Up Learn – A Level Chemistry (aqa) – Thermodynamics
Why are the units of entropy change ‘per mole’?
In this case, ‘per mole’ means ‘per the number of moles specified in the reaction equation’
More videos on Thermodynamics:
Want to see the whole course?
Last time we saw how to calculate the entropy change of a reaction
And now that we know how to calculate entropy changes, we’re a big step closer to figuring out why some reactions happen and others don’t.
But before we move onto that, there are a few loose ends we have to tie up…
And we’ll do that, now!
So, suppose we were asked to find the entropy change for this reaction. [4NH3 (g) + 5O2(g) → 4NO(g) + 6H2O(g)]
We used our equation, did all the calculations [don’t actually need to show] and got this answer [181 J K–1 mol-1]
That’s great, but, what does this answer actually represent?
Well, it doesn’t represent the entropy change when one mole of ammonia reacts
And it doesn’t represent the entropy change when one mole of oxygen reacts
Nor does it represent the entropy change when a total of 1 mole of reactants react
Or the entropy change when one mole of products form
So… what does it represent?
Well, the answer we worked out is actually the entropy change when exactly:
4 moles of ammonia react with 5 moles of oxygen
To produce 4 moles of NO and 6 moles of
So this raises the question – what on earth is this [mol-1] doing in the units of our answer?
Well, confusingly, this [“mol-1”] doesn’t mean ‘the entropy change per every one mole’
It really means ‘per the number of moles specified in the reaction equation’
So if the entropy change for this equation [2Al+3/2 O2(g) → Al2O3] was this [ΔS = -313.2 J K-1 mol-1], That means that when 2 moles of aluminium react with 1.5 moles of oxygen to produce 1 mole of aluminium oxide, the entropy change is -313.2 joules per kelvin per mole.
So, to sum up…
The units we use for the entropy change of a reaction are joules per kelvin per mole.
Where this ‘per mole’ [mol-1] means ‘per the number of moles specified in the reaction equation’.