CHRIS25 - 8-5-2014 at 03:23
Al2(SO4)3 + K2SO4 = 2KAl(SO4)2 (.12 water)
In solution: Al+3 + SO4-2 + K+1 + H+1 O-1 + H+1
I was trying to identify why this bond happened: Ruling out single replacement and Redox I was left with covalent and ionic bonding and the fact that
Al has a much more positive pull than K. Only two metals can form a non polar covalent bond so that is the metals sorted out I thought, but they are
not sharing anything as far as I can see? And an ionic bond can only be between non metal and metal so that explains the potassium and Aluminium
attached to a sulphate each. Then I wondered why this double compound is held together in the first place, evaporation leads to the solid state, in
other words a loss of water up to a certain point - so I looked at the water configuration: The oxygen on the sulphate ions has a negative 1 charge
and Hydrogen on the water molecule a positive 1 and there are 12 water molecules surrounding everything, yet I fail to grasp how these are bonding or
even why there should be 12.
I thought I had understood the basics of the bonding theories, until I looked at this one.
[Edited on 8-5-2014 by CHRIS25]
[Edited on 8-5-2014 by CHRIS25]
[Edited on 8-5-2014 by CHRIS25]
gdflp - 8-5-2014 at 03:55
The water molecules have nothing to do with the bonding, they are simply waters of hydration which arise because a complex forms between water and
this compound, similar to how a complex forms between ammonia and silver ions, but this one is stable in solution. Also, water doesn't noticeably
dissociate in solution and when it does, it is H+ and OH- not H+ and O-. Double salts don't really have to do with bonding at all they are still
ionic bonds, in solution there is not a difference. Double salts arise from a crystal lattice forming in such a way that formation of the double salt
is preferred.
blogfast25 - 8-5-2014 at 05:13
Alum is a double salt. There's no 'bonding' between the two salts, as such.
It just so happens that the three ionic species (Al3+, K+ and sulphate) and the crystal water can form a stable lattice, much like any other salt.
"The oxygen on the sulphate ions has a negative 1 charge and Hydrogen on the water molecule a positive 1".
No. The bonding between H and O on water is covalent. But O is more electronegative that H and tends to partly pull the bonding electrons a bit closer
to itself. As a result, the oxygen acquires a partial negative charge and hydrogen a partial positive charge. Because of water's bonding angles, the
water molecule is a so-called permanent bipole. And that explains why water is somewhat loosely 'bound' to ions a like Al3+ and how the water
ends up taking a place in the lattice.
CHRIS25 - 8-5-2014 at 05:31
Ok I am getting the impression that I have to dig yet another level downwards into to understand this - I have read about the shapes of molecules and
the lattice formation. It's still an ionic bond, thankyou. But if I have understood this correctly potassium and aluminium are not in any 'bond' per
se, they are just brought together by another kinetic, into this lattice arrangement? I will have to read more about these.
[Edited on 8-5-2014 by CHRIS25]
gdflp - 8-5-2014 at 07:56
Well aluminum and potassium are bonded, just not to each other. They both have ionic bonds to the sulfate ions.
CHRIS25 - 8-5-2014 at 08:25
Yes, that was the essential puzzle, I saw that they were bonded to the sulphate ions, but I was curious as to how the potassium sulphate and aluminium
sulphate stayed 'together' without any electron bonding. Now I understand this has to do with the lattice and the way crystals form, something I have
yet to learn.
blogfast25 - 8-5-2014 at 08:59
Perhaps this will help.
Remember the NaCl crystal lattice? That's essentially a three dimensional checkered flag: each sodium ion is surrounded by 6 chlorine ions, in the
same way each chlorine ion is surrounded by 6 sodium ions. Electrostatic attraction between the oppositely charged ions is what keeps it together, it
provides the bonding if you will.
Now suppose, hypothetically, that we replace half the sodium ions with potassium ions. Would that require any bonding between the sodium and potassium
ions? No.
The case of an alum crystal is geometrically far more complicated but the principle is the same.
CHRIS25 - 8-5-2014 at 09:06
Ok now I can visualize something, and yes that actually helps.