Thermodynamic - Entropy Change Predictions (A-Level Chemistry)
Entropy Change Predictions
Predicting Entropy Changes
As we have already explained, entropy can be defined as the randomness or dispersal of energy of a system. The more disordered a system is, the higher (the more positive) the value of entropy.
In a chemical reaction, an entropy change takes place as the reactants are changed into products. Before carrying out any calculations to find out its exact value, we can infer the direction that this entropy change go in by simply comparing reactants and products.
An entropy change will be positive when:
1. Number of gaseous molecules increases. The arrangement of particles is more random in gases than either in solids or liquids.
2. Overall number of particles increases. For an increased number of particles there is and increased number of ways in which these can be arranged and therefore an increased entropy. When in a reaction the number of moles of product formed is greater than the number of moles of reactants, there is an increase in entropy.
An entropy change will be negative when the opposite changes are seen.
Worked example: Predict the sign of the entropy change in the following reactions
Answer: There is an increase in the number of gas molecules so we predict that the entropy change will be positive
Answer: There is an decrease in the number of gas molecules so we predict that the entropy change will be negative
Answer: There is an decrease in the overall number of moles so we predict that the entropy change will be negative
Answer: There is an decrease in the overall number of moles so we predict that the entropy change will be negative
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