Question

During oxidation of ferrous sulphate using mixture of dil. \[{{\text{H}}_2}{\text{S}}{{\text{O}}_4}\]and potassium dichromate; oxidation state of chromium changes from _________.
A.\[ + 6{\text{ to }} + 2\]
B.\[ + 6{\text{ to }} + 3\]
C.\[ + 6{\text{ to }} + 1\]
D.\[ + 6{\text{ to }} + 4\]

Answer 1

Hint: The reaction of potassium chromate with ferrous sulphate yields chromium sulphate. The oxidation state of sulphate ion is \[ - 2\] and hence the oxidation state of chromium can be calculated using the formula.

Complete step by step answer:
The reaction of ferrous sulphate with potassium dichromate in the presence of the dilute sulphuric acid, chromium sulphate, iron sulphate, potassium sulphate and water forms. The balanced chemical equation can be written as:
\[6{\text{FeS}}{{\text{O}}_4} + {{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7} + 7{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to 3{\text{F}}{{\text{e}}_2}{\left( {{\text{S}}{{\text{O}}_4}} \right)_2} + {{\text{K}}_2}{\text{S}}{{\text{O}}_4} + {\text{C}}{{\text{r}}_2}{\left( {{\text{S}}{{\text{O}}_4}} \right)_3} + 7{{\text{H}}_2}{\text{O}}\]
We know the oxidation state of sulphate ion is considered as \[ - 2\], let us consider the oxidation state of chromium is x. Chromium sulphate is a neutral molecule hence addition of the oxidation state of each of the elements by multiplying it with the number of respective atoms will be equal to zero.
The oxidation state of chromium in \[{\text{C}}{{\text{r}}_2}{\left( {{\text{S}}{{\text{O}}_4}} \right)_3}\] can be calculated as:
\[2 \times {\text{x}} + 3 \times - 2 = 0\]
Keeping x on left hand side we will get,
\[{\text{x}} = \dfrac{6}{2} = 3\]
Hence the oxidation state changes to 3.
The oxidation of potassium is considered as \[ + 1\] and that of oxygen as \[ - 2\]
The oxidation state of potassium dichromate in \[{{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}\] can be calculated as:
\[2 \times 1 + 2 \times {\text{x}} + 7 \times - 2 = 0\]
Keeping x on left hand side we will get,
\[{\text{x}} = \dfrac{{12}}{2} = 6\]
Hence the oxidation state is 6.
The oxidation state changes from \[ + 6{\text{ to }} + 3\].

The correct option is B.

Note:
Potassium dichromate is an inorganic compound. It is an oxidizing agent and is red- orange in colour. It is used in cleaning glassware and in construction. Ferrous sulphate is a green colour salt with an oxidation state of iron equal to 2. Sulphuric acid is a mineral acid. In the above reaction the chromium is being reduced because of decrease in oxidation state.