Gibbs Free Energy is a term used to figure out if a reaction will be spontaneous. The equation to go with it is:

ΔG = ΔH – TΔS

If the value for ΔG is negative, then a reaction is spontaneous.

In what conditions can ΔG be guaranteed to be negative?

Just looking at the equation, if the value for ΔH is negative and the value for TΔS is positive, then you have a negative minus a positive:

- value – value = - value + (-value) = - value

So what are ΔH and TΔS?

ΔH is enthalpy. You’ve worked with enthalpy already.

When a reaction is exothermic, ΔH is negative.

When a reaction is endothermic, ΔH is positive.

T stands for temperature. The temperature is in Kelvin.

ΔS stands for change in entropy. In other words, did the reaction produce products that are more organized or less organized. If the reaction became more disorganized (less organized) then the ΔS is positive. You can think about it as the reaction being positive for becoming more disordered. Mother Nature likes things to become more disordered. If you ever see a wind organize a pile of leaves be afraid, be very afraid, because this means the laws of physics have reversed. It is “natural” for things to become more disorganized. It takes energy to organize things, it takes less energy for them to become disorganized. Entropy undoes what energy does. Or you can think of it as energy does what entropy undoes. It is almost like energy and entropy are opposing forces….entropy is the tendency toward disorder; energy is needed to make order out of randomness.

You get a – ΔH when a reaction is exothermic and a + ΔS when it becomes more disordered, so in exothermic reactions where the products are more disordered than the reactants, the reaction is spontaneous.

There is one other set of conditions that can definitely be determined…endothermic reactions that become more organized will never be spontaneous. In this situation, ΔH is positive and ΔS is negative so the equation becomes:

+ value – (-value) = + value + value = + value for ΔG

The other two situations are more difficult to determine and would have to be evaluated on a reaction by reaction basis.

For example, an exothermic (-ΔH) reaction that became more organized (-ΔS) would be:

- value – (-value) = - value + value = don’t know if it is + or -, it depends on the numbers for ΔH and TΔS. At low temperatures, it could be spontaneous, at higher temperatures it could be not spontaneous. More information (like the temperature) is needed to determine spontaneity.

The other situation is an endothermic (+ ΔH) reaction that became more disorganized (+ΔS):

+ value – (+ value) = + value – value = don’t know if it is + or -, it depends on the numbers for ΔH and TΔS. At high temperatures it could be spontaneous,…