Last time we saw how to take values for enthalpy change, entropy change and temperature, and use them to calculate dG
And, from that, we decided whether a reaction was feasible or not
But actually, in some cases, we don’t need to do all of that work to decide whether a reaction is feasible
For example, suppose we know that a reaction has a negative enthalpy change and a positive entropy change.
And so, looking at this equation ΔG = ΔH – TΔS , we can say that…
We can say that, because both of these numbers are positive [T and ΔS], this term [TΔS] is always positive
And so, overall, we’ve got some negative number…subtract some positive number
Which, in any case, means we’ll end up with a negative number overall
In other words, for reactions like this [ΔH = negative, ΔS = positive] delta g is always negative, and therefore they’re always feasible, at any temperature.
On the other hand, suppose we now know that a reaction has a positive enthalpy change and a negative entropy change.
We can say that this term [TΔS] is always negative
And so, overall, we’ve got some positive number…subtract some negative number
Which, in any case, means we’ll end up with a positive number overall
In other words, for all reactions like this [ΔH = positive , ΔS = negative] delta g is always positive, and therefore they’re never feasible, at any temperature.
So, using this information, which of these reactions are always feasible?
This reaction has a positive enthalpy change and a negative entropy change, so it’s never feasible.
But this reaction has a negative enthalpy change and a positive entropy change, so it’s always feasible
Take a second to let that sink in.
We only needed two pieces of information to decide that this reaction has never and will never happen… at any temperature, anywhere in the universe
And not only that, sometimes we don’t even need to be told whether the entropy change is positive or negative, we can predict it ourselves!
For example, we can predict that the entropy change for this reaction will be…
Will be positive, because there are more moles of products than reactants.
And so, since the enthalpy change is negative, this reaction should always be feasible.
This time…we only needed one value, plus the reaction equation, to determine that this reaction will always be feasible, at any temperature, anywhere in the universe.
It’s absolutely mind boggling…
So, to sum up…
Any reaction with a negative enthalpy change and a positive entropy change will always be feasible.
But any reaction with a positive enthalpy change and a negative entropy change will never be feasible.
