Aqa a level chemistry - entropy

Hi,
Just completed our lesson on Entropy and I have to say I'm confused by one thing... when there are more moles of an item on the products side, we say entropy has a likely positive change right? But how about when there are multiple changes of state in the meanwhile? For example a solid and a gas become aq (pretty random but hopefully my confusion is evident). I'm uncomfortable with the current method for predicting entropy changes. Any advice?

Reply 1

lordaxil

If you are only confused by one thing about entropy, that is impressive! 🙂
But seriously, when considering this rule of thumb method (and it is no more than that) for calculating sign of entropy changes, then solids can essentially be ignored (their entropy is very low), liquids less important (still lowish entropy) and gases are by far the most important at standard temperatures and pressures (their entropy is very high).
So, to take your example, solid + gas goes to liquid would most likely be a negative entropy change.
2 moles of gas going to 1 mole of gas + solid/liquid would also likely be a negative entropy change.
However, 2 moles of gas plus 1 mole of solid/liquid going to 3 moles of gas would likely be a positive entropy change.
Basically, it's all about the number of moles of gas (unless you have some very special liquids or solids). If the number of moles of gas on each side is equal, then this rule of thumb will not be sufficient.