Acid and Base Neutralisation
This topic covers neutralisation, the fundamental reaction between an acid and a base. For the ESAT, you must know that this process forms a salt and water, and that it typically releases heat, making it an exothermic reaction.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The reaction of an acid with a base is called neutralisation.
- The essential ionic reaction in the neutralisation of a strong acid by a strong alkali is H+(aq) + OH-(aq) → H2O(l).
- Neutralisation reactions are generally exothermic, meaning they release thermal energy into the surroundings.
- This energy release causes a measurable increase in the temperature of the solution.
- The exothermic nature is due to the formation of strong O-H bonds in the water molecule, which is an energetically favourable process.
Formulae
Acid + Base → Salt + Water To predict the chemical products of a typical neutralisation reaction.
H+(aq) + OH-(aq) → H2O(l) To represent the fundamental ionic change occurring when a strong acid reacts with a strong alkali.
Definitions
- Neutralisation
- A chemical reaction in which an acid and a base react quantitatively with each other to form a salt and water.
- Exothermic Reaction
- A process that releases energy, usually as heat, causing the temperature of the surroundings to rise.
Worked example
A student mixes a strong acid with a strong alkali in an insulated polystyrene cup and measures the temperature before and after the reaction. The initial temperature was 21.5 °C. Which of the following correctly identifies the type of reaction and a plausible final temperature?
- 1
Identify the reaction type:
The reaction between a strong acid and a strong alkali is a neutralisation reaction.
- 2
Recall the energy change:
Neutralisation reactions are exothermic, meaning they release heat.
- 3
Determine the effect of an exothermic reaction on temperature:
An exothermic reaction releases heat into the solution, causing its temperature to rise.
- 4
Evaluate the options:
The final temperature must be higher than the initial temperature of 21.5 °C.
The reaction type is exothermic.
Only one option matches both criteria.
Answer: Plausible final temperature: 28.0 °C; Reaction type: Exothermic
Common mistakes
- ×Confusing the direction of heat flow. For an exothermic reaction like neutralisation, heat is released *from* the reacting particles *into* the surrounding solution. This means the temperature reading on a thermometer placed in the solution will *increase*.
- ×Ignoring the context of diagrams showing experiments. If a diagram shows a thermometer in a beaker where an acid and base are mixed, expect to see a temperature rise, which signifies an exothermic process.
No-calculator tips
- ✓Remember the mnemonic 'EXO is EXIT'. In an EXO-thermic reaction, heat EXITs the chemical system and enters the surroundings (like the water in the beaker), making the thermometer reading go up.
- ✓Focus on the core process: H+ and OH- forming water. Making chemical bonds releases energy. Since this is the dominant process in neutralisation, the overall reaction releases energy.