Question

Predict the signs of ΔH, ΔS and ΔG of the system when a solid melts at 1 atmosphere and at 

(a) -55 °C 

(b) -95 °C 

(c) -77 °C, if the normal melting point of the solid is -77 °C.

Answer 1

Since the normal melting point of the solid is -77°C, to melt the solid at any temperature other than at -77 °C, the pressure is required to be changed. During the phase change, the system will be at equilibrium, hence Δ G = 0.

(a) In case a solid at -55 °C, 

The temperature should be decreased (ΔH < 0, ΔS < 0) to -77 °C and then it will melt, so ΔH > 0, ΔS > 0, ΔG = 0.

(b) In case of a solid at -95 °C, 

It represents supercooled system and the temperature should be raised to -77 °C (ΔH > 0, ΔS > 0) and then it will melt so ΔH > 0, ΔS > 0, ΔG = 0.

(c) At -77 °C, 

Solid will melt, solid and liquid will be at equilibrium. 

Melting involves absorption of heat,

ΔH >0, ΔS > 0, ΔG = 0.