Enthalpy Change, ΔH (CIE AS Chemistry)

• 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 energy than the reactants
• Heat energy is given off by the reaction to the surroundings
  • The temperature of the environment increases - this can be measured on a thermometer
  • The temperature of the system decreases

• There is an enthalpy decrease during the reaction so ΔH is negative
• Exothermic reactions are thermodynamically possible (because the enthalpy of the reactants is higher than that of the products)
• However, the rate may be too slow to observe any appreciable reaction
  • In this case the reaction is kinetically controlled
  • This means the reaction could have a high activation energy which is preventing the reaction from taking place.

The enthalpy change during an exothermic reaction

Exothermic reactions show a negative enthalpy change as energy is released

Endothermic reactions

• A reaction is endothermic when the products have more energy than the reactants
• Heat energy is absorbed by the reaction from the surroundings
  • The temperature of the environment decreases - this can be measured with a thermometer
  • The temperature of the system increases

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

The enthalpy change during an endothermic reaction

Endothermic reactions show a positive enthalpy change as energy is absorbed