Reactivity 1.4 - Entropy and spontaneity

• Entropy (S) refers to the distribution of available energy among the particles in a system
• The more ways the energy can be distributed, the higher the entropy
• Entropy is used as a measure of the disorder of a system
• Systems in nature tend towards an increase in entropy
• An increase in entropy (disorder) can result from:

  • Mixing different types of particles, e.g., the dissolving of sugar in water
  • A change in state where the distance between the particles increases, e.g., liquid water → steam
  • Increasing the number of particles, e.g., 2H2O2(l) → 2H2O(l) + O2(g)
• The greatest increase in disorder is usually found where the number of particles in the gaseous state increases
• The change in the disorder of a system is known as the entropy change, S
• The more disordered the system becomes, the more positive the value of S becomes
• Systems that become more ordered will have negative S values
• Absolute entropy values
• The standard entropy of a substance is the entropy change per mole from heating the substance from 0 K to the standard temperature of 298 K
• Absolute values of entropy can be measured
• The standard entropy change for a reaction is determined by calculating the difference between the entropy of the products and the entropy of the reactants: S = S(products) - S(reactants)
• A reaction is spontaneous if it causes a system to move from a less stable state to a more stable state
• Spontaneity depends on both the enthalpy change and the entropy change
• The Gibbs energy change, G, is expressed as: G = H - TS
• For a reaction to be spontaneous, G must have a negative value
• As a reaction proceeds, the composition of the reactants and products changes and the Gibbs energy moves closer to zero
• At equilibrium, G = 0, and the reaction will not proceed any further
• The equilibrium composition of a mixture depends on the value of G
• The equilibrium constant for a reaction, Kc, also depends on the value of G
• If H is negative or zero and S is positive, then G must always have a negative value
• If H is positive or zero and S is negative, then G must always be positive, and the reaction will never be spontaneous
• For some reactions, whether they are spontaneous depends on the temperature:

  • If H is positive or zero and S is positive, then G will only become negative at high temperatures when the value of TS exceeds the value of H