Why is the change in Gibbs energy negative?

1 Answer

Answer:

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.