At 25°C and pH 7.0, the concentrations of glucose 1-phosphate and glucose 6-phosphate are 2.0 mM and 38 mM, respectively, at equilibrium. The standard free energy change for the conversion of glucose 1-phosphate to glucose 6-phosphate is ______ J/mol. [R = 8.315 J mol$^{-1}$ K$^{-1}$]