Gibbs Free Energy

ITC equations gibbsGibbs Free Energy, or ΔG, is a term which tells you whether a reaction will happen or not on it’s own (or “spontaneously”).

Basically, you are describing how much “useful energy” (ΔG) is found in a reaction, by totalling the amount of energy released (ΔH) minus the energy that can’t be used because it increases the disorder of the reaction (TΔS).

If the value of ΔG is negative, then a reaction can occur spontaneously as you don’t need to put energy into the reaction to make it happen (There is “free energy” left over after you detract TΔS).  If it is positive, then the reaction needs energy to be put into it before it can begin, so it is not spontaneous.

As temperature increases (T), entropy (ΔS) has a bigger and bigger impact on whether a reaction will occur or not (ΔH will remain the same, you are detracting a larger and larger number from this)

Here’s a great quote if you’re still a bit confused:

“Time to think about making popsicles (ice lollies, to our european readers!). Never done it? You are missing out. You pour some juice into a mold, and freeze it. Easy as pie. Actually, way easier than pie. Whoever came up with that idiom had obviously never made popsicles. Freezing the juice decreases the entropy of the system because the molecules become more rigid and in an ordered state. For juice to freeze, a large amount of heat needs to be released from the system. When do you think that the popsicles will freeze? If you said “when you put it in the freezer,” congratulations, you win! But, why will the popsicles freeze only at temperatures below 0 °C? Because, only at temperatures below 0 °C will ΔH be negative enough to compensate for the negative TΔS value. “

Ref: http://www.shmoop.com/energy-flow-enzymes/free-energy.html

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