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An acid will usually release heat when it reacts.

In this reaction we are going to react an acid with an alkali and measure the temperature change during the reaction.

The acid will react to produce a salt and water – no gas is produced. Also, in this reaction there is no colour change. However, the pH will change – you could measure this as described in the previous reaction.

Apparatus and chemicals

• Polystyrene cup and lid
• 250 cm³ beaker
• Thermometer
• Measuring cylinder (25cm³)
• 1 mol/dm³ HCl (25cm³)
• 1 mol/dm³ NaOH (25cm³)

Method

1. Place the polystyrene cup into the 250 cm³ beaker to stabilise it.
2. Carefully measure 25 cm³ of hydrochloric and place it in the polystyrene cup.
3. Take the initial temperature of the hydrochloric acid and record it in the results table.
4. Wash out the measuring cylinder and rinse it with a little sodium hydroxide solution.
5. Then measure 25 cm³ of sodium hydroxide solution into the measuring cylinder.
6. Add the sodium hydroxide solution to the acid, stir with the thermometer, measure and record the highest temperature reached during the reaction.

During this reaction the H⁺ ions from the acid and the OH^- ions from the alkali react together to form water. As this happens, energy is released which is detected by the thermometer. The term we use to describe the release of energy is a chemical reaction is exothermic (the ‘exit’ of energy).