Quick Notes Total Entropy

  • The total entropy change that occurs in a reaction can be calculated using: equation to find total entropy change using entropy of systen change in enthalpy and temperature
  • During a chemical reaction, the entropy of reacting particles and their surroundings change.
    • How the entropies of the reacting particles change in a reaction is shown by entropy change of the system, ΔSsystem.
    • How the entropy of the surroundings changes in a reaction is shown by entropy change of the surroundings, ΔSsurroundings.
    • The entropy change of the system and the entropy change of the surroundings can be combined to describe the total entropy change of the reaction, ΔStotal.
  • The entropy of the surroundings is linked to the temperature of the reaction and the enthalpy change that occurs during the reaction.
    • It can be calculated by dividing the change in enthalpy (ΔH) by the temperature (T).
  • For a reaction to be possible, the change in total entropy must be positive (+ΔStotal).

Full Notes Total Entropy

This is for Edexcel and Salters A-level Chemistry only. If your course only mentions Free Energy or Gibbs Free Energy, this page is not for you.

When a chemical reaction happens, it’s not just the entropy of the reacting particles that changes, but also the entropy of the surroundings of the reaction.

The combined entropy of all particles involved in a reaction is referred to as the entropy of the chemical system, ΔSsystem. We can calculate the entropy change of the chemical system overall by subtracting the sum of entropies of the reactants from the sum of entropies of the products.

The more positive the value for the change in entropy of the chemical system, the more likely the reaction is to happen.

The entropy of surroundings, ΔSsurroundings, describes the entropy of the reaction’s surroundings. Entropy increases with temperature.

In an exothermic reaction (-ΔH), enthalpy is released from the reaction (chemical system) into the surroundings. This increases the entropy of the surroundings (remember 'nature' favours higher entropy).

The entropy of the surroundings is linked to the enthalpy change and temperature of a reaction.

equation to find total entropy change using entropy of system enthalpy temperature a-level chemistry

Feasibility

If we consider both the entropy of the chemical system and the surroundings, we have a total entropy that links the two.

deriving equation to find total entropy change using entropy of system enthalpy temperature a-level chemistry

For a reaction to occur, the total entropy needs to increase. If ΔStotal is a positive value, the reaction is feasible and spontaneous.

Be careful though, because even if a reaction looks like it should happen (due to enthalpy changes and entropy changes) in reality the reaction may still not happen. High activation energy barriers and very slow rates of reaction can stop a reaction from occurring, even if it ‘makes sense’ with regards to the enthalpy and entropy changes.

When calculating it’s important to remember the units. Entropy is given in J K-1 mol-1 and Enthalpy in kJ mol-1. For calculations, convert entropy into kJ.