I am not able to understand the mathematical expression of "change in Gibbs free energy", For a chemical reaction occurring at constant temperature and constant pressure, (ΔS)total = (ΔS)system + (ΔS)surrounding Considering that reaction is exothermic, ΔH be the heat supplied by system to surrounding at constant pressure and temperature, (ΔS)total = (ΔS)system + (-ΔH)/T - T * (ΔS)total = ΔH - T * (ΔS)system The term on left hand side is known as Change in Gibbs free energy ΔG. WHY? also, the temperature in above expression is Tsystem and not Tsurrounding , HOW? (ΔS)surrounding should be Q/Tsurrounding, where Q is heat added to surrounding. By intuition, ΔH amount of heat is available and T * ΔSsystem is the unavailable energy. Thus (ΔH-T*ΔSsystem) is the amount of energy that is available to be converted to work and should be Gibbs free energy right? I am not able to understand it from the mathematical equation. Please explain what am I missing out? and where am I wrong? Please tolerate me for any mistakes, this is my first post and I do not have chemical engineering background. Thank you.