Question

Write the formulae of the Ferric Chloride and state the valency of the cation​

Answer 1

Answer:

Think of iron(III) with valency 3 having 3 hooks.

Then Cl with valency 1 with only one hook.

So we just swap the numbers round and get Fe1Cl3.

If we had Fe(III) with O (valency 2) we would get F22O3.

But there is a catch with FeCl3.

With three very greedy (electronegative) Cl atoms on it, the Fe is very positive in the resulting molecule, which each Cl is very negative, and each Cl has THREE lone pairs of electrons bulging out from its outer shell. So each FeCl3 molecule, which we would expect to be planar trigonal (triangular shape) cuddles up to another one, rather like Trump trying to make friends with Putin, and one Cl in each FeCl3 molecules pushes onto the Fe of the other molecule, making a dative covalent (or coordinate) bond, in which BOTH electrons came from the same atom, and so the molecules join strongly, or dimerise, to make one Fe2Cl6 molecule. Thus, FeCl3 is only the empirical formule (simplest ratio) but Fe2Cl6 is the molecular formula! AlCl3 does the same thing, and both are used as catalysts in Friedel-Crafts alkylation and acylation of aromatic compounds, because they readily do this: AlCl3 + R.C=O.Cl ==> AlCl4- + RC=O+ with the + on the C, and that +ve ion is what attacks the otherwise highly stable and unreactive aromatic target. Cool, huh?