Question

Calculate ΔrG○ at 298 K for these reactions and predict the effect on ΔrG○ of lowering the temperature: a. NH3 (g) + HBr (g) → NH4Br (s) b. CaCO3 (s) → CaO (s) + CO2 (g) c. CH4 (g) + 3 Cl2 (g) → CHCl3 (g) + 3 HCl (g) (ΔfG○ for CHCl3 (g) is -70.4 kJ/mol)

Answer 1

Solution :-

Calculating the delta G of the reactions

a. NH3 (g) + HBr (g) → NH4Br (s)

Delta Go = sum of Delta G products – sum of delta G of reactants

                 = [NH4Br*1] - [(NH3*1)+(HBr*1)]

                 = [-8.08*1] – [(-16*1)+(-53.5 *1)]

                = 61.42 kJ/mol

Since the delta G is positive therefore increasing the temperature will decrease the reaction rate of forward reaction.

b. CaCO3 (s) → CaO (s) + CO2 (g)

Solution :-

Delta Go = sum of Delta G products – sum of delta G of reactants

                  =[(CaO*1)+(CO2*1)]-[CaCO3*1]

                  = [(-603.5*1)+(-394.4*1)]-[-1128.8*1]

                  = 130.9 kJ/mol

Decreasing the temperature will decrease the delta G of the reaction

c. CH4 (g) + 3 Cl2 (g) → CHCl3 (g) + 3 HCl (g)

solution :-

Delta Go = sum of Delta G products – sum of delta G of reactants

                  = [CHCl3*1)+(HCl*3)] –[(CH4*1)+(Cl2*3)]

                   = [(-70.4*1)+(-95.3*3)] –[(-50.81*1)+(0*3)]

                  = -305.49 kJ/mol

Lowring the temperature will increase the rate of forward reaction means lowers the delta G