3.2 - Strong & Weak Acids

Strong and weak acids

When an acid dissolves in water its particles ionise (dissociate): they split up and release hydrogen ions, H⁺, into the solution.

Strong acids

• A strong acid is completely ionised in aqueous solution. Every particle dissociates into ions.
• Examples: hydrochloric, nitric, and sulfuric acids.
• HCl(aq) → H⁺(aq) + Cl^-(aq)

Weak acids

• A weak acid is only partially ionised in aqueous solution. Only a small proportion of its particles dissociate into ions.
• Examples: ethanoic, citric, and carbonic acids.
• CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO^-(aq)

The reversible arrow (⇌) shows the ionisation is reversible: only a small fraction of the acid dissociates, setting up an equilibrium where most acid molecules stay whole.

Comparing strong and weak acids at the same concentration

• pH - A strong acid has a higher concentration of H⁺ ions, so it has a lower pH (around pH 0 to 2 for strong acids, pH 2 to 6 for weak acids).
• Rate of reaction - With more H⁺ ions in solution, a strong acid reacts faster. For example, magnesium fizzes vigorously in hydrochloric acid, but more slowly in ethanoic acid of the same concentration.

Concentration against strength

Strong/weak and concentrated/dilute describe different things:

• Strength is the proportion of dissolved acid molecules that ionise in water. It is a property of the acid itself and does not change when water is added.
• Concentration is the amount of acid dissolved in a given volume of solution.

The two are independent

• All four combinations exist: dilute strong, concentrated strong, dilute weak, and concentrated weak.
• Ethanoic acid remains a weak acid even when concentrated, and hydrochloric acid remains a strong acid even when dilute.

Both affect pH

• Both strength and concentration determine the concentration of H⁺ ions in solution:
  • Making any acid more concentrated increases the number of H⁺ ions in a given volume, so the pH falls.
  • A concentrated weak acid can have a lower pH than a very dilute strong acid.
• To compare the strength of two acids by their pH, they must be at the same concentration.

pH and hydrogen-ion concentration

• The pH of a solution is a measure of its hydrogen-ion concentration. The higher the concentration of H⁺ ions, the lower the pH.
• At pH 7, the concentration of hydrogen ions equals the concentration of hydroxide ions (neutral).

The pH scale is logarithmic

• Each change of 1 on the pH scale represents a tenfold (×10) change in hydrogen-ion concentration.
  • If pH falls by 1 unit, the hydrogen-ion concentration increases by a factor of 10.
  • If pH rises by 1 unit, the hydrogen-ion concentration decreases to a tenth.
• Changes of more than one unit multiply:
  • A fall of 2 pH units = a 100-fold increase (10 × 10)
  • A fall of 3 pH units = a 1,000-fold increase (10 × 10 × 10)

Worked example - A change of three pH units

A solution of an acid has a pH of 5. More acid is added and the pH falls to 2. By what factor has the hydrogen-ion concentration changed?