Exothermic and Endothermic Reactions

Every chemical reaction involves a transfer of energy between the reacting chemicals and their surroundings. The surroundings are everything around the reaction: the solution, the container and the air. You detect the energy transfer by measuring the temperature change of those surroundings.

Energy is always conserved. The total energy of the reactants and their surroundings before a reaction equals the total energy of the products and surroundings afterwards. Nothing is created or destroyed; energy is only moved between the chemicals and the surroundings.

If a reaction gives energy to the surroundings, the product molecules must store less energy than the reactant molecules did, by exactly the amount transferred out.

Reactions divide into two types based on the direction of this transfer:

• Exothermic reactions transfer energy to the surroundings.
• Endothermic reactions take in energy from the surroundings.

The whole of this topic rests on getting that direction right, so the sections that follow define each type by what happens to the temperature of the surroundings.

An exothermic reaction transfers energy to the surroundings. Because energy leaves the reaction and enters the surroundings, the temperature of the surroundings increases. If the reaction happens in a solution, the thermometer in that solution reads a higher temperature at the end than at the start.

Three groups of reactions you must recognise as exothermic:

Type of reaction	Everyday example	What you observe
Combustion	Burning methane or petrol	Releases heat and light
Oxidation (many)	A metal reacting with oxygen	Temperature rises
Neutralisation	Acid reacting with an alkali	Temperature of the mixture rises

Everyday uses put exothermic reactions to work when heat is wanted:

• Self-heating cans contain reactants (such as calcium oxide and water) kept apart until you press a button; when they mix, the exothermic reaction warms the drink or food.
• Hand warmers use a slow exothermic reaction, often the oxidation of iron, to release heat over a long period.

Judge exothermic by the surroundings, not the chemicals. The surroundings get hotter in an exothermic change.

An endothermic reaction takes in energy from the surroundings. Because energy leaves the surroundings and enters the reaction, the temperature of the surroundings decreases. A thermometer in the mixture reads a lower temperature at the end than at the start.

Reactions you must recognise as endothermic:

Type of reaction	Example	What you observe
Thermal decomposition	Heating calcium carbonate to break it down	Needs continuous heating to keep going
Acid with a carbonate salt	Citric acid with sodium hydrogencarbonate	Temperature of the mixture falls

A key everyday use is cooling:

• Sports injury packs (instant cold packs) use an endothermic process. When the pack is squeezed, chemicals mix and take in energy from the surroundings, so the pack goes cold and can be pressed against a sprain to reduce swelling.

Endothermic reactions are useful when you want something to get colder without a fridge. The surroundings get colder as energy is drawn into the reaction.

Note that measuring temperature change is all that is required here: calculating the energy change or ΔH is not part of this subsection. Those calculations belong to bond energies at Higher Tier.

Exam questions often give you data and ask whether a reaction is exothermic or endothermic. The rule is simple: look at what happens to the temperature of the surroundings.

• Temperature of the surroundings rises → energy released → exothermic.
• Temperature of the surroundings falls → energy absorbed → endothermic.

Worked example. A student mixes two solutions in a cup. The start temperature is 21.0 °C and the highest temperature reached is 27.5 °C. Is the reaction exothermic or endothermic?

$\Delta T=27.5-21.0=+6.5{}^{\circ }\text{C}$

The temperature rose, so energy was transferred to the surroundings. The reaction is exothermic.

Worked example. In a second experiment the temperature falls from 20.0 °C to 14.2 °C.

$\Delta T=14.2-20.0=-5.8{}^{\circ }\text{C}$

The temperature fell, so energy was taken in from the surroundings. The reaction is endothermic. Do not decide from the names of the chemicals; decide from the direction of the temperature change.

This required practical investigates the variables that affect the temperature change when solutions react. Suitable reactions include an acid with metals, an acid with carbonates, neutralisations, and displacement of metals.

Apparatus and method:

1. Place a polystyrene cup inside a beaker for support. The cup insulates the reaction.
2. Measure a fixed volume of one solution (for example 25 cm³ of dilute hydrochloric acid) into the cup with a measuring cylinder.
3. Record the starting temperature of the acid with a thermometer.
4. Add the second reactant, put a lid with a hole for the thermometer on the cup, and stir.
5. Record the highest temperature reached (for an exothermic reaction) or the lowest temperature reached (for an endothermic reaction).
6. Calculate the temperature change, $\Delta T=T_{\text{final}}-T_{\text{start}}$.

The variables:

Variable type	Example in this practical
Independent (you change)	Concentration or volume of the added reactant, or the metal used
Dependent (you measure)	Temperature change, $\Delta T$
Control (kept the same)	Volume of acid, starting temperature, same insulated cup

Why the polystyrene cup and lid? Polystyrene is a good insulator, so it reduces heat lost to the surroundings; the lid reduces heat lost by evaporation. Both make $\Delta T$ closer to the true value. Stirring makes sure the temperature is even throughout the solution before you read it.

To investigate concentration, repeat the experiment with different concentrations of acid while keeping every other variable the same, then plot temperature change against concentration.

1. Confusing the reaction and the surroundings

An exothermic reaction releases energy, so the surroundings heat up. Students sometimes write that the reaction "gets colder" for exothermic. State the temperature change of the surroundings, and say energy is transferred to or from the surroundings.

2. Guessing the type from the chemicals

Neutralisation sounds gentle but is exothermic; a reaction that fizzes can be endothermic. Do not guess from the substances. Use the direction of the temperature change: up means exothermic, down means endothermic.

3. Trying to calculate an energy change here

This subsection is limited to measuring temperature change. Calculating the energy released or ΔH is not required at this point, so a question here will not ask for a value in kJ. Bond energy calculations are a separate Higher Tier topic.

4. Forgetting to justify the polystyrene cup

A common practical question asks why the cup is polystyrene. The answer is insulation to reduce heat loss to the surroundings, not "so it does not break". Link the choice to reducing energy transfer.

5. Reading the wrong extreme temperature

For an exothermic reaction record the highest temperature; for an endothermic reaction record the lowest. Reading the temperature after the mixture has drifted back toward room temperature gives too small a value for $\Delta T$.