Year 12s: what will be the first way you research your future options? >>

Chemistry solubility question

Hey guys can anyone help? I cant figure out why magnesium sulphate is not soluble in water??

Magnesium sulfate is most certainly soluble in water! You are probably thinking of barium sulfate.

As to why some salts are soluble and others aren't, its due to the difference between lattice (free) energy and hydration energy. If the former is greater then the salt is insoluble.

Reply 2

Charliemosman

Ok, so this is my cycle but I don't understand why specifically these numbers make it soluble...

Reply 3

Sanjay.46

Original post by Charliemosman

Ok, so this is my cycle but I don't understand why specifically these numbers make it soluble...

if that ∆Hsol is -ve then it is soluble...
in this case ∆Hsol is +ve it means that we have to provide energy to make it soluble. hence it is not soluble

Reply 4

David Getling

Original post by Sanjay.46

if that ∆Hsol is -ve then it is soluble...
in this case ∆Hsol is +ve it means that we have to provide energy to make it soluble. hence it is not soluble

WRONG! Did you not read my post before jumping in, and see the "free" in brackets? It's ∆G. There are plenty of cases where ∆H is positive, but the salt still dissolves.

Reply 5

David Getling

Original post by Charliemosman

Ok, so this is my cycle but I don't understand why specifically these numbers make it soluble...

These numbers don't make it soluble. What you have done is calculate the enthalpy, but there is also an increase in entropy. When you put these together you find that the change in the Gibbs Free Energy is negative, and this is why magnesium sulfate is soluble in water.

Reply 6

MexicanKeith

Original post by Charliemosman

Ok, so this is my cycle but I don't understand why specifically these numbers make it soluble...

Your original question specifies Magnesium Sulfate, which is indeed soluble in water as other posters have pointed out.

This cycle you've draw is for Magnesium Sulfide which would react with water to form Hydrogen Sulfide and Magnesium Hydroxide rather than simply dissolving.

Be sure which of these compounds you actually mean.

If you mean the sulfate then the answers given by David are what you're looking for, but your cycle is no use to you.

If you mean sulfide then the answer is that it doesn't simply dissolve because sulfide ions in aqeous solution are unstable and will react with water molecules to form hydrogen sulfide and hydroxide ions.

Reply 7

Charliemosman

Hiya, sorry I do mean magnesium sulphide

Reply 8

Sanjay.46

Original post by David Getling

ok i agree, but in most of cases ∆H helps us out and in this case ∆H was soluble so i jumped and replied that ...and yes thanks for rectifying me bro!🤙