Required practical 10 · Chemistry · Chemistry Paper 1, both tiers
Temperature changes in reactions
Investigate the variables that affect temperature changes in reacting solutions, such as acid and alkali, acid and metal, or acid and carbonate.
The exam trap. Read the start temperature before mixing. The answer is a change, not the highest reading.
Variables
- Independent (you change)
- The variable changed, for example the volume of alkali added or the mass of metal
- Dependent (you measure)
- Temperature change (highest or lowest temperature minus starting temperature)
- Control (you keep the same)
- Volume and concentration of the acid
- Starting temperature
- Insulation (polystyrene cup with a lid)
Apparatus
- Polystyrene cup with a lid, in a beaker for stability
- Thermometer (0.1 °C resolution if possible)
- Measuring cylinder or burette
- Dilute hydrochloric acid and sodium hydroxide solution (or magnesium powder)
- Balance
Method, and why each step matters
Measure 30 cm³ of acid into the polystyrene cup and record its temperature.
Why The starting temperature must be read before the reaction begins.
Add a measured portion of the second reactant, stir and put the lid on.
Why Stirring mixes the reactants; the lid reduces energy loss.
Record the highest (or lowest) temperature reached.
Why The temperature change is calculated from the two readings.
Repeat with different amounts of the second reactant.
Why This shows how the variable affects the temperature change.
Repeat each test and calculate a mean.
Why Repeats reduce random error.
Plot temperature change against the variable and draw a line of best fit.
Why The graph shows the pattern, for example where neutralisation is complete.
Risks
| Hazard | How to reduce the risk |
|---|---|
| Acids and alkalis are irritants or corrosive. | Wear eye protection. |
| Glass thermometers break. | Stand the cup in a beaker and use a thermometer clamp. |
A worked set of results
| Volume of NaOH in cm³ | Start in °C | Highest in °C | Change in °C |
|---|---|---|---|
| 5 | 21.0 | 24.2 | 3.2 |
| 10 | 21.0 | 27.1 | 6.1 |
| 15 | 21.0 | 29.8 | 8.8 |
| 20 | 21.0 | 30.9 | 9.9 |
| 25 | 21.0 | 30.2 | 9.2 |
- Temperature rises as more alkali reacts, then falls once all the acid is neutralised and extra cold alkali is added.
- Energy transferred to the surroundings means the measured change is smaller than the true value.
Mistakes that cost marks
Measuring the starting temperature after the reaction has started.
Instead Read the starting temperature before mixing.
Giving the highest temperature instead of the temperature change.
Instead Change = highest temperature − starting temperature.
Forcing a straight line through points that curve or change direction.
Instead Draw two lines of best fit, or a smooth curve, as the data show.
Practice questions on this practical
- A student wants to investigate how the volume of sodium hydroxide solution added to 25 cm³ of dilute hydrochloric acid affects the temperature change.In Exothermic and endothermic reactions, reaction profiles
- A student adds 1.0 g of magnesium powder to 50 cm³ of copper sulfate solution in a glass beaker and records the temperature.In Exothermic and endothermic reactions, reaction profiles