Is #"FeCl"_2# an acid?

1 Answer
Truong-Son N.
Dec 19, 2017

Well, in a way, yes.

#"Fe"^(2+)# has an electron configuration of #[Ar]3d^6 4s^0#. #"Fe"^(2+)# has empty #4s# and partially-empty #d# orbitals that accept electron pairs.

As soon as you dissolve it in water, #"FeCl"_2# imparts iron(II) cations into solution. Thus, by definition it is a Lewis acid as a dissociated cation.

#"FeCl"_2(aq) -> "Fe"^(2+)(aq) + 2"Cl"^(-)(aq)#

Suppose the solution also had cyanide ions. Then:

#"Fe"^(2+)(aq) + 6"CN"^(-)(aq) -> ["Fe"("CN")_6]^(4-)(aq)#

with an equilibrium formation constant of #K_f ~~ 1.5 xx 10^35#. In this case, #"Fe"^(2+)# is the Lewis acid for the #"CN"^(-)# Lewis base, and they form the Lewis acid-base adduct hexacyanoferrate(II).

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