Introduction

Understanding the difference between strong and weak acids/bases, as well as their concentration levels, is fundamental to chemistry. This article explores these concepts using the Brønsted-Lowry theory as our framework.

The Brønsted-Lowry Theory

According to Brønsted-Lowry theory, acids are proton donors and bases are proton acceptors. This definition leads to two important reactions:

$\text{HF}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{H}_3\text{O}^+_{(aq)} + \text{F}^-_{(aq)}$

$$\text{NH}_{3(aq)} + \text{H}_{2}\text{O}_{(l)} \rightleftharpoons \text{NH}_{4(aq)}^{+} + \text{OH}_{(aq)}^{-}$$

Conjugate Acid-Base Pairs

When an acid donates a proton, it forms its conjugate base. Similarly, when a base accepts a proton, it forms its conjugate acid. The difference between a conjugate pair is always one proton. Consider acetic acid's reaction with water:

$\text{CH}_3\text{COOH}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{CH}_3\text{COO}^-_{(aq)} + \text{H}_3\text{O}^+_{(aq)}$

Acidic Hydrogen

Not all hydrogen atoms in a molecule can be donated as protons. Those that can are called acidic hydrogens. The ability to donate these hydrogens depends on the stability of the resulting conjugate base.

Strength vs. Concentration

Strong vs. Weak

Strong acids and bases completely ionize in water, while weak acids and bases only partially ionize. This is represented in their chemical equations:

Strong acid (HCl): $\text{HCl}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightarrow \text{H}_3\text{O}^+_{(aq)} + \text{Cl}^-_{(aq)}$

Weak acid (CH₃COOH): $\text{CH}_3\text{COOH}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{H}_3\text{O}^+_{(aq)} + \text{CH}_3\text{COO}^-_{(aq)}$

Concentrated vs. Dilute

Concentration refers to the amount of acid or base present per volume of solution, regardless of ionization. A solution can be:

Quantifying Strength: Ka and Kb

The strength of acids and bases can be quantified using dissociation constants: