Question

An example of a reversible reaction is: (a) \(\mathrm{KNO}_{3}\) (aq) \(+\mathrm{NaCl}(\mathrm{aq})=\mathrm{KCl}(\mathrm{aq})+\mathrm{NaNO}_{3}\) (aq) (b) \(2 \mathrm{Na}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(1)=2 \mathrm{NaOH}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g})\) (c) \(\mathrm{AgNO}_{3}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq})=\mathrm{AgCl}(\mathrm{s})+\mathrm{NaNO}_{3}(\mathrm{aq})\) (d) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{NaI}(\mathrm{aq})=\mathrm{PbI}_{2}(\mathrm{~s})+2 \mathrm{NaNO}_{3}(\mathrm{aq})\)

Short answer

The reversible reaction is (a) \( \mathrm{KNO}_{3} (aq) + \mathrm{NaCl} (aq) \rightarrow \mathrm{KCl} (aq) + \mathrm{NaNO}_{3} (aq) \).

Step-by-step solution

Understanding Reversible Reactions

A reversible reaction is a chemical reaction where the products can react to reform the reactants. In a chemical equation, this is often represented by a double arrow (⇌) instead of a single arrow (→). This implies that the reaction can proceed in both forward and backward directions.

Analysis of Option (a)

The equation: \( \mathrm{KNO}_{3} (aq) + \mathrm{NaCl} (aq) \rightarrow \mathrm{KCl} (aq) + \mathrm{NaNO}_{3} (aq) \) is a metathesis reaction (double displacement), where an exchange of ions occurs to form the products. These are often reversible but depend on conditions such as solubility, temperature, and concentration.

Analysis of Option (b)

The equation: \( 2 \mathrm{Na} (\mathrm{s}) + \mathrm{H}_{2} \mathrm{O} (l) \rightarrow 2 \mathrm{NaOH} (aq) + \mathrm{H}_{2} (\mathrm{~g}) \) is an irreversible reaction. The formation of hydrogen gas escapes as bubbles, driving the reaction to completion with little possibility to reverse under normal conditions.

Analysis of Option (c)

The equation: \( \mathrm{AgNO}_{3} (\mathrm{aq}) + \mathrm{HCl} (\mathrm{aq}) \rightarrow \mathrm{AgCl} (\mathrm{s}) + \mathrm{NaNO}_{3} (\mathrm{aq}) \) involves \( \mathrm{AgCl} (\mathrm{s}) \) forming a precipitate (solid), which typically means the reaction is irreversible in nature under normal conditions.

Analysis of Option (d)

The equation: \( \mathrm{Pb}(\mathrm{NO}_{3})_{2}(\mathrm{aq}) + 2 \mathrm{NaI}(\mathrm{aq}) \rightarrow \mathrm{PbI}_{2}(\mathrm{~s}) + 2 \mathrm{NaNO}_{3}(\mathrm{aq}) \) also forms a solid precipitate, \( \mathrm{PbI}_{2} (\mathrm{s}) \), indicating the reaction is irreversible.

Identifying the Reversible Reaction

Based on the nature of the reactions, the only equation where the reaction could potentially be reversible due to solubility and ion exchange is option (a): \( \mathrm{KNO}_{3} (aq) + \mathrm{NaCl} (aq) \rightarrow \mathrm{KCl} (aq) + \mathrm{NaNO}_{3} (aq) \).

Key concepts

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions in a reversible process are equal, leading to a stable balance of reactants and products. This is represented symbolically in equations with a double arrow (⇌), indicating that both the forward and backward reactions occur simultaneously.
When a system reaches equilibrium, the concentrations of reactants and products don't change, although reactions continue to happen. This doesn't mean that the amounts of reactants and products are equal, but rather stable.
Factors that influence equilibrium include:

• Concentration: Changes in the concentration of either reactants or products can shift the equilibrium position.
• Temperature: Increasing temperature generally favors the endothermic direction of a reaction.
• Pressure: Changes in pressure primarily affect gaseous reactions, where a shift will occur toward the side with fewer gas molecules.

Understanding equilibrium is crucial in predicting how changes in conditions might affect the outcome of a reversible chemical reaction.

Double Displacement Reactions

Double displacement reactions, also known as metathesis reactions, involve the exchange of ions between two compounds to form new products. These reactions typically occur in aqueous solutions and can be represented generally as:
\[ AB + CD \rightarrow AD + CB \]In these reactions, the ions "swap partners," which often leads to the formation of a precipitate, a gas, or a weak electrolyte. An example would be the reaction between potassium nitrate and sodium chloride:\[ \mathrm{KNO}_{3} (aq) + \mathrm{NaCl} (aq) \rightarrow \mathrm{KCl} (aq) + \mathrm{NaNO}_{3} (aq) \]Sometimes double displacement reactions are reversible, especially if no solid precipitate forms and both products stay in solution.
Common characteristics include:

• Occur mostly in aqueous solutions.
• Can lead to the formation of an insoluble precipitate.
• May produce a gas or water as one of the products.

These reactions play a significant role in various industrial processes and laboratory settings.

Irreversible Reactions

Irreversible reactions are chemical processes where reactants convert fully into products under the given conditions, with little to no reverse reaction occurring. These reactions proceed to completion, and the formation of products is typically favored without reaching an equilibrium. One common example is the reaction between sodium and water:\[ 2 \mathrm{Na} (s) + 2 \mathrm{H}_{2} \mathrm{O} (l) \rightarrow 2 \mathrm{NaOH} (aq) + \mathrm{H}_{2} (g) \]In this scenario, hydrogen gas is produced and escapes, driving the reaction forward and preventing it from reversing easily.
Key characteristics include:

• Formation of a gas, solid, or water, which prevents the reaction from reversing easily.
• Generally go to completion under normal conditions.
• Are often energy releasing (exothermic).

Understanding irreversible reactions is fundamental when predicting the final state of chemical processes and their potential applications in industry or everyday life.