Question

On reaction with silver nitrate solution, nitrite ion gives:
(A) a white coloured precipitate of silver nitrate
(B) a black precipitate of silver nitrite
(C) a black coloured solution of silver nitrite
(D) none of the above

Answer 1

The reaction between silver nitrate and nitrite ion is an example of a displacement reaction. Displacement reaction is a type of reaction in which an ion in a compound is displaced by an element (single displacement reaction) or an ion belonging to another compound (double displacement reaction).

Complete step by step solution:
In order to solve this question we first need to understand the meaning of displacement reactions. Displacement reaction is a type of reaction in which an ion in a compound is displaced by an element (single displacement reaction) or an ion belonging to another compound (double displacement reaction).
Below is an example of a single displacement reaction:
$A-B\quad +\quad C\quad \longrightarrow \quad C-B\quad +\quad A$
Where A and C are different metal species and B is an anion or
A and C are different halogen species and B is a cation.
Below is an example of double displacement reaction:
$A-B\quad +\quad C-D\quad \longrightarrow \quad A-D\quad +\quad C-B\quad$
Where A and C are cations while B and D are anions.
In the question, the reaction between silver nitrate and nitrite ion is an example of double displacement reaction. In double displacement reactions, an exchange of the anions belonging to different compounds takes place. So, basically, the reactants are two different compounds and the products are also two compounds that are different from the reactants. These reactions are also called double replacement reactions. Sodium nitrite is a compound which is soluble in water. When the aqueous solution of sodium nitrite is reacted with the aqueous solution of silver nitrate, then a double displacement reaction takes place such that we get an aqueous solution of sodium nitrate and a white precipitate of silver nitrate. The reaction is given below:
${ NaNO }_{ 2 }(aq)+{ AgNO }_{ 3 }(s)\rightarrow { NaNO }_{ 3 }(aq)+{ AgNO }_{ 2 }(s)\downarrow (White\quad ppt.)$
The solubility product of silver nitrite is $p{ K }_{ sp }=3.22$ which is very low and therefore it precipitates in water even at very low concentrations.

Hence the correct answer is (D) None of the above.

Note: Single displacement reactions are basically oxidation-reduction reactions wherein one species is oxidised and other is reduced. Not all chemical species can oxidise or reduce each other. Whether it will be oxidised or reduced by another species depends upon their redox potential which is a measure of the tendency of a species to accept or donate electrons to an electrode (generally the Standard hydrogen electrode) and thus be reduced or oxidised respectively.