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1. Hey!!

Please can you help me with the following Qs:

Why does solubility increase down Group 2 for Hydroxides?

What is the trend for solubility of sulphates for Group 2 and why?

How do you calculate the charge of a compound you do not know?

What is charge density?

Thanks guys!!
2. Woah a lotta questions....urm i'll try my best.

1. I'm not sure why solubility increases down Group 2 but it could be linked with reactivity as that increases down the group.

2. Group 2 sulphates have a declining trend of solubility but again i'm not sure why but i don't think you would need to know why.

3. How to calculate a charge on an unkown compound is quite difficult to explain but it's down to electronegativy of the atoms being bonded. The best way i can really explain is it look at

NH4 that has a positive charge
ALCL4 this has a negative charge

Both nitrogen and Aluminium normally make 3 bonds but depending on what their bonding to determines their charge.

4. Charge density = charge / ionic radius
Again i'm not sure why you'd be asked to calculate this unless you were given the value of the two variables.
3. (Original post by wigman15)
Hey!!

Please can you help me with the following Qs:

Why does solubility increase down Group 2 for Hydroxides?

What is the trend for solubility of sulphates for Group 2 and why?

How do you calculate the charge of a compound you do not know?

What is charge density?

Thanks guys!!

Charge density is the ratio between the size of an ion and its charge - ie Mg2+ has a higher charge density than, say, Rb+, because Mg2+ has a bigger charge spread over a smaller area.

The solubility of the Group II hydroxides increases down the group because of the following... Solubility is a balance between lattice enthalpy and hydration enthalpy: DH sol = DH hyd - DH lat (D meaning delta since I can't work out how to use greek letters on here!) If DH lat is much greater in magnitude than DH hyd, the solid is insoluble - and vice versa. As you go down Group II, both DH lat and DH hyd decrease in magnitude, but at different rates. The hydroxides become more soluble because DH hyd decreases more slowly than DH lat.

For similar reasons, the sulphates become LESS soluble going down the group - this time DH lat decreases in magnitude more slowly than DH hyd.

Not sure what you mean about calculating the charge of a compound you don't know; if it's ionic and you add up the charges of the individual ions you get the charge of the overall ion...?
4. Thank you very much for all your explanations.

MadNatSci- Could you please tell me what you mean by lattice enthalpy and hydration enthalpy.

So..the smaller the ion, the higher the charge density therefore the more soluble it is?
5. (Original post by wigman15)
Thank you very much for all your explanations.

MadNatSci- Could you please tell me what you mean by lattice enthalpy and hydration enthalpy.

OK... you're not an A level student? They're to do with the amount of energy required for reactions. Lattice enthalpy is the energy released when one mole of an ionic compound is formed from its constituent ions. Enthalpy of hydration is the energy released when one mole of a GASEOUS substance becomes fully hydrated - ie surrounded by water molecules. It's just A level terminology which gives a way of explaining trends in properties of elements etc.

So..the smaller the ion, the higher the charge density therefore the more soluble it is?
Yes, sort of. If the ion is small and has a high charge it will be more soluble than a larger one with a similar charge, because the charge density will be higher so water molecules will be attracted to the ion more.

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