9701 · 23.4
Gibbs free energy change, ΔG — FAQ
Frequently asked questions for 9701 Gibbs free energy change, ΔG. Direct answers first, then deeper explanation — then practise with marking.
What is the difference between ΔG and ΔG°?
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.
Why must temperature be in Kelvin for the Gibbs equation?
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.
Can a reaction with a positive ΔG ever happen?
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.
If ΔG is negative, does that mean the reaction will be fast?
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).