Calculating the entropy change for a phase change

ΔS = qrev
T
The magnitude of ΔS for a reversible process such as a phase change is calculated as shown at the right with the temperature being in Kelvin (°C + 273). Since entropy changes are much smaller than enthalpy changes, they are usually reported in J K–1 mol–1.

Examples of reversible processes are
  • Boiling: As temperature is constant, ΔS = ΔHvap/T
  • Melting: As temperature is constant, ΔS = ΔHfus/T
  • Heating a substance infinitely slowly with no phase change:
    In this case integration over the range of temperatures for one mole of substance gives:

    ΔS = nCp × ln T2
    T1
    Cp is the heat capacity of the substance at constant pressure
    n is the  amount in moles of substance being used
    T1 and T2 are temperatures in Kelvin
    ln is natural logarithm (similar to log - use the button on your calculator just like you would for log)

The absolute entropy (S°) of a substance at temperature T is the sum of the entropy changes on warming the substance from absolute zero to T.
Dr Sheila Woodgate
© The University of Auckland 2002