Exothermic & Endothermic Reactions

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Energy transfer in reactions

Energy is conserved: the total energy in the universe is the same before and after a reaction. Energy moves between the reacting chemicals and the surroundings
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases
- The products have less energy than the reactants, by the amount transferred
- Examples: combustion, many oxidation reactions, neutralisation
- Uses: self-heating cans and hand warmers
An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases
- Examples: thermal decomposition, citric acid with sodium hydrogencarbonate
- Uses: sports injury packs that go cold when squeezed
You tell them apart by the temperature change of the surroundings, measured with a thermometer in the reaction mixture
To evaluate a use: a hand warmer needs a reaction that is exothermic, safe, and lasts long enough; a reusable one uses a reversible change; a single-use one is cheaper but wasteful

Required Practical 4: temperature changes in reacting solutions

Investigate a variable that affects the temperature change when solutions react: for example the volume or concentration of acid added to a fixed volume of alkali, or the mass of metal added to acid
Method for neutralisation:
- Measure 25 cm³ of dilute sodium hydroxide with a measuring cylinder into a polystyrene cup standing in a beaker for support
- Record the starting temperature of the alkali
- Add 5 cm³ of dilute hydrochloric acid, put the lid on, stir with the thermometer through the lid, and record the highest temperature reached
- Add another 5 cm³ and repeat, up to 40 cm³ of acid
- Repeat the whole experiment and take means
Variables: independent is the volume of acid; dependent is the highest temperature reached; control variables are the volume and concentration of alkali, the concentration of acid, and the starting temperature
Result: the temperature rises with each addition until the acid has neutralised all the alkali, then falls as extra cold acid is added. The peak is the volume needed for neutralisation

The practical set-up: polystyrene cup in a beaker, lid with thermometer through it, acid being added from a measuring cylinder; beside it a sketch graph of highest temperature against volume of acid, rising to a peak then falling

The polystyrene cup and lid reduce energy transfer to the surroundings, so more of the energy released shows up as a temperature change
Sources of error: energy lost to the air and the cup; the thermometer read before the maximum; uneven stirring; the acid at a different starting temperature from the alkali