Question

The oxidation state of chromium in the final product formed in the reaction between $KI$ and acidified potassium dichromate solution is:
A.$ + 4$
B.$ + 6$
C.$ + 2$
D.$ + 3$

Answer 1

Hint: Oxidation is a number assigned to an element in a chemical combination which represents the number of electrons lost or gained, by an atom of that element in the compound. If the oxidation number is positive, then this means that the atom loses electrons, and if it is negative, it means the atom gains electrons. If it is zero, then the atom neither gains nor loses electrons.

Complete answer:
Potassium dichromate is an oxidizing agent, and is milder than potassium permanganate. It is used to oxidize alcohol. It converts primary alcohols into aldehydes and, under more forcing conditions, into carboxylic acids.
The reaction of potassium dichromate with sulphuric acid and potassium iodide gives chromium sulfate, potassium sulfate and water.
 ${K_2}C{r_2}{O_7} + 7{H_2}S{O_4} + 6KI \to C{r_2}{(S{O_4})_3} + 3{I_2} + 7{H_2}O + 4{K_2}S{O_4}$
 The reaction between $KI$and acidified dichromate solution is as follows:
 $
  6{e^ - } + C{r_2}{O_7}^{2 - } \to 2C{r^{3 + }} \\
  2{I^\theta } \to {I_2} + 2{e^ - } \\
$
 The oxidation state of a free element (uncombined element) is zero. For a simple (monoatomic) ion, the oxidation state is equal to the net charge on the ion.
 The oxidation number $Cr = + 3$.
So, the correct answer is (D) $ + 3$.

Note:
There is a slight difference between the oxidation state and oxidation number. Oxidation state refers to the degree of oxidation of an atom in a molecule. Oxidation numbers are used in coordination complex chemistry. They refer to the charge the central atom would have if all ligands and electron pairs shared with the atom were removed.