Solubility of Group 2 Sulphates and Group 2 Hydroxides (Edexcel, Paper 4 - Energetics

I cannot for the life of me understand this topic.

I'm pretty bad at most topics to do with enthalpy changes because I find it very hard to put together all of the facts andpicture the process in my mind, i.e. sulphates, hydroxides, charge, size, lattice energy, hydration enthalpy, endothermic, exothermic, solubility etc...

Can someone PLEASE help me and give me a step by step guide as to the theory behind this topic. You would truly be saving my arse.

Thanks

Rob

Reply 1

Zygroth

I might be making all this up, but it makes sense at least :p:

Using barium sulphate as an example, both ions are big, so the lattice is more ordered, the distance between ions is small, so strong forces and higher lattice energy. To dissolve it, the lattice has to be broken up which requires more energy than the energy released when the ions form weak bonds with water. Similarly, the hydroxide ion is small, like magnesium, so magnesium hydroxide is insoluble.

When the sizes of ions are different, the lattices aren't as ordered so the distance between ions isn't as small = weaker forces and lower lattice enthalpy. This is less than the energy released when the ions form bonds with water, so these dissolve.