Enthalpy Change, ΔH (Cambridge International A-Level Chemistry)

Enthalpy Changes

• 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

Exam Hint

• It is important to specify the physical states of each species in an equation when dealing with enthalpy changes, as any changes in state can cause very large changes of enthalpy.
• For example:

Na⁺Cl^-(s) → Na⁺(aq) + Cl^-(aq) ΔH = +4 kJ mol^-1

Na⁺Cl^-(g) → Na⁺(g) + Cl^-(g) ΔH = + 500 kJ mol^-1

• Also, remember that the system is the molecules that are reacting (i.e. the reaction itself) and the surroundings are everything else (e.g. the flask the reaction is taking place in).