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What is Gibbs Free Energy?

Gibbs free energy allows you to link together the two factors entropy and enthalpy, these factors together determine if a chemical reaction is spontaneous or not.  For a reaction to be spontaneous then the change in free energy must be negative.  The change in Gibbs free energy is calculated using:

 

dG = dH - TdS

 

dH = Enthalpy change,

T   = Temperature (K)

dS = Change in Entropy.

 

 

As dG depends on the term TdS this means that as the temperature increases the effect that dS has becomes more and more important. This results in some reactions only being spontaneous if the temperature is within the correct range.

 

Enthalpy

Entropy

Spontaneous?

Positive

Positive

Only below a certain temperature

Negative

Positive

Yes, at all temperatures

Positive

Negative

No

Negative

Negative

Only above certain temperatures

 

There is also a critical temperature; this is the temperature at which the reaction changes from being spontaneous to not being spontaneous. At this temperature dG = 0, hence meaning that

 

T= dH/dS

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