the standard enthalpy change of neutralization for each reaction.

Questions

1. The enthalpy change of neutralization for a very dilute strong acid (i.e. an acid which is completely ionised in solution) 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. Why is the value constant?
2. Experimental results for hydrochloric acid are usually a little less negative than – 57.6 kJ mol^-1. Suggest two reasons for this.
3. Ethanoic acid is a weak acid, i.e. it is not completely ionised in solution. Suggest a reason why the heats of neutralization for reactions involving weak acids and/or weak bases are always less negative than for strong acids and bases