Question

1. Explain the effect of temperature on the spontaneity of a process based on Gibbs equation. 

2. For a reaction 2A(g) + B(g) → 2D(g), enthalpy and entropy changes are – 20.5 kJ mol^-1 and – 50.4 J K^-1 mol^-1  respectively. Predict whether the reaction occurs at 25 °C.

Answer 1

1. If ∆H is -ve and ∆S is +ve, ∆G would certainly be -ve and the process will be spontaneous at all temperatures. 

If both ∆H and ∆S are – ve ∆G would be -ve if ∆H > T∆S 

If both ∆H and ∆S are + ve ∆G would be -ve if T∆S > ∆H 

If ∆H is +ve and AS is -ve, ∆G would certainly be +ve and the process will be non-spontaneous at all temperatures.

2. According to Gibbs equation, 

∆G = ∆H – T∆S ∆G 

= (-20.5 × 10³) - (298 ×-50.4) 

= – 20500 + 15019.2 = – 5480.8 J mol^-1 

Since ∆G is -ve, the process is spontaneous.