Question

Why is the change in Gibbs energy negative?

Answer 1

Well, it is not always negative...

Explanation:

If the change in Gibbs free energy, `DeltaG`, is a negative value, a reaction is spontaneous at a given temperature. It is non-spontaneous if `DeltaG` takes a positive value at a given temperature.

The majority of reactions studied in laboratories are often spontaneous at room temperature- so it might seem that the majority of reactions has a value for Gibbs free energy which is negative, but this is not necessarily true.

The change in Gibbs free energy is given at constant temperature as:

`DeltaG=DeltaH-TDeltaS`

For a given reaction with given constant temperature `T` in `"K"`, entropy change of the reaction `DeltaS` (`"J/mol"cdot"K"`) and enthalpy change of the reaction, `DeltaH ("kJ/mol")`.

From this, it can be established that reactions which have a positive `DeltaH` and a negative `DeltaS` are not going to be spontaneous at any temperature.

However, reactions with a negative `DeltaH` and a positive `DeltaS` are going to be spontaneous at any temperature.

Other combinations of `DeltaS` and `DeltaH` are temperature dependent.