Consider the following three compounds which contain complex ions that involve iron I [Fe(NH₃)₆]Cl₃ II K₃[FeCl₆] III K₃[FeCl₄] The oxidation state of the iron in each of these compounds is A - VCE - SSCE Chemistry - Question 19 - 2005 - Paper 1

In K₃[FeCl₆], the compound overall must remain neutral, and potassium (K) has a +1 charge. With three potassium ions, this gives +3 total. The six chloride ions (Cl-) bring a total charge of -6. Therefore, the iron must have an oxidation state of:

total oxidation state: x + 6(-1) + 3(+1) = 0
⇒ x - 6 + 3 = 0
⇒ x = +3

Thus, the oxidation state of iron in II is +3.

In K₃[FeCl₄], similarly to the previous compound, the three potassium ions give a +3 charge, while the four chloride ions contribute -4 total:

total oxidation state: x + 4(-1) + 3(+1) = 0
⇒ x - 4 + 3 = 0
⇒ x = +1

Thus, the oxidation state of iron in III is +2.