3.1 Strength of Acids and Bases

The strength of an acid or base depends on the extent to which it ionizes in water. This is quantified by the acid dissociation constant ($K_a$) and the base dissociation constant ($K_b$).

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Strong vs Weak Acids

Strong Acids

A strong acid ionizes completely in water. The degree of ionization is approximately 1 (100%).

$HA\to H^{+}+A^{-}$

Examples: $HCl$, $HN{O}_3$, $H_{2}S{O}_4$, $HCl{O}_4$

For strong acids, $K_a$ is very large (effectively $\gg 1$), so the equilibrium lies entirely to the right.

Weak Acids

A weak acid ionizes only partially in water, establishing a dynamic equilibrium:

$HA\rightleftharpoons H^{+}+A^{-}$

The acid dissociation constant is:

$K_a=\frac{[H^{+}][A^{-}]}{[HA]}$

A larger $K_a$ means greater ionization → stronger acid.

Examples:

The $p{K}_a$ Scale

$p{K}_a=-{\log }_{10}(K_a)$

• Lower $p{K}_a$ → larger $K_a$ → stronger acid
• A difference of 1 $p{K}_a$ unit = 10-fold difference in $K_a$

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Strong vs Weak Bases

Strong Bases

A strong base dissociates completely in water. Degree of ionization ≈ 1.

$MOH\to M^{+}+O{H}^{-}$

Examples: $NaOH$, $KOH$, $Ca(OH{)}_2$

Weak Bases

A weak base ionizes only partially in water:

$B+H_{2}O\rightleftharpoons B{H}^{+}+O{H}^{-}$

The base dissociation constant is:

$K_b=\frac{[B{H}^{+}][O{H}^{-}]}{[B]}$

A larger $K_b$ means greater ionization → stronger base.

Example: Ammonia ($N{H}_3$), $K_b=1.8\times 10^{-5}$

$N{H}_3+H_{2}O\rightleftharpoons N{H}_4^{+}+O{H}^{-}$

The $p{K}_b$ Scale

$p{K}_b=-{\log }_{10}(K_b)$

• Lower $p{K}_b$ → larger $K_b$ → stronger base

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Summary: Comparing Acid/Base Strength

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Worked Example

Q: Ethanoic acid has $K_a=1.8\times 10^{-5}$. Calculate its $p{K}_a$ and classify it.

Solution: $p{K}_a=-\log (1.8\times 10^{-5})=-(-4.74)=4.74$

Since $K_a\ll 1$ and $p{K}_a>0$, ethanoic acid is a weak acid with low degree of ionization.

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Past Paper Questions