Positive and Negative Entropy Changes

So far, we’ve talked lots about entropy, and how it increases and decreases. 

But we haven’t looked at how chemists discuss and represent these changes on paper.

So let’s look at that now. 

First up, chemists tend not to talk about entropy increasing or decreasing, but instead about positive or negative ‘entropy changes’

For example, when we melt ice into liquid water, instead of saying the “entropy increases”, chemists say “the entropy change is positive”

And if we refreeze that water, instead of saying the “entropy decreases”, chemists say “the entropy change is negative”

So now, using that language, how would we describe these processes?

For these processes the entropy change is positive

But for this process the entropy change is negative

Second, chemists also have a nice shorthand for representing entropy changes

For example, when we melt this ice into liquid water, we represent the positive entropy change like this

And when we refreeze that water we represent the negative entropy change like this

This ‘S’ means entropy, and this greek letter delta means ‘change in’.

So, now that we can represent how entropy changes during a reaction, we’re one step closer to figuring out whether that reaction can actually happen.

The next step is being able to predict how entropy changes during a reaction

And we’ll look at how to do that, next!

But first, to sum up…

Instead of describing processes by saying entropy increases or decreases, chemists instead say…

Chemists instead describe processes by saying they have a positive or negative entropy change

And they represent those entropy changes like this [ΔS = positive, ΔS = negative]