A2-Level Enthalpy and Entropy

  • During a chemical reaction, the entropy of reacting particles and the surroundings change.

  • How the entropies of the reacting particles change in a reaction is called the entropy of the system, ∆Ssystem

  • How the entropy of the surroundings changes in a reaction is called the entropy of the surroundings, ∆Ssurroundings

  • Entropy change of the system and entropy change of the surroundings can be combined to describe the total entropy change of the reaction, ∆Stotal.

  • The entropy of 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 (∆H) by the temperature.

  • The total entropy change that occurs in a reaction can be calculated using:



     

  • For a reaction to be possible, change in total entropy must be positive.

QUICK NOTES

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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 is not just the entropy of the reacting particles that changes but also the entropy of the surroundings of the reaction. 

 

The combined entropy of particles involved in a reaction is referred to as the entropy of the chemical system, ∆Ssystem. We can calculate the entropy of the chemical system overall by subtracting the entropy of the reactants from the entropy 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 the surroundings, ∆Ssurroundings, describes the entropy of the reaction’s surroundings. Entropy increases with temperature. The warmer the system gets, the higher its entropy.

 

In an exothermic reaction, enthalpy is released from the reaction (chemical system) into the surroundings. This increases the entropy of the surroundings and makes the surroundings more stable (remember nature favours higher entropy).

 

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

 

 

 

 

 

 

 

Feasibility

 

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

 

 

 

For a reaction to occur, total entropy needs to increase and if ∆Stotal 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 is important to remember units. Entropy is given in J K  mol  and Enthalpy in kJ mol .  For calculations, convert entropy into kJ.

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