A good way to become familiar with thermodynamic processes is to start with a very simple system and consider how various changes might affect it. Calculate the change in molar Gibbs free energy, ΔG_m, for the process H₂O(s) → H₂O(l) at 1 atm and

(a) 10. °C;

(b) 0. °C. Decide for each temperature whether melting is spontaneous or not. Treat ΔH_fus and ΔS_fus as independent of temperature.

ANTICIPATE Because the solid and liquid phases are in equilibrium at the melting point, you should expect to find that ΔG = 0 at 0.°C. Above that temperature the melting of the solid state is favored, so you should expect that ΔG will be negative at 10.°C.

PLAN Find the enthalpy of fusion of water in Table 4C.1 (the values there are for 1 atm and the transition temperature). The entropy of fusion of water can be determined from the values in Table 4G.1. Use Eq. 3 to calculate the change in Gibbs free energy.

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AG=AH - TAS at constant temperature (3)