Test Index

NCERT Class XII Chemistry
Chapter - The d- and f-Block Elements
Questions with Solutions

© examsnet.com
Question : 15
Total: 49
Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with
(i) iodide
(ii) iron(II) solution and
(iii) H2S.
Solution:  
K2Cr2O7 is a powerful oxidising agent. In dilute sulphuric acidmedium the oxidation state of Cr changes from +6 to +3. The oxidisingaction can be represented as follows:
Cr2O72–+14H++6e–→2Cr3++7H2O
(i) Iodide : Iodide ion (I–) is oxidised to I2 by the acidfied solution ofK2Cr2O7.
Reaction :
Cr2O72–+14H++6e–→2Cr3++7H2O
‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌6I–→3I2+6e–
–––––––––––––––––––––––––––––––––––––––––
Cr2O72–+14H++6I–→3I2+2Cr3++7H2O
–––––––––––––––––––––––––––––––––––––––––
(ii) Iron(II) solution : Ferrous salts (Fe2+) are oxidised to ferric (Fe3+)saltswhen they are treated with acidified K2Cr2O7.
Reaction :
Cr2O72–+14H++6e–→2Cr3++7H2O
‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌6Fe2+→6Fe3++6e–
–––––––––––––––––––––––––––––––––––––––––––––
Cr2O72–+6Fe2++14H+→2Cr3++6Fe3++7H2O
–––––––––––––––––––––––––––––––––––––––––––––
(iii) H2S : H2S is oxidised to sulphur.
Cr2O72–+3H2S+8H+→2Cr3++7H2O+3S
© examsnet.com
Go to Question:
Prev Question
Next Question