Up Learn – A Level Chemistry (aqa) – Thermodynamics
Gibbs Free Energy: Why do some Feasible Reactions not happen?
Some reactions are feasible, but have a enough high activation energy that their rate is effectively zero.
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Last time we saw that just because the entropy change of the universe means a reaction can happen, that doesn’t mean that it will.
For example, this reaction [C60H120 + 90 O2→ 60CO2 + 60H2O] between a long chain hydrocarbon [black tarry substance] and oxygen gas [just show a translucent box] would increase the entropy of the universe.
But when you mix the two together, nothing happens
..even if we left it for years. [clock ticking]
On the other hand, this reaction [Ba(OH)2 + 2NH4Cl → BaCl2 + 2NH3 + 2H2O] between barium hydroxide and ammonium chloride also increases the entropy of the universe.
And it happens as soon as you mix them together
So what’s going on?
They’re both feasible, but only one of them actually happens.
Well, to find out, we need to consider their activation energies.
Now, this reaction is feasible, but it has a really high activation energy.
That means when particles collide…it’s incredibly unlikely that they will have enough kinetic energy to react with one another.
In fact, we could leave this for millions of years before even one successful collision happened, and trillions upon trillions of years before the reaction went to completion.
So technically, this reaction is happening, but the rate is so slow that we as humans would never ever notice.
On the other hand, this reaction has a much lower activation energy
This means when particles collide, it’s much more likely they’ll have enough kinetic energy to react with one another.
And therefore this reaction only takes minutes to go to completion.
So, for a reaction to actually happen it must…
For a reaction to actually happen, It must increase the entropy of the universe
…And occur at a reasonable rate.
So, to sum up…
Some reactions are feasible, but when scientists mix the reactants in a lab nothing happens.
That’s because the activation energy for the reaction is too high
And this means the reaction rate is exceptionally slow
So slow that, within the scientists entire lifetimes nothing would happen.