# Why is the change in Gibbs energy negative?

Mar 30, 2018

Well, it is not always negative...

#### Explanation:

If the change in Gibbs free energy, $\Delta G$, is a negative value, a reaction is spontaneous at a given temperature. It is non-spontaneous if $\Delta G$ 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:

$\Delta G = \Delta H - T \Delta S$

For a given reaction with given constant temperature $T$ in $\text{K}$, entropy change of the reaction $\Delta S$ ($\text{J/mol"cdot"K}$) and enthalpy change of the reaction, $\Delta H \left(\text{kJ/mol}\right)$.

From this, it can be established that reactions which have a positive $\Delta H$ and a negative $\Delta S$ are not going to be spontaneous at any temperature.

However, reactions with a negative $\Delta H$ and a positive $\Delta S$ are going to be spontaneous at any temperature.

Other combinations of $\Delta S$ and $\Delta H$ are temperature dependent.