# AQA Chem5: Entropy and Free Gibbs Energy

Revision notes on entropy and free gibbs from the physical section of AQA Chem5

• Created by: anna
• Created on: 09-06-13 18:28

## Spontaneous Events

• Spontaneous events occur without having any energy supplied to them from the surroundings
• Many spontaneous reactions are exothermic, this is as the products reach a lower energy (more stable) state
• Endothermic reactions can also be spontaneous however they result in a higher energy product so energy and enthalpy cannot explain this alone, entropy must also be considered...
1 of 5

## Entropy

• Entropy shows how disordered something is
• Most spontaneous reactions are accompanied by an increase in disorder
• The degree of disorder of a substance is measured by its entropy
• The symbol for entropy is S
• The greater the disorder the higher the entropy
• Solids have lower entropy than liquids which have lower entropy than gases
• Complex substances have higher entropy values than simple ones
• Entropy increases with temperature
2 of 5

## Entropy Values

• Values of entropy are measured in JK-1mol-1
• Under standard conditions (298K and 100kPa) there is a standard entropy for a reaction
• A change in entropy (/\S) for a reaction is calculated by:
• /\S = sum of products S - sum of reactants S
• for an increase in entropy /\S is positive
• for an decrease in entropy /\S is negative

/\ = delta

3 of 5

## Free Gibbs Energy

• Free energy is related to the enthalpy and entropy (and temperature) of a chemical reaction or a change in state
• The symbol is /\G
• The equation is /\G = /\H - T/\S
• The units used in the equation are: /\G = kJmol-1  /\H = kJmol-1  T = K  /\S = kJK-1mol-1 (JK-1mol-1/1000)
• A reaction is said to be spontaneous/feasible when /\G is equal or less than 0.
• When /\G = 0 then T = /\H//\S this is the minimum temperature the reaction is feasible
4 of 5

## Graphs of /\G = /\H - T/\S

• When /\H is -ve and /\S is +ve as T increases /\G decreases
• When /\H is -ve and /\S is -ve as T increases /\G increases
• When /\H is +ve and /\S is +ve as T increases /\G decreases
• When /\H is +ve and /\S is -ve as T increases /\G increases
5 of 5