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...
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  • 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
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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

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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
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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
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