Last time we said that if we can predict the entropy change of a chemical reaction, we’ll be one step closer to figuring out whether that reaction can actually happen.
But so far we’ve not really discussed entropy changes for actual chemical reactions
We’ve seen how entropy changes when a substance’s state changes or if we increase its temperature….
…but those are physical changes, rather than chemical reactions.
In fact the only chemical reactions we’ve investigated the entropy changes for are ones like this.. [S 8 → 4S 2 ]
…Where one substance breaks down into two or more substances.
Now we found that the entropy change for reactions like this was.
We found that the entropy change for reactions like this was positive…
…And we justified that by saying the number of products was higher than the number of reactants.
Well, as it turns out, that line of reasoning isn’t just helpful for reactions like this one…
It’s actually helpful for all reactions!
For example…
In this reaction [2NH 3 → N 2 + 3H 2 ] we’re going from 2 moles of reactants to 4 moles of products
So its entropy change is likely to be positive
In this reaction [C 6 H 6 +3O 2 → 6CO + 3H 2 ] we’re going from 4 moles of reactants to 9 moles of products
So its entropy change is likely to be positive
And in this reaction [2H 2 + CO → CH 4 O ] ..
In this reaction, we’re going from 3 moles of reactants to 1 mole of products.
And that means its entropy change is likely to be negative.
So, during any chemical reaction, if the number of particles in the products is higher than the number of particles in the reactants, the entropy change for that reaction is likely to be positive
Now, so far, we’ve been a bit non-committal with our language.
Rather than saying the entropy change will be positive, we’ve said it’s ‘likely’ to be positive.
That’s not just because we at Up Learn love sitting on the fence.
It’s actually because, as we’ve seen, the number of particles isn’t the only factor which determines the entropy change of a reaction
And we need to consider these other factors before we can be certain our prediction will be a good one.
And we’ll do that next!
But first to sum up…
If a reaction has more moles of products than reactants, it’s entropy change is likely to be positive.
