Thermodynamics

 46) (a) H₂(g) + Cl₂(g) 2HCl (g)

             ΔG°_rxn = ΣΔG_f°(products) - ΣΔG_f°(reactants)

             ΔG°_rxn = [2ΔG_f°(HCl(g))] - [ΔG_f°(H₂(g)) + ΔG_f°(Cl₂(g))]

             ΔG°_rxn = [2 mol HCl(g) x -95.27 kJ/1 mol HCl(g)] - 0 = -190.54 kJ

             The reaction will be spontaneous at this temperature because the reactants and products are in their standard states and ΔG°_rxn < 0.

       (b) MgCl₂(s) + H₂O(l) MgO(s) + 2HCl(g)

            ΔG°_rxn = ΣΔG_f°(products) - ΣΔG_f°(reactants)

            ΔG°_rxn = [ΔG_f°(MgO(s)) + 2ΔG_f°(HCl(g))] - [ΔG_f°(MgCl₂(s)) + ΔG_f°(H₂O(l))]

            ΔG°_rxn = [1 mol MgO(s) x -569.6 kJ/1 mol MgO(s) + 2 mol HCl(g) x -95.27 kJ/1 mol HCl(g)] -

                            [1 mol MgCl₂(s) x -592.1 kJ/1 mol MgCl₂(s) + 1 mol H₂O(l) x -237.13 kJ/1 mol H₂O(l)] = +69.1 kJ

            The reaction will be nonspontaneous at this temperature because ΔG°_rxn < 0.

       (c) 2NH₃(g) N₂H₄(g) + H₂(g)

            ΔG°_rxn = ΣΔG_f°(products) - ΣΔG_f°(reactants)

            ΔG°_rxn = [ΔG_f°(N₂H₄(g)) + 0] - [2ΔG_f°(NH₃(g)) + 0]

            ΔG°_rxn = [1 mol N₂H₄(g) x 159.4 kJ/1 mol N₂H₄(g) + 0] - [2 mol NH₃(g) x -16.66 kJ/1 mol NH₃(g)] = +192.7 kJ

           The reaction will be nonspontaneous at this temperature because ΔG°_rxn < 0. 

       (d) 2NOCl(g) 2NO(g) + Cl₂(g)

            ΔG°_rxn = ΣΔG_f°(products) - ΣΔG_f°(reactants)

            ΔG°_rxn = [2ΔG_f°(NO(g)) + 0] - [2ΔG_f°(NOCl(g))]

            ΔG°_rxn = [2 mol NO(g) x 86.71 kJ/1 mol NO(g) + 0] - [2 mol NOCl(g) x 66.3 kJ/1 mol NOCl(g) + 0] = +40.8 kJ

            The reaction will be nonspontaneous at this temperature because ΔG°_rxn < 0.