Question

An example of reversible reaction is:
A.$Pb{\left( {N{O_3}} \right)_2}\left( {aq} \right) + 2NaI\left( {aq} \right) \to Pb{I_2}\left( s \right) + 2NaN{O_3}\left( {aq} \right)$
B.$AgN{O_3}\left( {aq} \right) + HCl\left( {aq} \right) \to AgCl\left( s \right) + HN{O_3}\left( {aq} \right)$
C.$2Na\left( s \right) + {H_2}O\left( l \right) \to 2NaOH\left( {aq} \right) + {H_2}\left( g \right)$
D.$KN{O_3}\left( {aq} \right) + NaCl\left( {aq} \right) \to KCl\left( {aq} \right) + NaN{O_3}\left( {aq} \right)$

Answer 1

We could say chemical changes are of two kinds. They are reversible and irreversible changes. In reversible change, there is an alteration in physical properties, shape, and size of the material. Folding of paper, spring elongation are some examples of reversible changes. When a portion of wood is burned to ash, it cannot be made to wood again. This is an example of irreversible change.

Complete step by step solution:
We must remember that a reversible reaction can occur in either direction from reactants to products or form products to reactants. The forward reaction happens from left to right as drawn. The reverse reaction happens from right to left as drawn.
Example:
$C{O_{\left( g \right)}} + {H_2}{O_{\left( l \right)}} \rightleftarrows C{O_2}_{\left( g \right)} + {H_{2\left( g \right)}}$
The forward reaction is,
$C{O_{\left( g \right)}} + {H_2}{O_{\left( l \right)}}\xrightarrow{{}}C{O_2}_{\left( g \right)} + {H_{2\left( g \right)}}$
The reverse reaction is,
$C{O_2}_{\left( g \right)} + {H_{2\left( g \right)}}\xrightarrow{{}}C{O_{\left( g \right)}} + {H_2}{O_{\left( l \right)}}$
The given reaction (A) is,
$Pb{\left( {N{O_3}} \right)_2}\left( {aq} \right) + 2NaI\left( {aq} \right) \to Pb{I_2}\left( s \right) + 2NaN{O_3}\left( {aq} \right)$
In this reaction, lead nitrate and sodium iodide reacts together to form sodium nitrate and lead iodide as precipitate. Due to precipitation reaction lead iodide does not react with sodium nitrate. So, the reaction is irreversible.
Therefore, the option (A) is incorrect.
The given reaction (B) is,
$AgN{O_3}\left( {aq} \right) + HCl\left( {aq} \right) \to AgCl\left( s \right) + HN{O_3}\left( {aq} \right)$
In this reaction, silver nitrate and nitric acid reacts together to form nitric acid and silver chloride as products. Due to the stable nature of silver chloride nitric acid and silver chloride does not form reactants. So, the reaction is irreversible. Option (B) is incorrect.
The given reaction (C) is,
$2Na\left( s \right) + {H_2}O\left( l \right) \to 2NaOH\left( {aq} \right) + {H_2}\left( g \right)$
In this reaction sodium and water react to form sodium hydroxide with liberation of hydrogen. The released hydrogen gas is not present in the reacting system anymore and does not go in the reverse direction. So, the reaction is irreversible. Option (C) is incorrect.
The given reaction (D) is,
$KN{O_3}\left( {aq} \right) + NaCl\left( {aq} \right) \to KCl\left( {aq} \right) + NaN{O_3}\left( {aq} \right)$
In this reaction, potassium nitrate and sodium chloride reacts together to form sodium nitrate and potassium chloride as products. Both sodium and potassium show close reactivity, so they replace each other both the reactant side and the product side. So, the reaction is reversible.
Therefore, the option (D) is correct.
The reversible reaction is written as,
$KN{O_3}\left( {aq} \right) + NaCl\left( {aq} \right) \rightleftarrows KCl\left( {aq} \right) + NaN{O_3}\left( {aq} \right)$

Note:
We have to know that irreversible reactions occur only in forward direction. Irreversible reactions could proceed to completion. Some of the irreversible reactions are neutralization reactions, double decomposition reactions (or) precipitation reactions, thermal decompositions, and redox reactions.