Neutralisation Releases Heat

Introduction

When an acid and an alkali react, you usually cannot see much happening. There are no bubbles, and if both solutions are colourless the mixture stays colourless. Yet if you place a thermometer in the mixture, the reading climbs. The mixture has become warmer. This temperature rise is quiet evidence that a chemical change has taken place and that energy has been released. In this page we look at neutralisation as an exothermic change and at how chemists detect the heat it gives out.

Core explanation

Energy and chemical change. Every chemical reaction involves energy. In some reactions, energy is given out to the surroundings; in others, energy is taken in. A reaction that gives out energy, usually as heat, is called exothermic . A reaction that takes in energy, so the surroundings get colder, is called endothermic .

Neutralisation is exothermic. When an acid reacts with an alkali, the products are a salt and water. As the reaction happens, energy is released into the solution, and the solution warms up. For example, when dilute hydrochloric acid is mixed with dilute sodium hydroxide solution, the temperature of the mixture rises by several degrees Celsius.

Where does the heat come from? Acids in water contain hydrogen ions (H⁺) and alkalis contain hydroxide ions (OH⁻). In neutralisation these ions join together to form water molecules: H⁺ + OH⁻ → H₂O. Forming the new bonds in water releases energy. That energy spreads out into the water around the particles, making them move faster, which we measure as a higher temperature.

Detecting the change. Because the heat is invisible, we detect it with a thermometer or temperature probe. We record the starting temperature of the solutions, mix them, and record the highest temperature reached. The difference is the temperature rise.

What affects the temperature rise?

Factor Effect on temperature rise --- --- More concentrated acid and alkali Larger rise, because more particles react in the same volume Larger total volume of liquid, same amount reacting Smaller rise, because the heat is shared among more water Poor insulation Smaller measured rise, because heat escapes to the air Adding alkali until just neutral Greatest rise, because the most acid has reacted

A maximum at neutral. If alkali is added to acid a little at a time, the temperature rises with each portion while acid is still left to react. Once all the acid has been neutralised, adding more alkali produces no further reaction. The extra cool alkali actually lowers the temperature slightly. So the highest temperature marks the point where neutralisation is complete.

Step-by-step reasoning

To decide whether a reaction is exothermic:

1. Measure the temperature of the starting solutions. 2. Mix them in an insulated container and stir. 3. Record the highest (or lowest) temperature reached. 4. If the temperature has gone up, energy has been released: the change is exothermic. 5. If it has gone down, energy has been taken in: the change is endothermic.

Visual explanation

Imagine a graph with volume of alkali added on the horizontal axis and temperature on the vertical axis. The line climbs steadily as each portion of alkali is added, reaches a peak, then slopes gently downwards. The peak sits exactly above the volume of alkali needed to neutralise the acid.

Real-world analogy

Think of two magnets snapping together. As they move towards each other they speed up and hit with a click, giving out energy. When H⁺ and OH⁻ ions join to form water, they too "snap together", and the energy they release shows up as warmth in the solution.

Real-world example

Some self-heating food cans and hand warmers rely on exothermic reactions to produce heat when needed. Although most use other reactions rather than neutralisation, the principle is the same: a chemical change releases stored energy, and the surroundings warm up. Neutralisation in a beaker is a small, gentle example of the same idea.

Why?

Why does concentrated acid and alkali give a bigger temperature rise than dilute solutions of the same volume? More concentrated solutions contain more H⁺ and OH⁻ ions in each cubic centimetre. More ions react, more water forms, and more energy is released into the same volume of liquid, so the temperature climbs higher.

Common misconception

"The mixture gets hot because the acid is burning the alkali." Nothing is burning. Neutralisation releases energy because new bonds form in water molecules. Burning (combustion) is a different reaction that needs oxygen and usually produces a flame.

Worked example

Question: A student mixes 25 cm³ of dilute hydrochloric acid at 19.5 °C with 25 cm³ of dilute sodium hydroxide solution at 19.5 °C. The highest temperature reached is 26.0 °C. Calculate the temperature rise and state the type of change.

Reasoning: temperature rise = highest temperature − starting temperature = 26.0 − 19.5 = 6.5 °C. The temperature went up, so energy was released.

Answer: The temperature rise is 6.5 °C, and the reaction is exothermic.

Quick check

1. Is neutralisation exothermic or endothermic? Answer: Exothermic, because it releases heat and the temperature of the mixture rises.

Exam focus

Be ready to explain how a thermometer shows that neutralisation is exothermic, to calculate a temperature rise by subtraction, and to suggest why a polystyrene cup with a lid gives more reliable results than an open glass beaker: it reduces heat loss.

Advanced insight

For many strong acids and strong alkalis, the energy released per unit of water formed is almost the same, about 57 kJ for every mole of water produced. This is because in each case the same reaction really happens: H⁺ + OH⁻ → H₂O. With weak acids or weak alkalis, slightly less energy is measured, because some energy is used in breaking up the weak acid or alkali particles.

Summary

Neutralisation is an exothermic change: as H⁺ and OH⁻ ions join to form water, energy is released and the solution warms up. The temperature rise is detected with a thermometer and found by subtracting the starting temperature from the highest temperature. The rise is larger with more concentrated solutions and good insulation, and the maximum temperature is reached when the acid has just been neutralised.

Practice questions

1. Define an exothermic change. Answer: A change that releases energy to the surroundings, usually as heat, so the temperature of the surroundings rises. 2. Why is a polystyrene cup better than a thin glass beaker for measuring the temperature rise in neutralisation? Answer: Polystyrene is a good insulator, so less heat escapes and the measured temperature rise is closer to the true value. 3. A student adds alkali to acid in portions. The temperature rises, then falls slightly after a certain volume. Explain the fall. Answer: Once all the acid has been neutralised, no more reaction occurs, so no more heat is released; adding cooler alkali lowers the temperature. 4. The starting temperature is 20.0 °C and the highest temperature is 24.5 °C. Calculate the temperature rise. Answer: 24.5 − 20.0 = 4.5 °C.