Section 1 of 7Measuring energy change in solution
Idea
- Idea When a reaction takes place in water, the energy transferred heats up or cools down the solution,
- so the temperature change tells us about the energy change.
- This works well for neutralisation reactions and for displacement reactions, and for solids dissolving in water.
Insulation
- Insulation The reaction is done in a polystyrene cup, often with a lid, because it is a very good insulator and reduces heat lost to the surroundings.
- Even so, the measured temperature change is usually smaller than the true one, so the energy is underestimated.
Assumption about density
- Assumption about density The solution is assumed to behave like water, so 1 cm3 has a mass of 1 g and the specific heat capacity is 4.18 J/g/°C.
Section 2 of 7Method
1
Place a polystyrene cup in a beaker to keep it steady, and add a measured volume of the first solution.
2
Record its starting temperature with a thermometer.
3
Add an excess of the second reagent, which may be a solid or a measured volume of solution, and stir continuously.
4
Watch the thermometer and record the highest temperature reached for an exothermic reaction, or the lowest for an endothermic one.
5
Work out the temperature change, ΔT, by subtracting the start temperature from the final temperature.
Section 3 of 7The equation
| Symbol | Quantity | Unit |
|---|---|---|
| Q | Heat energy change | J |
| m | Mass of the solution being heated | g |
| c | Specific heat capacity of the solution (4.18 for water-based solutions) | J/g/°C |
| ΔT | Temperature change | °C |
Section 4 of 7Worked example
1Find the mass
- Find the mass 100.0 cm3 of copper(II) sulfate solution behaves like water, so its mass is 100 g.
2Substitute
- Substitute Excess iron powder gives a temperature rise of 7.5 °C, so Q = 100 × 4.18 × 7.5.
3Calculate
- Calculate Q = 3135 J, which is 3.14 kJ to three significant figures.
4Give the sign
- Give the sign The temperature rose, so the reaction is exothermic and the energy change is written as −3.14 kJ.
Section 5 of 7Improving the experiment
Reducing error
Less heat loss
- Less heat loss Use a lid, a second nested cup, or a vacuum flask calorimeter.
Better temperature reading
- Better temperature reading Use a digital thermometer or a temperature probe with a data logger, and stir so the temperature is even.
Repeat and average
- Repeat and average Repeat the experiment and use a mean value to reduce the effect of random errors.
Excess reagent
- Excess reagent Using an excess of one reactant ensures that the other is fully used and the maximum temperature change is obtained.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Exothermic | A reaction that transfers energy to the surroundings, so the temperature rises. |
| Endothermic | A reaction that takes in energy from the surroundings, so the temperature falls. |
| Specific heat capacity | The energy needed to raise the temperature of 1 g of a substance by 1 °C. |
| Calorimeter | An insulated container used to measure the energy transferred in a reaction. |
| Excess | More of a reactant than is needed to react completely with the other reactant. |
Section 7 of 7Exam technique
1
Using the mass of the solid added instead of the mass of the whole solution in Q = m × c × ΔT.
2
Forgetting to convert joules to kilojoules by dividing by 1000 when the question asks for kJ.
3
Leaving out the negative sign for an exothermic reaction, even though the temperature rose.
4
Using the final temperature instead of the temperature change (final minus initial).
5
Saying that heat loss makes the result too high; in fact it makes the measured temperature rise too small.