Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

An example of a reversible reaction is (a) \(\mathrm{KNO}_{3}(\mathrm{aq})+\mathrm{NaCl}(\mathrm{aq})=\mathrm{KCl}(\mathrm{aq})+\mathrm{NaNO}_{3}(\mathrm{aq})\) (b) \(2 \mathrm{Na}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})=2 \mathrm{NaOH}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g})\) (c) \(\mathrm{AgNO}_{3}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq})=\mathrm{AgCl}(\mathrm{s})+\mathrm{NaNO}_{3}\) (aq) (d) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{NaI}(\mathrm{aq})=\mathrm{PbI}_{2}(\mathrm{~s})+2 \mathrm{NaNO}_{3}\) (aq)

Step-by-Step Solution

Answer

Option (a) is an example of a reversible reaction.

Step 1: Define Reversible Reaction

A reversible reaction is one where the reactants and products are in equilibrium, meaning the reaction can proceed in both the forward and reverse directions under certain conditions.

Step 2: Analyze Each Reaction Option

(a) Involves ions in solution and can shift back and forth, potentially reversible. (b) Produces gas and is a vigorous reaction, not easily reversible. (c) Forms a solid precipitate, limiting reversibility. (d) Also forms a precipitate, limiting reversibility.

Step 3: Identify Reaction with Equilibrium

Reversible reactions often involve ionic solutions that can re-dissolve, like in option (a) where no gas or solid is formed permanently, unlike the other options which involve a solid or gas.

Step 4: Confirm Reversibility

In reaction (a), the reactants and products are both fully aqueous, supporting the potential for the reaction to reverse under appropriate conditions, fitting the characteristic of a reversible reaction.

Key Concepts

Chemical Equilibrium

Chemical equilibrium occurs in a reversible reaction when the rate at which reactants turn into products equals the rate at which products revert to reactants. At this point, the concentrations of reactants and products remain constant, although the molecular activities continue. This dynamic state of balance is crucial, especially in understanding how reactions can proceed both forwards and backwards.
In a reversible reaction, like that described in option (a) in the exercise, reactants and products are in an aqueous state, allowing them to reach equilibrium under specific conditions. This means that the ions continue to interact without resulting in a permanent state change, such as forming a gas or a solid. Instead, they can easily transfer back into their original forms, contributing to the balance that defines chemical equilibrium.
This exchange process is highly dependent on factors such as temperature, pressure, and concentration of substances in the reaction mixture. When equilibrium is disturbed by changing these factors, the system will shift to minimize that change, a principle known as Le Chatelier's Principle. This adaptability ensures that reversible reactions like option (a) can exist in equilibrium.

Ionic Reactions

Ionic reactions occur when ions in an aqueous solution interact, often resulting in the formation of new compounds. These are significant because they often involve the exchange of ions to form weak electrolytes, precipitates, or gases.
Ionic reactions can be reversible, as seen in the interaction in option (a). The ions in a solution—such as \( ext{K}^+\), \( ext{Na}^+\), \( ext{Cl}^-\), and \( ext{NO}_3^-\)—charge around based upon their interactions with water molecules and other ions present. When no gases or irreversible precipitates are formed, as in this option, the reaction is more likely to be reversible.
In contrast, options (c) and (d) in the exercise illustrate typical irreversible ionic reactions because they form precipitates, such as \( ext{AgCl}\) and \( ext{PbI}_2\), respectively. Once these solids form, the reactions significantly slow in the reverse direction, as the precipitate tends to settle out of the solution and not dissolve back. This attribute is what distinguishes reversible ionic reactions from those that are not.

Aqueous Solutions

Aqueous solutions play a vital role in reversible reactions, particularly because they provide the ideal medium for ions to move freely and react. The solutes in these solutions are usually ions or molecules that are dissolved in water, making up an essential part of chemistry involving reversible and equilibrium reactions.
In an aqueous solution, water acts as the solvent, and numerous solutes can dissolve within it to form homogenous mixtures. This is key in reversible reactions, like option (a), where the reactants and products remain soluble in the solution without forming solid or gaseous states permanently. The aqueous nature of the solution allows ions to dissociate and recombine, enabling reversibility.
The flexibility of ions in aqueous solutions is what facilitates the establishment of equilibrium in reversible reactions. Recognizing when a reaction is in an aqueous solution can help in predicting its behavior, such as solubility and reactivity, influencing how reversible it may be and whether equilibrium can be achieved.