Exothermicity
Definition and meaning of Exothermicity in chemistry.
Exothermicity is the property of a process that releases energy into its surroundings. This energy usually leaves the system as heat or light during a chemical reaction. The starting materials end up with more potential energy than the final products.
In more detail
A chemical process shows exothermicity when it gives off more energy than it absorbs. Every reaction requires a starting boost of energy to break existing chemical bonds. Chemists call this initial energy barrier the activation energy of the reaction.
Once the starting bonds break, the atoms rearrange to form brand new bonds. Making new chemical bonds always releases energy back into the surrounding environment. If these new bonds are stronger than the old ones, extra heat is released.
Chemists measure this heat release as a negative change in enthalpy, written as ΔH. Many students think that all exothermic reactions must happen very quickly. However, the speed of a reaction depends only on the activation energy barrier.
A reaction can release a lot of heat while occurring at a slow pace. We can map this energy path using a simple reaction coordinate diagram. On this graph, the products sit lower than the starting reactants.
Key facts
| Field | Physical Chemistry |
|---|---|
| Sign Convention | ΔH < 0 for a negative enthalpy change |
| Opposite Process | Endothermicity with ΔH > 0 |
| Measurement Tool | Calorimeter device |
| Energy Profile | Products have lower potential energy than reactants |
| Common Examples | Combustion of fuels and freezing of water |
The combustion of natural gas is a familiar example of an exothermic reaction. Methane gas reacts with oxygen to produce carbon dioxide gas and liquid water. The chemical equation is CH4(g) + 2O2(g) → CO2(g) + 2H2O(l). This chemical process releases about 890 kilojoules of heat per mole of methane. The reaction releases this heat because the new product bonds are very stable. We use this released thermal energy to heat homes and cook food daily. You can feel the air around the flame get warm during this process. This temperature rise is a clear sign that energy is leaving the system.
Frequently asked questions
Does an exothermic reaction always happen quickly?
No, exothermicity only describes the overall release of energy from the chemical system. The speed of the reaction depends entirely on the activation energy barrier.
How can you tell if a reaction is exothermic from an energy diagram?
The diagram plots potential energy against the progress of the chemical reaction. In an exothermic process, the products sit lower on the graph than the reactants.
Why do chemists consider freezing water to be an exothermic process?
Liquid water molecules must lose kinetic energy to slow down and form ice. This lost energy leaves the water and enters the surroundings as heat.