Strong and Weak Acids and Bases
What Makes an Acid or Base "Strong" or "Weak"?
- Acids and bases exhibit different behaviors when dissolved in water due to their ability to ionize, or break apart into charged particles called ions.
- This ionization determines whether an acid or base is classified as "strong" or "weak."
Definitions and Ionization Behavior
Strong Acids and Bases
Definition
Strong acids and bases
Strong acids and bases ionize completely in water. All their molecules dissociate into ions, leaving no undissociated molecules in the solution.
- Hydrochloric acid (HCl) dissociates fully in water:
$$\text{HCl (aq)} \rightarrow \text{H}^+ \text{(aq)} + \text{Cl}^- \text{(aq)}$$ - Similarly, sodium hydroxide (NaOH), a strong base, dissociates completely:
$$\text{NaOH (aq)} \rightarrow \text{Na}^+ \text{(aq)} + \text{OH}^- \text{(aq)}$$
Weak Acids and Bases
Definition
Weak acids and bases
Weak acids and bases only partially ionize in water. At equilibrium, the solution contains both ions and undissociated molecules.
- Ethanoic acid (CH₃COOH) ionizes partially:
$$\text{CH}_3\text{COOH (aq)} \rightleftharpoons \text{H}^+ \text{(aq)} + \text{CH}_3\text{COO}^- \text{(aq)}$$ - Ammonia (NH₃), a weak base, also ionizes partially:
$$\text{NH}_3 \text{(aq)} + \text{H}_2\text{O (l)} \rightleftharpoons \text{NH}_4^+ \text{(aq)} + \text{OH}^- \text{(aq)}$$
- Weak acids and bases are often mistaken as being dilute.
- However, "weak" refers to their partial ionization, not their concentration.
Position of Equilibrium in Strong and Weak Acids and Bases
The position of equilibrium in acid-base reactions varies depending on the strength of the acid or base.
- Strong Acids and Bases:
- Since they ionize completely in water, the equilibrium lies far to the right, favoring the formation of ions with negligible amounts of undissociated molecules remaining.
- Weak Acids and Bases:
- Weak acids and bases only partially ionize, so the equilibrium lies to the left, favoring the undissociated form.
- The concentration of ions in solution is much lower compared to strong acids and bases.
- The direction of acid-base equilibria favors the formation of the weaker conjugate acid or base.
- Strong acids produce weak conjugate bases (like $Cl^-$), while weak acids (like ethanoic acid) form stronger conjugate bases (like $CH_3COO^-$).
Always use a reversible arrow $ \rightleftharpoons $ for weak acids and bases to represent partial ionization.
Examples of Strong and Weak Acids/Bases
Strong Acids
- Hydrochloric acid (HCl): Found in stomach acid.
- Nitric acid (HNO₃): Used in fertilizers.
- Sulfuric acid (H₂SO₄): Common in car batteries.
- Hydrobromic acid (HBr): Used for reducing certain compounds in industrial processes.
