Energy Changes in Neutralisation

Introduction

If you mix dilute hydrochloric acid with dilute sodium hydroxide solution in an insulated cup, the thermometer climbs by several degrees within seconds. Nothing fizzes and nothing changes colour, yet energy has clearly been released. Neutralisation is an exothermic reaction. Understanding where this energy comes from helps explain why all strong acid–strong alkali neutralisations release almost exactly the same energy per mole of water, and how a temperature reading can even be used to find the point at which neutralisation is complete.

Core explanation

Where the energy comes from. Neutralisation between an acid and an alkali in solution is the ionic reaction:

H⁺(aq) + OH⁻(aq) → H₂O(l)

Forming the new O–H bond in a water molecule releases energy. Very little energy is needed to bring the separate ions together, so overall more energy is released than taken in. The released energy is transferred to the surroundings — mainly the water of the solution — and the temperature rises. That is what exothermic means.

The same reaction every time. For any strong acid reacting with any strong alkali, the ions that are not part of H⁺ + OH⁻ (such as Na⁺ and Cl⁻) are spectator ions and do not change. So hydrochloric acid with sodium hydroxide, nitric acid with potassium hydroxide, and sulfuric acid with sodium hydroxide are all really the same reaction. The energy released is about 57 kJ for every mole of water formed . This value is called the enthalpy of neutralisation, written as −57 kJ/mol, where the minus sign shows that energy leaves the reacting system.

Temperature rise depends on amount. The more moles of water that form, the more energy is released. Doubling the concentrations of both solutions (with the same volumes) doubles the moles of water formed and roughly doubles the temperature rise. However, doubling the volumes at the same concentration gives the same temperature rise, because twice the energy is shared among twice the mass of solution.

Temperature and the end point. If alkali is added in portions to a fixed volume of acid, the temperature rises with each addition while acid remains. Once all the acid has reacted, extra alkali has nothing to react with. Adding it now just mixes in cooler liquid, so the temperature levels off and then slowly falls. The highest temperature marks the point of exact neutralisation.

Weak acids release less. When a weak acid such as ethanoic acid is neutralised, most of its molecules are not yet ionised. Some energy is used to break the O–H bonds in the acid molecules, so the overall energy released is a little less, about 56 kJ/mol for ethanoic acid, and noticeably less for some very weak acids.

Hazard note. Some acid–base processes release enough heat to be dangerous. Diluting concentrated sulfuric acid, for example, releases a large amount of heat and can cause boiling and spitting, which is why it is only ever handled by trained people with suitable precautions.

Step-by-step reasoning

To explain a temperature change in neutralisation:

1. Write the ionic equation H⁺ + OH⁻ → H₂O. 2. State that bond formation releases energy, so the reaction is exothermic. 3. Energy passes to the solution, so its temperature rises. 4. The rise continues only while both H⁺ and OH⁻ are available to react.

Visual explanation

Sketch a graph of temperature (y-axis) against volume of alkali added (x-axis). It shows a straight rising line, a peak at the neutralisation point, then a gently falling line. The two lines, extended, cross exactly at the volume needed for neutralisation.

Real-world analogy

A campfire only heats you while there is wood to burn. Adding more matches after the wood has gone gives no more heat. In the same way, once the acid is used up, adding more alkali releases no more energy.

Real-world example

Some self-heating food cans and hand warmers use exothermic reactions to release heat on demand. Neutralisation is also why industrial plants that treat acidic waste with alkalis must control mixing carefully: large volumes reacting quickly can warm the tanks noticeably.

Why?

Why does the temperature fall after the maximum? Beyond neutralisation, no reaction takes place, so no more energy is released. Meanwhile, the warm mixture loses energy to the air and container, and cooler alkali is still being added, so the temperature drops.

Common misconception

"Energy is released because bonds are broken." Breaking bonds always takes in energy. Neutralisation is exothermic because a new bond forms when H⁺ joins OH⁻, and bond making releases energy.

Worked example

Question: 25 cm³ of an acid and 25 cm³ of an alkali, each 1.0 mol/dm³, give a temperature rise of 6.8 °C. Predict the rise if both solutions are 2.0 mol/dm³ with the same volumes.

Reasoning: Twice as many moles of water form in the same 50 cm³ of solution, so twice the energy heats the same mass.

Answer: About 13.6 °C (roughly double).

Quick check

1. Is neutralisation exothermic or endothermic, and what happens to the temperature of the solution? Answer: Exothermic; the temperature of the solution rises.

Exam focus

Be ready to explain temperature–volume graphs: rising line while acid remains, maximum at neutralisation, falling line afterwards. State that for strong acids and alkalis the energy released per mole of water is about the same, because the reaction is always H⁺ + OH⁻ → H₂O.

Advanced insight

Measured values are usually a little below 57 kJ/mol because heat escapes to the cup, thermometer and air. Using an insulated polystyrene cup with a lid, stirring, and extrapolating the cooling line back to the moment of mixing all reduce this error. These ideas form the basis of simple calorimetry, where energy change = mass × specific heat capacity × temperature change.

Summary

Neutralisation is exothermic because forming O–H bonds in water releases energy. For strong acids and alkalis the reaction is always H⁺ + OH⁻ → H₂O, releasing about 57 kJ per mole of water. The temperature rise depends on the moles of water formed per mass of solution. The highest temperature marks neutralisation; weak acids release slightly less energy.

Practice questions

1. Write the ionic equation for neutralisation and explain why it is exothermic. Answer: H⁺(aq) + OH⁻(aq) → H₂O(l); a new O–H bond forms, and bond formation releases energy. 2. Why do hydrochloric acid and nitric acid give the same energy change per mole of water when neutralised by sodium hydroxide? Answer: Both are strong acids, so the only reaction is H⁺ + OH⁻ → H₂O; the other ions are spectators. 3. On a temperature–volume graph, what does the highest temperature show? Answer: The volume of alkali at which the acid has been exactly neutralised. 4. Suggest why ethanoic acid releases slightly less energy on neutralisation than hydrochloric acid. Answer: Ethanoic acid is weak and mostly un-ionised, so some energy is used to break its O–H bonds before H⁺ can react.