fireplace 2909225 1280 Energy Changes In Chemical Reactions

Energy Changes In Chemical Reactions


During a chemical reaction, there is always an energy change. Energy is given out or taken in, in form of heat, light, electricity or sound.

Enthalpy Change

The enthalpy change of a reaction is the amount of energy given out or taken in during the reaction and is represented by the symbol ΔH.

The enthalpy change of a chemical reaction can be calculated as follows:

Δ H = Total energy of products – Total energy of reactants

There are two different types of enthalpy change:

  • exothermic change
  • endothermic change

Exothermic Change

An exothermic change is a chemical or physical change in which heat is released into the surroundings causing a rise in temperature of the surroundings.

An exothermic change can be described as:

reactants → products + energy(lost to the surroundings)

Due to the conservation of energy, the total energy should be the same on each side of the arrow in a chemical reaction. Therefore, in exothermic reactions, the products have lower energy than the reactants. Examples of exothermic changes include:

  • condensation (physical change)
  • freezing (physical change)
  • neutralisation reactions (chemical change)
  • combustion (chemical change)
  • respiration (chemical change)

For an exothermic reaction, the total energy of the reactants is greater than the total energy of the products because some of the energy is lost to the surroundings as heat energy. Therefore the enthalpy change, ΔH, of an exothermic reaction is negative.


Example of an exothermic reaction between iron and sulphur

Fe(s) + S(s) → FeS(s) ΔH = -100 kJ

ΔH = -100 kJ means that 100 kJ of energy is given out (in form of heat) when 1 mole of iron reacts with 1 mole of sulphur to produce 1 mole of iron sulphide. The minus sign shows that the products have less energy than the reactants because energy is given out.

Endothermic change

An endothermic change is a chemical or physical change in which heat is absorbed from the surroundings causing as a drop in temperature of the surroundings.

An endothermic change can be described as:

reactants + energy(absorbed from the surrounding) → products

Since the total energy should be the same on each side of the arrow in a chemical reaction, in exothermic reactions the products have higher energy than the reactants. Examples endothermic changes include:

  • evaporation (physical change)
  • melting (physical change)
  • dissolving of ammonium chloride (physical change)
  • photosynthesis (chemical change)
  • thermal decomposition (chemical change)

For an endothermic reaction, the total energy of the products is greater than the total energy of the reactants because some of the energy is absorbed from the surroundings during the reaction. Therefore, the enthalpy change, Δ H of an endothermic reaction is positive.

Example of an endothermic reaction, the decomposition of calcium carbonate

CaCO3(s) → CaO(s) + CO2(g) ΔH = +178 kJ

ΔH = +178 kJ means that 178kJ of energy is absorbed to decompose 1 mole of CaCO3 into 1 mole of CaO and 1 mole of CO2 gas. The plus sign shows that the products have more energy than the reactants because the energy is taken in.