Question

What is the condition for a reaction to be spontaneous? If a reaction is spontaneous at one temperature is it spontaneous under all conditions? [10 marks] ii) Given the following information ?fH°(CH3OH(g)) = -201.0 kJ mol-1 / S°(CH3OH(g)) = 239.70 J mol-1 K-1 ?fH°(CH2O(g)) = -108.6 kJ mol-1 / S°(CH2O(g)) = 218.95 J mol-1 K-1 ?fH°(H2(g)) = 0 / S°(H2(g)) = 130.70 J mol-1 K-1 determine ?rH°, ?rS° and ?rG° for the formation of formaldehyde from the dehydrogenation of methanol at 298 K. [32 marks] iii) Is this reaction always spontaneous, never spontaneous, spontaneous at 298 K but not at higher temperatures, or not spontaneous at 298 K but spontaneous at higher temperatures? Justify your answer.

Answer 1

For the formation of formaldehyde from dehydrogenation of methanol , reactant is methanol and product is formaldehyde .

Therefore

Enthalpy of reaction is = enthalpy of products - enthalpy of reactants = -108.6-(-201.0) = 92.4kj

Entropy change = entropy of products - entropy of reactants = 130.7 + 218.95 - 239.7 = 109.95 j

Gibbs free energy change = enthalpy change - (entropy change)(temperature) = 92400-(109.95×298) = 59634.9j

This reaction is not spontaneous as gibbs free energy change is positive .

The reaction is not spontaneous for 298k . But it is spontaneous for 840k and higher .