The reaction you've provided is the formation reaction for both H(aq)+ and Cl(aq)- from their elements in their standard states. According to the convention we discussed earlier, the standard Gibbs free energy of formation ΔfG° for H(aq)+ is defined as zero. Therefore, the given ΔfG° of -131.23 kJ/mol for the reaction is entirely due to the formation of Cl(aq)- . So, we can say that the standard Gibbs free energy of formation for Cl(aq)- , ΔfGCl(aq)-° , is -131.23 kJ/mol. This is the maximum non-expansion work that can be extracted from the system at constant pressure and temperature when 1 mole of Cl(aq)- is formed from its elements in their standard states.