Exothermic & Endothermic Reactions (DP IB Chemistry: HL): Revision Note

Exothermic & endothermic reactions

• The total chemical energy inside a substance is called the enthalpy (or heat content)

• When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes

• An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy)

• An enthalpy change can be positive or negative

Exothermic reactions

• A reaction is exothermic when the products have less enthalpy than the reactants

• Heat energy is given off by the system to the surroundings

  • The temperature of the surroundings increases

  • The temperature of the reaction mixture (what a thermometer measures) increases

• There is an enthalpy decrease during the reaction so ΔH is negative

• Exothermic reactions are thermodynamically favourable in terms of enthalpy (the products are lower in enthalpy than the reactants)

  • Although whether a reaction is spontaneous also depends on entropy and temperature (Gibbs energy)

• However, if the rate is too slow, the reaction may not occur

  • In this case the reactants are kinetically stable (the activation energy is high)

• In simple terms, energy flows out of the system to the surroundings

Energy profile for an exothermic reaction

Endothermic reactions

• A reaction is endothermic when the products have more enthalpy than the reactants

• Heat energy is absorbed by the system from the surroundings

  • The temperature of the surroundings decreases

  • The temperature of the reaction mixture (what a thermometer measures) decreases

• There is an enthalpy increase during the reaction so ΔH is positive

• In simple terms, the system takes in energy, so the surroundings feel colder

Energy profile for an endothermic reaction