|Gibbs Free Energy
|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. Gibbs Free energy is defined as.
|It is also defined as ΔGo = Gproducts
- Greactants. But this equation is only valuable to
you of the reaction is carried out at standard temperature and pressure. Any
other temperature requires the use of the first equation.
|Take a look again at Go = ΔHo - T ΔSo|
|A change can only be spontaneous if it is accompanied by a decrease
in free energy. In other words, for a change to be spontaneous, G must
be negative. What does this mean in terms of H and S?
|When a change is exothermic and is also accompanied by an increase
in entropy, both factors favour spontaneity.
|ie. H is negative (-ve)
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.
|For similar reasons, when H and S are both negative, G will be
negative only at relatively low temperatures. The freezing of water is an
|H2O(l) ----> H2O(s)
|Energy is released as the solid is formed and the entropy decreases.
You know, of course, that water freezes spontaneously at low temperatures,
that is, below 0oC.