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

Question

An example of a reversible reaction is (a) \(\mathrm{KNO}_{3}\) (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) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{NaI}(\mathrm{aq})=\mathrm{PbI}_{2}(\mathrm{~s})+2 \mathrm{NaNO}_{3}(\mathrm{aq})\)

Step-by-Step Solution

Answer

Reaction (a) is reversible.

Step 1: Understanding Reversible Reactions

In a reversible reaction, the chemical reaction can proceed in both directions; products can revert back to reactants under certain conditions. This typically involves equilibrium where forward and reverse reactions occur at the same rate.

Step 2: Analyzing Option (a)

The reaction given is: \[\mathrm{KNO}_{3} (aq) + \mathrm{NaCl} (aq) \rightleftarrows \mathrm{KCl} (aq) + \mathrm{NaNO}_{3} (aq)\]This reaction involves ionic compounds in solution, suggesting that all species can dissociate and re-associate, which makes it a potential reversible reaction.

Step 3: Analyzing Option (b)

The reaction given is: \[2 \mathrm{Na} (s) + \mathrm{H}_{2} \mathrm{O} (l) \rightarrow 2 \mathrm{NaOH} (aq) + \mathrm{H}_{2} (g)\]This reaction is not reversible because it involves the formation of a gas \( \mathrm{H}_{2} \), which escapes the reaction mixture, and metallic sodium reacts violently with water forming non-reversible products.

Step 4: Analyzing Option (c)

The reaction given is: \[\mathrm{AgNO}_{3} (aq) + \mathrm{HCl} (aq) \rightarrow \mathrm{AgCl} (s) + \mathrm{HNO}_{3} (aq)\]The formation of a solid precipitate \(\mathrm{AgCl} (s)\) suggests that the reaction is not easily reversible under normal conditions.

Step 5: Analyzing Option (d)

The reaction given is: \[\mathrm{Pb}(\mathrm{NO}_{3})_{2} (aq) + 2 \mathrm{NaI} (aq) \rightarrow \mathrm{PbI}_{2} (s) + 2 \mathrm{NaNO}_{3} (aq)\]This reaction results in the formation of a solid precipitate \(\mathrm{PbI}_{2} (s)\), which is not reversible as the solid cannot easily revert to the ions in aqueous solution under normal conditions.

Step 6: Conclusion

Among all the given options, only reaction (a) can be considered as an example of a reversible reaction under typical conditions, since it involves the exchange of ions in solution without the formation of a precipitate or a gas.

Key Concepts

Ionic Compounds

Ionic compounds are chemical compounds composed of ions held together by electrostatic forces termed ionic bonding. These compounds are generally formed between metals and non-metals. When in solid form, ionic compounds are structured as tightly packed crystal lattices. However, ionic compounds display different properties when dissolved in water.

• Ions in Solution: When ionic compounds dissolve in water, they dissociate into their component ions. For example, salt (NaCl) dissociates into sodium ions (Na⁺) and chloride ions (Cl⁻).
• Electric Conductivity: The mobility of these ions allows the solution to conduct electricity, a property unique to ionic solutions.
• High Melting Points: Due to the strong attractions between ions, ionic compounds have high melting and boiling points in their solid state.

Understanding these properties helps in analyzing reactions involving ionic compounds, especially in aqueous solutions where these compounds can participate in reversible reactions by re-associating into different ionic combinations.

Chemical Equilibrium

Chemical equilibrium refers to the state of a reversible reaction where the rate of the forward reaction equals the rate of the backward reaction, and the concentrations of reactants and products remain constant over time. This dynamic balance signifies that, at equilibrium, the chemicals participate in ongoing reactions but without any net change in concentration.

• Dynamic Nature: Even at equilibrium, reactions occur continuously but with no overall effect on concentration levels.
• Le Chatelier's Principle: This principle elaborates that if an external change is applied to a system at equilibrium, the system will adjust itself to counteract the effect of the change and restore a new equilibrium.
• Factors Influencing Equilibrium: Concentration, temperature, and pressure can all influence the position of equilibrium. For instance, increasing the concentration of reactants typically favors the forward reaction.

Chemical equilibrium is crucial in reversible reactions, allowing for predictions about reaction behavior when conditions change.

Aqueous Solutions

Aqueous solutions are those where water acts as the solvent. Water's unique properties make it an ideal medium for chemical reactions involving ionic compounds and other solutes. Understanding the behavior of aqueous solutions is essential for predicting chemical reactivity and the conditions under which reversible reactions occur.

• Solvation: This is the process of surrounding solute ions with solvent molecules, facilitating dissolving of ionic compounds.
• Polarity: Water's polar nature allows it to dissolve various ionic and polar substances, making it an excellent solvent.
• pH Levels: The concentration of hydrogen ions in an aqueous solution determines its acidity or alkalinity, which can affect reaction pathways and solubility.

Mastering aqueous solutions broadens your understanding of how ions interact in water, highlighting their capability to react and shift between forms depending on external conditions, such as in reversible reactions.