# Hard soft acid base theory, i'm not understanding it

1. Oct 28, 2011

### lil_angel_007

1. The problem statement, all variables and given/known data

the question asks to determine if the equilibrium is greater, equal or less than 1. one of the equation is the following:
[CuI4]2- (aq) + [CuCl4]3- (aq) ↔ [CuCl4]2-(aq) + [CuI4]3- (aq)

2. Relevant equations

3. The attempt at a solution

the solution given was:
the answer using the HSAB theory is:
HA-SB + SA-HB <-> HA-HB + SA-SB
(respectively) so equilibrium lies towards the products. I don't know how they labeled some of these elements:
i understand that in the first product Copper with an oxidation state of 2+ is a hard acid, Iodine is a soft base with oxidation state of 1-. In the second product, copper is a soft acid with an oxidation state of 1+ (am i right) and Cl is a soft base. Then in the products: i see copper is a hard acid with an oxidation state of 2+ but why is Cl a hard base??? in the second product copper is a soft acid (ox state of 1+) and I is a soft base..

basically my question is why is the chloride ion a hard base in the second product?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution