Gibbs free energy

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I don’t get what free energy is and how change in entropy affects it

In: Chemistry

2 Answers

Anonymous 0 Comments

Free energy is the energy that can be used to _perform work_. Increase of entropy lowers the free energy of a system.

So imagine, that you sit in a room, that is completely isolated from outside world. You have some amount of red-hot iron and a bucket of cold water. You can now use the heat to produce steam and power a turbine (perform work). After the temperature of everything in you room evens out, you can no longer power the turbine, but the total energy of the system is the same. So the free energy went down and the entropy went up.

Anonymous 0 Comments

So from the definition, gibbs free energy is a combination of the contributions of enthalpy and entropy to a system. G = H – TS. Note the minus, the more entropy S, the lower the G.

…and yeah, that doesn’t explain much, that’s a very common problem. So rather than focusing on WHAT it is, let’s discuss how and why it’s used.

It’s often used as the “delta”, i.e. the change in free energy, because that’s when it really matters. We want to see the difference in the internal energy between two states.

We want to know that, because it lets us predict whether reactions will spontaneously occur in specific circumstances. If one system has a lower energy than another, it will transform into it on its own.

Enthalpy is effectively (as in not STRICTLY true, the definition is a bit more complex, but that’s how it’s generally used) a measure of whether a change in state will absorb or generate heat. Exothermic and endorthermic reactions.

So you’d think that a reaction that absorbs heat will never occur on its own, because that would beeak thermodynamics right? Hot things becoming cold on their own? And yet that happens. It’s physically possible.

That’s because of the second contribution, entropy. Entropy is the measure of disorder of a system, and systems prefer to be more disordered, that’s just your bog standard thermodynamics.

So now it’s possible that if the change in a system increases entropy enough, it can decrease the energy enough that suddenly even a reaction that absorbs heat is favourable. 

For example, mixing ice and salt makes a solution which has a higher entropy, is more disordered, than the crystals of ice and salt alone. The ice melts and the saltwater solution actually gets colder than the starting solids.