Question

Ammonium dichromate on heating gives:
(A) Chromic oxide and nitrogen
(B) Chromic acid and ammonia
(C) Chromic acid and nitrogen
(D) Chromic oxide and ammonia

Answer 1

Hint: Transition metal and their oxide are coloured due to the presence of partially filled d-orbitals. The unpaired electron present in the partially filled d-orbital to the higher energy d- orbital by absorbing energy from visible light.
Potassium dichromate, on heating gives oxygen $\text{(}{{\text{O}}_{\text{2}}}\text{)}$ and chromic oxide $\text{(C}{{\text{r}}_{2}}{{\text{O}}_{3}}\text{)}$, which is represented by following reaction. $\text{2}{{\text{K}}_{\text{2}}}\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}\xrightarrow{\Delta }\,\text{4}{{\text{K}}_{\text{2}}}\text{Cr}{{\text{O}}_{\text{4}}}\,\text{+}\,\text{3}{{\text{O}}_{\text{2}}}\,\text{+}\,\text{2C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$

Complete step by step solution:
Dichromate is an inorganic divalent anion which is obtained by the deprotonation of chromic acid. Ammonium dichromate is the ammonium salt of dichromic acid. Ammonium dichromate on heating gives green colour powder of $\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ (chromic oxide). Chromic oxide are coloured because in this oxide chromium ions have three unpaired electrons and appear green coloured due to d-d transition. In this reaction nitrogen gas release instate of oxygen $\text{(}{{\text{O}}_{\text{2}}}\text{)}$ gas.
When chromate salt of ammonium is present is small amount and heated at very high temperature is produces ${{\text{N}}_{\text{2}}}$ (nitrogen gas) along with the green colour $\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ (chromic oxide) and water vapour. This reaction is represented by following manner – ${{\text{(N}{{\text{H}}_{4}})}_{\text{2}}}\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}\xrightarrow{\Delta }\,\,\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{+}\,{{\text{N}}_{\text{2}}}\,+\,4\,{{\text{H}}_{\text{2}}}\text{O}$

So, the option (A) will be the correct answer.

Note: The decomposition of ammonium dichromate is a highly exothermic process. This reaction is not similar to the decomposition of other metal dichromate salt. This reaction produces a green spark of chromium (III) oxide, and also produces steam and nitrogen gas. While other metal dichromate salts give oxygen gas with oxide.
These salts are mainly used in fireworks.