Gibbs free energy change, ΔG — common mistakes

The most common error in ΔG calculations is forgetting to convert the units of ΔS from J K⁻¹ mol⁻¹ to kJ K⁻¹ mol⁻¹ (by dividing by 1000) to match the units of ΔH, which are typically in kJ mol⁻¹.

ΔG is the Gibbs free energy change under any set of non-standard conditions. ΔG° is the standard Gibbs free energy change, which refers specifically to a reaction carried out under standard conditions (298 K, 100 kPa pressure, 1 mol dm⁻³ concentrations). ΔG determines spontaneity under the actual conditions, while ΔG° is a fixed reference point for a given reaction.

The Gibbs equation relates to absolute energy and disorder. The Kelvin scale is an absolute temperature scale where 0 K represents the theoretical point of zero entropy (Third Law of Thermodynamics). Using Celsius would lead to mathematical errors, such as having a zero or negative temperature term, which is physically meaningless in this context.

A reaction with a positive ΔG is non-spontaneous, meaning it will not proceed on its own. However, it can be forced to occur by coupling it with another, highly spontaneous reaction (with a large negative ΔG) or by supplying continuous external energy, such as in electrolysis.

No. Gibbs free energy only tells us about the thermodynamic feasibility (if a reaction can happen), not about its kinetics (how fast it will happen). A spontaneous reaction can be extremely slow if it has a high activation energy (e.g., the conversion of diamond to graphite is spontaneous but takes millions of years).