5.2.2 - enthalpy and entropy

• Entropy is a measure of the dispersal of energy in a system which is greater when the system is more disordered.
• The symbol for entropy is S
• Liquids are more disordered than solids and gases are more disordered than liquids
• The unit for standard entropy is JK-1mol-1
• Temperature affects entropy as the greater temperature particles have more energy and move more. so the arrangement of particles becomes more random and more disordered.

  More disorder = higher entropy
• When a solid ionic lattice dissolves in solution, entropy increases because the ions are more disordered.
• increase in number of gas molecules = increase in entropy
• decrease in number of gas molecules = decrease in entropy
• the equation to calculate entropy change is:

  ^S reaction = sum S products - sum S reactants
• The gibbs free energy equation is

  ^G = ^H - T^S
• ^G is gibbs free energy or available energy and is measured in kJ mol-1
• ^H is enthalpy change and is measured in kJmol-1
• T is temperature and is measured in K
• ^S is entropy change and is measured in JK-1mol-1
• ^H = negative

  ^S = positive

  ^G = always negative

  feasibility of spontaneous change = reaction feasible
• ^H = positive

  ^S = negative

  ^G = always positive

  feasibility of spontaneous change = reaction never feasible
• ^H = positive

  ^S = positive

  ^G = negative at high temperatures

  feasibility of spontaneous change = feasible at high temperatures
• ^H = negative

  ^S = negative

  ^G = negative at low temperatures

  feasibility of spontaneous change = feasible at low temperatures
• For a reaction to occur spontaneously, ^G must be negative
• The limitations of the predictions of feasibility made by using ^G is that the reaction may have a high activation energy or the rate of reaction may be very slow