Exothermic and endothermic reactions explained

A reaction profile shows the energy of the reactants and products, and the energy needed to start the reaction.

• Exothermic: the products are lower than the reactants. The drop is the energy released.
• Endothermic: the products are higher than the reactants. The rise is the energy taken in.
• In both, the curve rises to a peak first. The height from the reactants to the peak is the activation energy.

A catalyst lowers the peak but doesn't change the overall energy change.

During a reaction, bonds in the reactants break and new bonds form in the products.

• Breaking bonds is endothermic: energy must be supplied.
• Making bonds is exothermic: energy is released.

If more energy is released making bonds than is used breaking them, the reaction is exothermic and the energy change is negative.

Hydrogen reacts with chlorine: $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$.

Bond energies (kJ/mol): H–H 436, Cl–Cl 242, H–Cl 431.

1. Bonds broken: $436 + 242 = 678$ kJ/mol.
2. Bonds made: two H–Cl bonds, $2 \times 431 = 862$ kJ/mol.
3. Energy change: $678 - 862 = -184$ kJ/mol.

The answer is negative, so the reaction is exothermic. Check calculations like this with our bond energy calculator.

Mix reactants (for example, hydrochloric acid and sodium hydroxide) in a polystyrene cup with a lid, to reduce energy lost to the surroundings. Measure the starting temperature, add the second reactant, stir and record the highest temperature reached. Repeat with different volumes or concentrations. A temperature rise shows an exothermic reaction; a fall shows an endothermic one.

• Hand warmers use an exothermic reaction, such as the oxidation of iron. Some can be reused by reversing the reaction.
• Cold packs dissolve a salt like ammonium nitrate in water, an endothermic process.
• Cooking often uses endothermic changes: heat must be supplied to cook food.

• Saying breaking bonds "releases energy". It takes energy in.
• Getting the sign wrong: exothermic changes are negative.
• Forgetting to multiply bond energies by the number of bonds (two H–Cl in the example).
• Thinking endothermic reactions don't need activation energy. All reactions do.

1. The temperature of a reaction mixture falls from 21 °C to 15 °C. Is the reaction exothermic or endothermic?
2. In a reaction, breaking bonds needs 1200 kJ/mol and making bonds releases 1500 kJ/mol. Calculate the energy change.
3. What happens to the energy change if a catalyst is added?

Answers: 1. Endothermic (the temperature fell). 2. $1200 - 1500 = -300$ kJ/mol, so exothermic. 3. Nothing: a catalyst lowers the activation energy, not the overall energy change.