Energy Changes in Reactions
Every chemical reaction involves an energy change. The energy is usually transferred as heat.
- Exothermic reactions release heat to the surroundings → the surroundings get warmer.
- Endothermic reactions absorb heat from the surroundings → the surroundings get colder.
The temperature change you feel or measure tells you which type of reaction is happening.
Exothermic Reactions
In exothermic reactions, the products have less energy than the reactants. The extra energy is released as heat (and sometimes light).
Common examples:
| Reaction | Observation |
|---|---|
| Combustion (burning) | Flame and heat produced |
| Neutralisation (acid + alkali) | Solution warms up |
| Oxidation (rusting) | Slow warmth released over time |
| Hand warmers | Packet heats up |
| Respiration (in cells) | Body heat produced |
Testing neutralisation
Mix 25 cm³ of hydrochloric acid with sodium hydroxide solution. The thermometer shows the temperature rising from 20 °C to 28 °C. This confirms it is exothermic.
Endothermic Reactions
In endothermic reactions, the products have more energy than the reactants. Energy is taken in from the surroundings, which cool down.
Common examples:
| Reaction | Observation |
|---|---|
| Thermal decomposition | Requires continuous heating |
| Photosynthesis | Absorbs light energy |
| Dissolving ammonium nitrate | Solution cools down |
| Cool packs (sports injuries) | Pack gets cold |
| Citric acid + sodium bicarbonate | Solution cools down |
Cool pack demonstration
Dissolve ammonium nitrate in water inside a bag. The bag feels cold because the dissolving process absorbs heat from your hand and the water.
Energy Profile Diagrams
An energy profile diagram shows the energy of reactants and products:
- Exothermic: reactants are higher on the energy axis than products; energy is released.
- Endothermic: products are higher than reactants; energy is absorbed.
The activation energy is the energy needed to start the reaction — shown as the hump on the diagram.
Remember: Exothermic = energy exits to the surroundings. Endothermic = energy enters from the surroundings.
Measuring Temperature Change
You can investigate energy changes by:
- Measuring the temperature before the reaction.
- Mixing the reactants and stirring.
- Recording the highest (exothermic) or lowest (endothermic) temperature reached.
- Calculating ΔT = T_final − T_initial.
A positive ΔT = exothermic. A negative ΔT = endothermic.
Real world: Hand warmers use exothermic oxidation
Disposable hand warmers contain iron powder that slowly oxidises (rusts) when exposed to air: 4Fe + 3O₂ → 2Fe₂O₃ + energy. This exothermic reaction releases heat steadily for several hours, reaching 50–60 °C. The same iron oxidation that destroys cars over years is engineered here into a controlled, safe heat source — exothermic chemistry put to practical use.
Real world: Instant cold packs for sports injuries
Instant cold packs used in sports medicine contain ammonium nitrate and water in separate compartments. When the inner bag is broken and the contents mix, the ammonium nitrate dissolves endothermically — it absorbs heat from the surroundings, dropping the pack temperature to around 2–4 °C within seconds. No freezer needed: the endothermic chemistry does the cooling.
Real world: Burning fuels heats homes (exothermic)
Gas central heating burns methane (CH₄ + 2O₂ → CO₂ + 2H₂O) — a highly exothermic reaction. The heat released warms water in a boiler, which circulates through radiators. Every unit of energy in your home heating bill ultimately comes from the exothermic energy released when carbon-hydrogen bonds in methane are replaced by stronger bonds in CO₂ and H₂O.
Where do you see this in real life? Hand warmers and gas fires (exothermic), sports cold packs and sweating to cool down (endothermic), photosynthesis absorbing sunlight (endothermic), burning coal in a power station (exothermic) — exothermic and endothermic energy changes are behind heating, cooling, and powering the modern world.
Practice Problems
A hand warmer gets hot when activated. Is the reaction exothermic or endothermic? Explain.
Show answer
Photosynthesis converts carbon dioxide and water into glucose using light. Is it exothermic or endothermic?
Show answer
When citric acid and sodium bicarbonate dissolve together in water, the mixture cools. What does this tell you about the energy change?