Chromium and its reactions

In a nutshell

Chromium ions usually exist in a $$+2, \ +3$$​ and $$+6$$​ oxidation state. Chromium ions in the $$+3$$​ state are the most stable. Chromium ions can be oxidised and reduced. Chromium hydroxide precipitate is amphoteric.

Oxidation numbers

Chromium ions can exist in the $$+2, \ +3$$​ and $$+6$$​ oxidation state. The most stable chromium ion is in the $$+3$$​ state.  

Chromium and oxygen can form chromate (VI) ions $$(CrO_4^{2-})$$, and dichromate (VI) ions $$(Cr_2O_7^{2-})$$​. Chromium is in the $$+6$$​ state.

Chromate (IV) ions and dichromate (IV) ions are good oxidising agents and can easily be reduced to $$Cr^{3+}$$ ions.

Six water ligands can surround a  Cr^{3+}Cr3+ ion to form an aqueous complex; these complexes appear violet. Impurities in water can substitute the the water ligands in the complex ion and turn the solution green.

Example 

$$Cl^-$$ ions in water can substitute the water ligands coordinating with chromium ions. 

Oxidation and reduction  

Chromium can take part in various redox reactions.

Dichromate (VI) ions can be reduced to $$Cr^{3+}$$ions with zinc and dilute acids as reducing agents. 

​$$Cr_2O_7 ^{2-}{(aq)} \space + \space 14H ^+{(aq)} \space + \space 3Zn(s) \rightarrow 3Zn^{2+}{(aq)} \space + \space 2Cr ^{3+}{(aq)} \space +\space 7H_2O{(l)} \space \space \newline E^{\circ}=+2.09 \, V$$

​$$Cr ^{3+}$$ can be reduced further to $$Cr ^{2+}$$ using zinc. This must be carried out in an inert atmosphere as $$Cr ^{2+}$$ is very unstable and in air it will oxidise back to $$Cr ^{3+}$$.

​$$2Cr ^{3+}{(aq)} \space + \space Zn{(s)} \rightarrow Zn^{2+}{(aq)}\space + \space 2Cr ^{2+}{(aq)} \space \space \newline E^{\circ}=+0.35 \, V$$

​​$$Cr ^{3+}$$ can be oxidised to chromate (VI) ions using hydrogen peroxide in alkaline solution. 

​$$2Cr ^{3+}{(aq)} \space + \space 10OH^-{(aq)} \space + \space 3H_2O_2{(aq)}\rightarrow 2CrO_4 ^{2-}{(aq)} \space +\space 8H_2O{(l)} \\ E^{\circ}=+1.08 \, V$$​​

Acid can be added to the chromate (VI) ions to form dichromate (VI) ions. This reaction is reversible and equilibrium lies in the middle. Equilibrium will shift to the right if an acid is added, and to the left if an alkali is added.

​$$2CrO_4 ^{2-}{(aq)} \space + \space 2H ^{+}{(aq)} \rightleftharpoons Cr_2O_7 ^{2-}{(aq)} \space + \space H_2O{(l)}$$

Acid and base reactions 

An aqueous solution of chromium (III) ions can react with sodium hydroxide $$(NaOH)$$ or aqueous ammonia $$(NH_3)$$​ to form a chromium hydroxide precipitate $$(Cr(OH)_3(H_2O)_3)$$. The starting solution is green and the precipitate is grey-green. 

​$$[Cr(H_2O)_6]^{3+}{(aq)} \space + \space 3OH^-{(aq)} \rightarrow [Cr(OH)_3(H_2O)_3] {(s)} \space + \space 3H_2O {(l)}$$

$$[Cr(H_2O)_6]^{3+}{(aq)} \space + \space 3NH_3{(aq)} \rightarrow [Cr(OH)_3(H_2O)_3] {(s)} \space + \space 3NH_4 ^+ {(aq)}$$

​When an excess of sodium hydroxide is added to chromium hydroxide precipitate, the water ligands deprotonate and $$[Cr(OH)_6]^{3-}$$ ions form. A dark green solution forms. 

$$[Cr(OH)_3(H_2O)_3] {(s)} \space + \space 3OH^- {(aq)} \, \rightarrow [Cr(OH)_6]^{3-} {(aq)} + \, 3H_2O {(l)}$$​​

​

Adding acid to chromium hydroxide will protonate the $$OH^-$$ ligands and result in $$[Cr(H_2O)_6]^{3+}$$ ions forming. A green solution forms. 

​$$[Cr(OH)_3(H_2O)_3] {(s)} \space + \space 3H^+{(aq)} \, \rightarrow [Cr(H_2O)_6]^{3+}{(aq)}$$​​

These are acid-base reactions and not ligand exchanges; $$H^+$$ ions are being added or removed.  

Ligand exchange reaction    

Adding an excess of ammonia to chromium hydroxide precipitate results in a ligand exchange reaction. A purple solution forms containing [$$Cr(NH_3)_6]^{3+}$$ ions.

​$$[Cr(OH)_3(H_2O)_3] {(s)} \space + \space 6NH_3 {(aq)} \, \rightarrow [Cr(NH_3)_6]^{3+} {(aq)} \space + \space \, 3OH^- {(aq)} \space + \space 3H_2O {(l)}$$​​