Gibbs Free Energy Is a Criterion for Spontaneity and Takes Into Account the Entropy Change

Gibbs Free Energy

Gibbs Free Energy is a criterion for spontaneity and takes into account the entropy change of the surroundings.

The Gibbs Free Energy Function is defined by the equation G = H - TS

For change at constant temperature the equation appears as G = H - T S

Based on G reaction behavior can be predicted accurately.

Free energy or reaction can be calculated as follows :

The value of Gibbs Free Energy for a reaction is based upon its enthalpy and entropy. Both of these factors can have either a negative or positive effect on whether or not a reaction will occur spontaneously or at all. We will even be able to predict at what temperature a nonspontaneous reaction will become spontaneous.

It is called free energy because it is the energy that will be released or freed up to do work.

Consider the following example: H is negative (-ve) and S is positive (+ve)
G = H - TS
= (-) - T(+)

In such a change, G will be negative regardless of the value of the absolute temperature, T (which can only have positive values). Therefore, the change will occur spontaneously at all temperatures.

On the other hand, if a change is endothermic and is accompanied by a decrease in entropy, both factors work against spontaneity.

i.e. H is positive (+ve)
S is negative (-ve)
G = H - TS
= (+) - T(-)

In this case, G will be positive at all temperatures and the change will always be nonspontaneous.

When H and S have the same sign, the temperature becomes critical in determining whether or not an event is spontaneous.

If H and S are both positive,
G = (+) - T(+)

Only at relatively high temperatures will the value of TS be larger than the value of H so that their difference, G, is negative. A familiar example is the melting of ice.

H2O(s) ----> H2O(l)

Here is a change that we know is endothermic and occurs with an increase in entropy. At temperatures above 0oC (when the pressure is 1 atm), ice melts because the TS term is bigger than the H term. At lower temperatures, ice doesn't melt because the smaller value of T gives a smaller value for TS and the difference H-TS, is positive.

SCH4U Gibb's Free Energy Entropy Worksheet