AQA GCSE Chemistry · Energy Changes
Temperature changes required practical
Investigate the variables that affect the temperature change when an acid neutralises an alkali.
Neutralisation gives out heat. Adding more sodium hydroxide makes the mixture hotter until the acid is used up; after that, extra alkali only cools it down. Where the two lines meet is the exact neutral point.
Equipment
- Polystyrene cup with a lid, standing in a beaker
- Thermometer
- Dilute hydrochloric acid and sodium hydroxide solution
- Two measuring cylinders
Variables
- Independent you change
- Volume of sodium hydroxide added
- Dependent you measure
- Highest temperature reached
- Control you keep the same
- Volume of hydrochloric acid (30 cm³)
- Concentrations of acid and alkali
- Starting temperature
- Same insulated cup and lid
Method
- 1
Measure 30 cm³ of hydrochloric acid into a polystyrene cup standing in a beaker to keep it steady.
- 2
Record the starting temperature of the acid.
- 3
Add 5 cm³ of sodium hydroxide, put the lid on and stir.
- 4
Record the highest temperature reached.
- 5
Repeat with 10, 15, 20 … 40 cm³ of sodium hydroxide, using fresh acid each time.
- 6
Plot highest temperature against volume and draw two lines of best fit.
Worked example
30 cm³ of acid each time, starting at 20.0 °C.
| NaOH added (cm³) | Highest temp (°C) | Rise (°C) |
|---|---|---|
| 5 | 23.1 | 3.1 |
| 10 | 25.9 | 5.9 |
| 15 | 29.0 | 9.0 |
| 20 | 32.1 | 12.1 |
| 25 | 34.8 | 14.8 |
| 30 | 35.6 | 15.6 |
| 35 | 35.0 | 15.0 |
| 40 | 34.0 | 14.0 |
Working
Temperature rise = highest temperature − starting temperature
20 cm³: 32.1 − 20.0 = 12.1 °C
Conclusion
The reaction is exothermic. The temperature rises until about 27 cm³ of sodium hydroxide, where the acid is exactly neutralised (the lines meet). After that no more reaction happens, so extra alkali just cools the mixture.
Risks
- Sodium hydroxide and hydrochloric acid irritate skin and eyesWear eye protection and rinse off splashes.
- Polystyrene cup can tip overStand it in a beaker.
Common mistakes
- Using a glass beaker on its own. Heat escapes quickly, so the temperature rise is too small.
- Recording the temperature at a fixed time instead of the highest temperature.
- Joining the points dot-to-dot. Draw two straight lines of best fit and find where they cross.
Better results
- Put a lid on the cup to reduce heat loss to the surroundings.
- Use smaller volume steps (e.g. 1 cm³) near the highest point to find it more precisely.
- Repeat each volume and take a mean.
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7 exam questions on this practical.
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