1 D G o (a delta G, with a superscript o), is the free energy change for a reaction, with everything in the standard states (gases at 1 bar, and solutions at 1 M concentration), and at a specific temperature (usually 25°C). It is important to distinguish between them. 2  D G o (a delta G, with a superscript o), is the free energy change for a reaction, with everything in the standard states (gases at 1 bar, and solutions at 1 M concentration), and at a specific temperature (usually 25°C). 3  D G (just delta G).