Question

Amphoteric oxides of Manganese and Chromium are:
A) $MnO$ and $CrO$
B) $M{n_2}{O_3}$ and $C{r_2}{O_3}$
C) $Mn{O_2}$ and $C{r_2}{O_3}$
D) $M{n_2}{O_7}$ and $Cr{O_3}$

Answer 1

Hint: The word amphoteric was derived from the greek word ‘amphoterio’ which means ‘both’.Amphoteric oxides can either be a base or an acid to form water and salt respectively. Amphoterism mainly depends upon the oxidation states of the chemical species.

Complete step by step answer:
Amphoteric oxides are the one that can act as an acid as well as base.
Metallic oxides are generally basic whereas nonmetallic oxides are acidic in nature.
There are some metallic oxides that show both acidic and basic nature.
The elements which form amphoteric oxides have characteristics of both metals and nonmetals.
Amphoteric oxides react with acids and base to give water and salt respectively.
Salt and water are the main products of these oxides.
Amphoterism mainly depends on the oxidation state of an oxide.
There are various metals that form amphoteric oxides namely: manganese, chromium, titanium, aluminium, zinc, lead and tin.
Some examples of the metals that form amphoteric oxides with acids and bases are as follows: a) Zinc oxide b) Aluminium oxide
We will see the reaction of the metals that form amphoteric oxides given below:

1.Zinc oxide
With acid: $ZnO + 2{H^ + } \to Z{n^{2 + }} + {H_2}O$
With base: $ZnO + {H_2}O + 2O{H^ - } \to {\left[ {Zn{{\left( {OH} \right)}_4}} \right]^{2 - }}$

2.Aluminium oxide
With acid: $A{l_2}{O_3} + 3{H_2}O + 6{H_3}{O^ + }_{\left( {aq} \right)} \to 2{\left[ {Al{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}_{\left( {aq} \right)}$
With base: $A{l_2}{O_3} + 3{H_2}O + 2O{H^ - }_{\left( {aq} \right)} \to 2{\left[ {Al{{\left( {OH} \right)}_4}} \right]^ - }_{\left( {aq} \right)}$
With base:

3.Manganese oxide:
With acid: $Mn{O_2} + 4HCl \to MnC{l_2} + C{l_2} + {H_2}O$
with base: $2Mn{O_2} + 4KOH + {O_2} \to 2{K_2}Mn{O_4} + 2{H_2}O$

4.Chromium oxide
With acid: $6C{r_2}{O_3} + 24{H^ + } \to {\left[ {12Cr{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$
With base: $C{r_2}{O_3} + 2NaOH \to 2NaCr{O_2} + {H_2}O$

Now, we will see the characteristic of manganese and chromium as follows:
1.Manganese: the only oxidation state of manganese that is amphoteric is $ + 4$ .
So the amphoteric oxide of manganese with $ + 4$ oxidation state is $Mn{O_2}$ .
2.Chromium: the only oxidation state of chromium that is amphoteric is $ + 3$ .
So the amphoteric oxide of chromium with $ + 3$ oxidation state is $C{r_2}{O_3}$ .

So the correct answer for the above question is option c) $Mn{O_2}$ and $C{r_2}{O_3}$ .

Note: The higher oxidation state of manganese is $ + 7$ which is highly acidic whereas the lowest oxidation state of manganese is $ + 2$ which is basic. Though chromium with oxidation state $ + 3$ is insoluble in water, it still dissolves well in acid.