question_answer

Acidified potassium dichromate is treated with hydrogen sulphide. In the reaction, the oxidation number of chromium [MP PET 1996]

A) Increases from + 3 to + 6

B) Decreases from +6 to +3

C) Remains unchanged

D) Decreases from +6 to +2

Correct Answer: B

Solution :

Decreases from + 6 to + 3. \[{{K}_{2}}C{{r}_{2}}{{O}_{7}}+4{{H}_{2}}S{{O}_{4}}\,\to {{K}_{2}}S{{O}_{4}}+C{{r}_{2}}{{(S{{O}_{4}})}_{3}}+4{{H}_{2}}O+3O\] \[[{{H}_{2}}S+[O]\,\to \,S+{{H}_{2}}O]\times 3\] \[\overset{+6}{\mathop{{{K}_{2}}C{{r}_{2}}{{O}_{7}}}}\,+4{{H}_{2}}S{{O}_{4}}+3{{H}_{2}}S\,\to \]\[{{K}_{2}}S{{O}_{4}}+\overset{+3}{\mathop{Cr}}\,{{(S{{O}_{4}})}_{3}}+7{{H}_{2}}O+3S\]