Required Practical: Investigating Temperature Changes (AQA GCSE Chemistry): Revision Note

Required practical 4: Investigating temperature changes

Objective

To perform a calorimetry study of the reaction between HCl and NaOH, and investigate the variables that affect temperature change in chemical reactions

Hypothesis

• The temperature change will vary depending on the volumes of acid and alkali used

• The temperature will:

  • Rise as more alkali is added

  • Reach a peak

  • Then fall when one reactant becomes in excess

Materials

• 2 mol/dm³ dilute hydrochloric acid

• 2 mol/dm³ dilute sodium hydroxide solution

• Styrofoam (polystyrene) calorimeter & lid

• 250 cm³ beaker (to stabilise the calorimeter)

• 10 cm³ and 25 cm³ measuring cylinders

• Thermometer

• Stirrer

Calorimetry apparatus

Diagram showing the apparatus for the calorimetry investigation for displacement, dissolving and neutralisation

Method

1. Measure 25.0 cm³ of hydrochloric acid and pour it into the polystyrene cup

2. Stand the cup in a beaker to provide support

3. Measure and record the temperature of the acid

4. Measure 5.0 cm³ of sodium hydroxide solution using a measuring cylinder

5. Add the sodium hydroxide to the cup, fit the lid and stir gently

6. Record the highest temperature reached when the thermometer reading stabilises

7. Repeat steps 1–6

   1. Add an additional 5.0 cm³ of sodium hydroxide each time

   2. Repeat until a maximum of 40.0 cm³ of sodium hydroxide is added

8. Record the temperature each time in a suitable table:

9. Repeat the entire experiment for a second trial

Results

Example results table 

Analysis

• Calculate the mean maximum temperature reached for each volume of sodium hydroxide

*30.8 is not included in the mean calculation as it is an anomalous result

• Plot a graph:

  • x-axis: volume of sodium hydroxide added (cm³)

  • y-axis: mean temperature

• Draw two straight lines of best fit:

  • One showing the initial increase

  • The other showing the fall after the maximum

• The intersection of these two lines gives the volume at which neutralisation is complete

  • From this graph, neutralisation is complete when roughly 23.0 cm³ sodium hydroxide is added

Evaluation

• This reaction is exothermic because it releases heat

• Initially, temperature increases because neutralisation is occurring

• Once all of the limiting reactant is used up:

  • The other reactant is now in excess

  • So, further addition of the other reactant dilutes the solution

  • This causes the temperature to fall

Conclusion

• The greatest temperature change occurs when 25.0 cm³ of acid and 23.0 cm³ of alkali are mixed

• After this point:

  • The excess alkali absorbs heat

  • The excess alkali does not contribute to further reaction

  • This leads to a fall in temperature