Exothermic Reaction

Summary: An exothermic reaction is a chemical reaction that releases thermal energy to its surroundings, resulting in a temperature increase of the surroundings and a negative enthalpy change (ΔH). Tags: chemistry igcse energetics exothermic enthalpy Created: 2026-07-13

An exothermic reaction is a chemical reaction in which energy, primarily in the form of thermal energy (heat), is transferred from the reacting system to its surroundings, causing the temperature of the surroundings to rise. In an exothermic reaction, the total energy stored in the chemical bonds of the products is less than the total energy stored in the chemical bonds of the reactants, meaning that the excess energy is released to the environment. The Enthalpy Change (ΔH) for an exothermic reaction is always negative, because the system loses energy — this is often written as ΔH < 0 or given a negative sign (e.g. ΔH = −286 kJ/mol for the combustion of hydrogen). On a Reaction Pathway Diagram, the products sit at a lower energy level than the reactants, with the vertical drop representing the energy released to the surroundings. Exothermic reactions are ubiquitous in both the laboratory and everyday life, encompassing processes such as combustion, neutralisation, respiration, and the dissolving of certain substances like sodium hydroxide in water.

Characteristics of Exothermic Reactions

• Temperature increase: The surroundings gain thermal energy, so a thermometer placed in the reaction mixture will register a rise in temperature.
• Negative ΔH: The enthalpy change is negative because the system loses energy to the surroundings.
• Products are lower in energy: The chemical bonds in the products contain less stored energy than the bonds in the reactants.
• Often self-sustaining: Once initiated, many exothermic reactions (such as combustion) continue without further external energy input, as the energy released can provide the Activation Energy for more reactant particles.

The Reaction Pathway Diagram

A Reaction Pathway Diagram (also called an energy profile diagram) for an exothermic reaction shows:

• The reactants on the left, at a higher energy level.
• The products on the right, at a lower energy level.
• An activation energy hump between them — the minimum energy required for the reaction to start.
• The ΔH labelled as the vertical drop from reactants to products, with a negative value.

Energy
  ↑
  |   Reactants
  |     |  \
  |     |   \  Activation energy
  |     |    \
  |     |     \
  |     |      \_____ Products (lower energy)
  |     |   ΔH (−)
  |     |______________________
  |
  +————————————————————————→ Reaction progress

The key takeaway: in an exothermic reaction, the products have less energy than the reactants; the difference is released as heat.

Bond Breaking and Bond Making

The energetics of any chemical reaction can be understood through the processes of Bond Breaking and Bond Making:

• Bond breaking is endothermic — energy must be absorbed from the surroundings to overcome the attractive forces holding atoms together.
• Bond making is exothermic — energy is released when new bonds form, because the bonded state is more stable (lower in energy) than separated atoms.

A reaction is exothermic overall when the energy released during bond making is greater than the energy absorbed during bond breaking. In other words:

Energy released in forming new bonds > Energy absorbed in breaking existing bonds → ΔH is negative → reaction is exothermic.

Common Examples of Exothermic Reactions

Other notable examples include the thermite reaction (Fe₂O₃ + 2Al → 2Fe + Al₂O₃), used in welding railway tracks, and the reaction of calcium oxide (quicklime) with water, which produces enough heat to boil water.

Everyday Applications

• Hand warmers: Reusable hand warmers contain a supersaturated solution of sodium acetate. Clicking a metal disc triggers crystallisation — an exothermic process that releases heat for several hours.
• Self-heating cans: These contain a compartment of calcium oxide separated from water. When the seal is broken and the two mix, the exothermic reaction heats the contents of the can (e.g. coffee or soup).
• Hot packs: Single-use hot packs use the exothermic oxidation of iron powder (similar to rusting, but accelerated by salt and water).
• Central heating and cooking: The combustion of natural gas (methane) in boilers and stoves is an exothermic reaction.

Measuring Exothermic Reactions

In an IGCSE laboratory experiment, exothermic reactions can be identified and compared using simple calorimetry:

1. Place a known volume of a reactant solution in a polystyrene cup (a calorimeter).
2. Record the initial temperature.
3. Add the second reactant, stir, and record the highest (or lowest, for endothermic) temperature reached.
4. Calculate the temperature change (ΔT).

For an exothermic reaction, the final temperature will be higher than the initial temperature. The heat energy change can be calculated using:

q = m × c × ΔT

Where q is the heat energy transferred (J), m is the mass of the solution (g), c is the specific heat capacity (typically 4.18 J/g°C for aqueous solutions), and ΔT is the temperature change (°C).

The enthalpy change per mole (ΔH, in kJ/mol) can then be found by dividing q by the number of moles of the limiting reactant and converting joules to kilojoules.

Comparison with Endothermic Reactions

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus, Section 5.1: Energetics of Reactions
• Cambridge IGCSE Chemistry Coursebook (4th Edition), Chapter 6: Chemical Energetics
• Reaction Pathway Diagram
• Bond Making
• Enthalpy Change

Common Misconceptions