Question

A) Calculate the value of ΔrG° for the reaction: N2 (g) + 3 H2 (g) → 2 NH3 (g) Write your answer in kJ to four significant figures

Is the reaction spontaneous at 25°C?

B) Calculate  ΔrG° at   540 ∘C

Is the reaction spontaneous at   540 ∘C  ?

Answer 1

Gibbs free energy change= 2* Gibbs free energy change of NH3-{1( gibbs free energy change of H2+ 3* gibbs free energy change of H2}

the enthalpy change of H2 and N2 are Zeros since they are elements in their standard state.

deltaG298=2*(-16.5)-0- 0=-33 Kj/mol

deltaH 298= 2*-46.1-0 =-92.2 Kj/mol

since gibbs free energy change is -ve, the reaction is spontaneous at 25 deg.c

at 540 deg.c

gibbs free energy change at 540 K = 540{ deltaG298/298)+ deltH298*(1/540-1/298)}

=540{-33/298-92.2*(1/540-1/298)}=15.07 KJ

the reaction is not spontaneous since gibbs free energy change is +ve.