Question

The neutralization of \(\mathrm{NaOH}\) by \(\mathrm{HCl}\) is represented in equation (1), and the neutralization of \(\mathrm{NH}_{3}\) by HCl in equation (2). 1. \(\mathrm{OH}^{-}+\mathrm{H}_{3} \mathrm{O}^{+} \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O} \quad K=?\) 2\. \(\mathrm{NH}_{3}+\mathrm{H}_{3} \mathrm{O}^{+} \rightleftharpoons \mathrm{NH}_{4}^{+}+\mathrm{H}_{2} \mathrm{O} \quad K=?\) (a) Determine the equilibrium constant \(K\) for each reaction. (b) Explain why each neutralization reaction can be considered to go to completion.

Short answer

The equilibrium constant K for the first reaction can be represented as \(1/[OH-][H3O+]\) and for the second reaction it can be represented as \([NH4+] / [NH3]\). Each neutralization reaction can be considered to go to completion as these reactions typically proceed until one of the reactants is exhausted due to the high reactivity of both acids and bases.

Step-by-step solution

Calculate equilibrium constant K for the first reaction

For the first reaction, \(\mathrm{OH}^{-}+\mathrm{H}_{3} \mathrm{O}^{+} \to 2 \mathrm{H}_{2} \mathrm{O}\), the equilibrium constant \(K\) can be determined by setting it equal to the concentrations of the products divided by the concentrations of the reactants. However, since water is a product and it remains fairly constant, the equilibrium constant is primarily determined by the concentration of the reactants, which is \([OH-][H3O+]\). Therefore, for this reaction, \(K = [H2O] / [OH-][H3O+] = 1/[OH-][H3O+]\).

Calculate equilibrium constant K for the second reaction

For the second reaction, \(\mathrm{NH}_{3}+\mathrm{H}_{3} \mathrm{O}^{+} \to \mathrm{NH}_{4}^{+}+\mathrm{H}_{2} \mathrm{O}\), the equilibrium constant \(K\) can be determined in a similar manner. Since the concentration of water remains fairly constant, the equilibrium constant is primarily determined by the concentration of the reactants, which is \([NH3][H3O+]\). Therefore, for this reaction, \(K = [NH4+][H2O] / [NH3][H3O+] = [NH4+] / [NH3]\).

Explain why each neutralization reaction can be considered to go to completion

An acid-base neutralization reaction can be considered to go to completion because it is a process where an acid and a base react quantitatively with each other. In such reactions, the acid and base become less acidic and less basic, respectively, as they react with each other, leading to the formation of water and a type of salt. This reaction proceeds until one of the reactants is exhausted. Due to the high reactivity of both acids and bases, these reactions typically proceed to completion, meaning that there are no leftover reactants.

Key concepts

Neutralization Reaction

In a neutralization reaction, an acid reacts with a base, resulting in the formation of water and a salt.
This type of reaction is essential because it leads to the transformation of reactants into neutral products.
In chemical terms, a neutralization reaction can be represented as:

• An acid (often generating hydronium ions, \( \mathrm{H}_3\mathrm{O}^+\)), and
• A base (often generating hydroxide ions, \( \mathrm{OH}^-\)) come together.

When these ions meet, they combine to form water(\(\mathrm{H}_2\mathrm{O}\)), which significantly lowers the acidity (or alkalinity) of the solution.
So in essence, the completion to water molecules as products is what defines a neutralization reaction.

Acid-Base Reaction

Acid-base reactions are a fundamental part of chemistry where an acid donates a proton (Hgsup>+) to a base.
These reactions can create a variety of outcomes, not just water and salt.
In the case of \( \mathrm{NaOH}\) neutralizing \( \mathrm{HCl}\), both are strong, meaning the reaction is clear-cut and quick.For equation (2) with \( \mathrm{NH}_3\) being less basic, the completion means that all available \( \mathrm{NH}_3\) molecules will have grabbed protons from \( \mathrm{H}_3\mathrm{O}^+\) to become \( \mathrm{NH}_4^+\), nicely defining the completion aspect.
In general, acid-base reactions have:

• Proton transfer as the core,
• Formation of a conjugate acid-base pair, and
• Tendency towards an irreversible step under certain conditions.

Understanding the direction and extent of these reactions can be deciphered using the equilibrium constant.

Reaction Completion

Reaction completion refers to the point where the reactants have fully transformed into products and no further reaction occurs.
Unlike some other chemical reactions that may reach a balance (equilibrium) with a mix of products and reactants, neutralizations generally "go to completion."
This is evident because:

• The reactants - acids and bases - are entirely consumed,
• Water and salt are predominantly stable, and
• The driving force of these reactions is strong and nearly irreversible under ordinary conditions

The idea of completion is characterized by the lack of leftover reactants.
When one of the reactants is completely used up, the reaction is complete, resulting in only products in equilibrium, pushing the equation heavily in one direction.
If you encounter a neutralization reaction in practice, you can expect no backwards "undoing" like in other reversible reactions.