Question

How can you determine if an anion will act as a weak base? Write a generic equation showing the reaction by which an anion, \(A^{-},\) acts as a weak base.

Short answer

To determine if an anion will act as a weak base, check if it is the conjugate base of a weak acid. The general equation for an anion acting as a weak base is: \( A^- + H_2O \rightleftharpoons HA + OH^- \).

Step-by-step solution

Understanding Bronsted-Lowry Base Concept

Recognize that according to the Bronsted-Lowry definition, a base is a substance that can accept a proton (H+). An anion, which is a negatively charged ion, can act as a base if it has the ability to accept a proton.

Examining the Anion's Tendency to Accept a Proton

To determine if an anion will act as a weak base, check if it is the conjugate base of a weak acid. If it is indeed the conjugate base of a weak acid, it has a tendency to accept a proton, thereby acting as a weak base.

Writing the Generic Equation

Write a generic equation showing how an anion acts as a weak base by accepting a proton in a reversible reaction. The equation will be: \[ A^- + H_2O \rightleftharpoons HA + OH^- \] where \( A^- \) is the anion acting as a weak base and \( HA \) is the weak acid from which the anion is derived.

Key concepts

Bronsted-Lowry Base

When studying acids and bases, it's essential to understand the Bronsted-Lowry theory, which provides a clear definition of how acids and bases behave. A Bronsted-Lowry base is defined as any substance that can accept a proton (which is a hydrogen ion, symbolized as \(H^+\)). It's all about the transfer of protons. For example, when an anion \(A^-\), a negatively charged ion, comes into contact with water \(H_2O\), it may accept a proton from the \(H_2O\).

This behavior is significant because it extends the concept of bases beyond just hydroxide ions \(OH^-\). In the Bronsted-Lowry view, many more substances can be classified as bases. A good way to identify such bases is by looking at reactions where a proton is handed off to another species — that receiver is the Bronsted-Lowry base.

Conjugate Base of Weak Acid

Diving deeper into acid-base chemistry, the term 'conjugate base' surfaces often. The conjugate base of a weak acid is the species left over after the acid has donated a proton. To visualize this, imagine a weak acid \(HA\). When \(HA\) loses a proton \(H^+\), it becomes \(A^-\). This \(A^-\) is the conjugate base of the weak acid \(HA\).

What makes \(A^-\) a 'weak' base is rooted in its origin as a weak acid. Weak acids partially dissociate in solution, meaning they don't completely give up their protons. Because their conjugate bases come from these incomplete reactions, they aren't aggressively seeking protons; hence, they only weakly act as bases. When considering if an anion will be a weak base, always check if it's the conjugate base of a weak acid — this gives you a clue about its base-like behavior.

Weak Base Reaction Equation

Understanding the behavior of weak bases in terms of equations is fundamental in chemistry. The weak base reaction equation demonstrates how a weak base accepts a proton in the aqueous environment. Representing this generically, the equation is \[ A^- + H_2O \rightleftharpoons HA + OH^- \].

This equation shows an equilibrium process; the double arrows mean the reaction can go both ways. On the left, \(A^-\) represents the weak base anion while \(H_2O\) is water acting as the proton donor. When \(A^-\) accepts a proton \(H^+\) from \(H_2O\), it forms \(HA\), the weak acid from which \(A^-\) was originally derived, and \(OH^-\), a hydroxide ion. This reversible reaction is crucial because it explains how weak bases can influence the pH of a solution — by shifting the balance, more or fewer hydroxide ions can be formed, thus affecting the solution's acidity or basicity.