Question

Explain why enthalpy data alone cannot be used to predict whether a reaction can occur spontaneously or not?

Answer 1

Since the spontaneity of a reaction depends on the Gibbs' free energy term and not on the enthalpy term alone.

Explanation:

The Gibbs' free energy equation goes as follows:

`DeltaG=DeltaH-TDeltaS`

where `DeltaG` is the change in Gibbs' free energy, `DeltaH` is the change in enthalpy, `DeltaS` is the change in entropy and `T` is the temperature.

• If `DeltaG` is less than 0 then the reaction will occur spontaneously.
• If `DeltaG` is larger than 0, the reaction will not occur spontaneously.
• If `DeltaG = 0` the forward and backward reaction are in a thermodynamical equilibrium and both occur at the same rate.

As you can see one has to pay attention to the entropy term as well in order to predict the spontaneity of a reaction.