Skip to content

5.2.2 Enthalpy and entropy

Definitions

Term Definition
Standard entropy (\(S^\ominus\)) The entropy of one mole of a substance under standard conditions. Unit \(J \cdot K^{-1} \cdot mol^{-1}\)
Entropy (\(S\)) A measure of the dispersal of energy within the chemicals that make up the chemical system; the greater the entropy, the more disordered a system. Unit \(J \cdot K^{-1} \cdot mol^{-1}\)
Free energy change The balance between enthalpy, entropy and temperature for a process given by \(\Delta G = \Delta H - T\Delta S\). A process is feasible when \(\Delta G < 0\).
Feasibility of reaction Whether a reaction is able to happen and is energetically feasible

Entropy

When does entropy increase

  • Solid \(\rightarrow\) liquid \(\rightarrow\) gas
  • More complex substances with more atoms \(\rightarrow\) simpler substances with fewer atoms
  • Increasing the number of molecules (especially gas)
  • Increasing the temperature (at 0 K there is no energy and all substances have entropy of 0)
  • Exported image

Calculating entropy changes

  • \(\Delta S^\ominus = \Sigma S^\ominus (\text{products}) - \Sigma S^\ominus (\text{reactants})\)

Free energy

Free energy change (\(\Delta G\))

  • Overall energy change during a chemical reaction
  • Made up of enthalpy change (\(\Delta H\)) and entropy change at the temperature of the reaction (\(T\Delta S\))

The Gibbs' equation

  • \(\Delta G = \Delta H - T \Delta S\)
  • (Note: Convert the unit of \(\Delta S\) to \(kJ \cdot K^{-1} \cdot mol^{-1}\) to match \(\Delta H\))

Gibbs' equation graph

  • Graph of \(\Delta G\) against \(T\)
  • y-intercept = \(\Delta H\)
  • x-intercept = temperature where feasibility changes (\(\Delta G = 0\))
  • Gradient = \(-\Delta S\)

Conditions for feasibility

  • There must be a decrease in free energy: \(\Delta G < 0\)
  • Depends upon enthalpy change (\(\Delta H\)) and entropy change + temperature (\(T\Delta S\))

Limitations or predictions made for feasibility

  • Many reactions with \(\Delta G < 0\) don't take place
  • \(E_a\) is too high
  • The rate is too slow
  • Catalysts can be used to overcome the high \(E_a\) so the reaction can take place