Chemical Thermodynamics
- For the reaction \(\mathrm{N_2(g)+3H_2(g)\rightarrow2NH_3(g)}\) at 298 K, \(\Delta H-\Delta U\) is approximately Answer: A. -4.96 kJ Δ n_g=2-4=-2 ; Δ H-Δ U=Δ n_gRT=-2×8.314×298≈-4955 J. 1
- For a reaction \(\Delta H=+30\) kJ mol-1 and \(\Delta S=+100\) J K-1 mol-1. The reaction becomes spontaneous above Answer: B. 300 K Δ G<0 when T>Δ H/Δ S=30000/100=300 K. 1
- Given: \(\mathrm{C(s)+O_2(g)\rightarrow CO_2(g)}\), \(\Delta H=-393.5\) kJ; \(\mathrm{CO(g)+\frac12O_2(g)\rightarrow CO_2(g)}\), \(\Delta H=-283.0\) kJ. The standard enthalpy of formation of CO(g) is Answer: D. -110.5 kJ mol-1 Subtract the second equation from the first: Δ_fH(CO)=-393.5-(-283.0)=-110.5 kJ mol⁻¹. 1