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Chemistry - solubility and enthalpy of hydration

I was looking over the solubility of group 2 hydroxides and saw that they become more soluble down the group and so more alkaline in solution. But now having done A2 chemistry, this seems suprising as I would've expected the enthalpy of hydration to become less exothermic down the group as atomic radius increases whilst charge remains the same, and so more soluble in aqueous solution. Can anyone help with where I'm going wrong here?
Original post by cata03
I was looking over the solubility of group 2 hydroxides and saw that they become more soluble down the group and so more alkaline in solution. But now having done A2 chemistry, this seems surprising as I would've expected the enthalpy of hydration to become less exothermic down the group as atomic radius increases whilst charge remains the same, and so more soluble in aqueous solution. Can anyone help with where I'm going wrong here?

You are correct, but solubility is a combination of several factors.
1. The lattice enthalpy
2. The hydration enthalpy
3. The entropy change

The first factor is endothermic and the second exothermic. Both are also affected by changing the ionic radius.
The endothermic lattice enthalpy gets smaller as the ion increases in size, but the exothermic hydration enthalpy also gets smaller.
It is the difference between these two that give the trend (if we assume that entropy change remains more or less the same)

So, if the lattice enthalpy decreases MORE than the hydration enthalpy, then overall dissolution is more favourable.
Reply 2
Original post by charco
You are correct, but solubility is a combination of several factors.
1. The lattice enthalpy
2. The hydration enthalpy
3. The entropy change

The first factor is endothermic and the second exothermic. Both are also affected by changing the ionic radius.
The endothermic lattice enthalpy gets smaller as the ion increases in size, but the exothermic hydration enthalpy also gets smaller.
It is the difference between these two that give the trend (if we assume that entropy change remains more or less the same)

So, if the lattice enthalpy decreases MORE than the hydration enthalpy, then overall dissolution is more favourable.

Thank you so much, that’s so helpful!!
Reply 3
Original post by charco
You are correct, but solubility is a combination of several factors.
1. The lattice enthalpy
2. The hydration enthalpy
3. The entropy change

The first factor is endothermic and the second exothermic. Both are also affected by changing the ionic radius.
The endothermic lattice enthalpy gets smaller as the ion increases in size, but the exothermic hydration enthalpy also gets smaller.
It is the difference between these two that give the trend (if we assume that entropy change remains more or less the same)

So, if the lattice enthalpy decreases MORE than the hydration enthalpy, then overall dissolution is more favourable.
Is the change of Enthalten of lattice negative?
Original post by Rui Jin
Is the change of Enthalten of lattice negative?
I assume you mean enthalpy?

There are two definitions of lattice enthalpy with opposite sense, so they must be described correctly.
Lattice enthalpy of dissociation is positive
Lattice enthalpy of formation is negative.
Reply 5
Original post by charco
You are correct, but solubility is a combination of several factors.
1. The lattice enthalpy
2. The hydration enthalpy
3. The entropy change

The first factor is endothermic and the second exothermic. Both are also affected by changing the ionic radius.
The endothermic lattice enthalpy gets smaller as the ion increases in size, but the exothermic hydration enthalpy also gets smaller.
It is the difference between these two that give the trend (if we assume that entropy change remains more or less the same)

So, if the lattice enthalpy decreases MORE than the hydration enthalpy, then overall dissolution is more favourable.
Are you sure that the Enthalten change of lattice energy is positive
Original post by Rui Jin
Are you sure that the Enthalten change of lattice energy is positive
Read my previous post, the sign depends on the definition.
Reply 7
Original post by charco
Read my previous post, the sign depends on the definition.
wow,ok, I suddenly realize what you mean. I was a bit mixed up.
Thank you for replying me!

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