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Madex Economics
Director
Jan 12, 2022
In Chemistry
If you’ve studied A-Level chemistry, you’ve heard of the Gibbs free energy equation. ΔG = ΔH – TΔS You’ve used this equation to determine whether a reaction is spontaneous, non-spontaneous or at equilibrium, with reactions that have a negative ΔG being spontaneous, those that have a positive ΔG being non-spontaneous and those with ΔG = 0 being at equilibrium. Firstly, what does it mean. Well, as suggested by the name, Gibbs free energy shows the energy available in a chemical reaction for work to be done. That’s why the units of Gibbs free energy are J/mol or kJ/mol. So when you see ΔG you are told how much work a reaction is capable of doing. However, you were likely never told how a number can determine the spontaneity of a reaction. Here is how ΔG was derived: For this we need to establish that for a chemical reaction to occur spontaneously, it must increase the disorder of the universe. This is the 2nd Law of Thermodynamics. ΔSuniverse = ΔSsystem + ΔSsurroundings However, it is extremely difficult to measure the ΔS of the surroundings. So, to get around this, we find an alternative for the ΔSsurroundings. Since ‘S = qsur/T’ then, ‘S = -qsys /T’ and qsys = ΔH we can create a new relationship of: S = -ΔH/T. If we now substitute this in the ΔSuniverse equation, we get: ΔSuniverse = ΔSsystem - ΔH/T. We can now rearrange this in the more familiar version: -TΔSuniverse = -TΔSsystem + ΔH (Multiplying both sides by T) This is the same as writing: -TΔSuniverse = ΔH + TΔSsystem From here was born the term ΔG which is equal to -TΔSuniverse . ΔG therefore tells us if a reaction is spontaneous or not because it tells us whether the entropy of the universe has increased or decreased. By Zonndaz
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Madex Economics
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Jan 06, 2022
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Madex Economics
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Dec 13, 2021
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Madex Economics
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Dec 13, 2021
In Welcome to the Forum
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