Chemical Reactions in SolutionOxford AQA IGCSE Chemistry: Revision notes
Section 1
How do we measure energy change in solution?
When a reaction happens in solution, the energy transferred can be found by measuring the temperature change of the solution before and after mixing. The reagents are mixed in an insulated container (such as a polystyrene cup) to reduce energy loss to the surroundings, so the temperature change reflects the reaction as accurately as possible.
- A thermometer records the starting temperature of the solution(s)
- The reagents are mixed and the maximum (or minimum) temperature reached is recorded
- The temperature change (ΔT) is calculated by subtracting the starting temperature from the final temperature
Examiners want you to name the insulated container (e.g. polystyrene cup) and explain WHY it is used — to minimise energy loss to the surroundings.
Section 2
Which reactions can this method be used for?
This method applies to reactions where the reactants end up dissolved or mixed in the same solution:
- Reactions of solids with water — e.g. dissolving a solid such as ammonium nitrate or anhydrous copper sulfate, or a metal reacting with water
- Neutralisation reactions — an acid reacting with an alkali in solution
In both cases, the temperature of the solution rises (exothermic) or falls (endothermic) as the reaction proceeds, and this change is measured directly with a thermometer.
Adding solid sodium hydroxide to water: the temperature of the water rises steadily as the solid dissolves and reacts, showing an exothermic process.
Section 3
How is the experiment carried out?
- Measure a fixed volume of one reagent (e.g. acid) into the insulated container using a measuring cylinder
- Record its starting temperature
- Add the second reagent (e.g. alkali, or a solid) and stir
- Record the highest (or lowest) temperature reached
- Repeat and take a mean to improve reliability
- Calculate the temperature change
Students often forget to stir the mixture — without stirring, the thermometer may not record the true maximum or minimum temperature of the whole solution.
Section 4
What can the results tell us?
A larger temperature change generally means more energy was transferred per mole of reactant, for a fixed volume and concentration of solution. This allows different reactions (e.g. different acids, different concentrations, different metals) to be compared for how exothermic or endothermic they are.
| Result | Meaning |
|---|---|
| Temperature rises | Reaction is exothermic |
| Temperature falls | Reaction is endothermic |
| Bigger ΔT (same conditions) | More energy transferred |
Must Know
- Energy change in a solution reaction is found from the temperature change measured with a thermometer
- An insulated container (e.g. polystyrene cup) reduces energy loss to the surroundings
- This method is used for solids dissolving/reacting with water and for neutralisation reactions
- Temperature rises = exothermic; temperature falls = endothermic
- Repeating and averaging results improves reliability
- Stirring ensures an accurate, even temperature reading
That's the notes covered.
Carry on to the next subtopic.