How to calculate entropy change using standard molar entropies.
Transcript
Last time we looked at tables of standard molar entropies
And, now that we’ve seen what they are and how to use them, that means we can finally
move from predicting entropy changes to actually calculating them!
For example, say we wanted to find the entropy change of this reaction [N 2 + 3H 2 → 2NH 3 ]
We would use this table of values to find the total entropy of the products, which is…
And then use the same table to find the total entropy of the reactants, which is…
And subtract one from the other, to give this.
And this approach doesn’t just work for this reaction
In fact, to find the entropy change of any reaction, we just need to find the sum of entropies of the products
which we represent like this [ΣS products ]
…and subtract the sum of entropies of the reactants
which we represent like this [ΣS reactants ]
So, using this equation and this table , what’s the entropy change of this reaction? [H 2 (g) + Cl 2 (g) → 2HCl (g)]
The total entropy of the products is this [pause]
The total entropy of the reactants is this [pause]
And so the entropy change of this reaction is this [pause]
And what about this reaction? [CH 4 (g) + 2O 2 (g) → CO 2 (g)+ 2H 2 O (g) ]
The total entropy of the products is this [pause]
The total entropy of the reactants is this [pause]
And so the entropy change of this reaction is this [pause]
So, to sum up…
To calculate the entropy change of any reaction, we can use this formula
Where this [ΣS ⊖ products ]represents the sum of the entropies of the products
And this [ΣS ⊖ reactants ] represents the sum of the entropies of the reactants