Question

Acidified potassium dichromate is treated with hydrogen sulphide. In the reaction, the oxidation number of chromium.

• A. Increases from + 3 to + 6
• B. Decreases from +6 to +3
• C. Remains unchanged
• D. Decreases from +6 to +2

Answer 1

Answer: (B)

Decreases from +6 to +3

Answer 2

Decreases from + 6 to + 3.

$K_{2}Cr_{2}O_{7} + 4H_{2}SO_{4} \rightarrow K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 4H_{2}O + 3O$

$\lbrack H_{2}S + \lbrack O\rbrack \rightarrow S + H_{2}O\rbrack \times 3$

––––––––––––––––––––––––––––––––––

$\overset{+ 6}{K_{2}Cr_{2}O_{7}} + 4H_{2}SO_{4} + 3H_{2}S \rightarrow$

$K_{2}SO_{4} + \overset{+ 3}{Cr}(SO_{4})_{3} + 7H_{2}O + 3S$