Strong vs weak
The strength of an acid or alkali is the degree to which it ionises in water. It is not the same as concentration.
- A strong acid ionises completely in water, giving a high [H+]. Examples: hydrochloric acid HCl, nitric acid HNO3, sulphuric acid H2SO4.
- A weak acid ionises partially, so only a small fraction of molecules release H+. Example: ethanoic acid CH3COOH, carbonic acid H2CO3.
Key idea / 要点
Strong acid: HCl(aq) → H+(aq) + Cl−(aq) (complete, single arrow)
Weak acid: CH3COOH(aq) ⇌ CH3COO−(aq) + H+(aq) (partial, reversible arrow)
Alkalis and comparing
Strong alkalis (NaOH, KOH) ionise fully to give a high [OH−]; weak alkalis such as aqueous ammonia NH3 ionise partially. At the same concentration, a strong acid has a lower pH, reacts faster with magnesium, and conducts electricity better than a weak acid.
It is important not to confuse a strong acid with a concentrated one. A dilute strong acid (for example 0.001 mol dm−3 HCl) can have a higher pH than a concentrated weak acid, because pH depends on the actual [H+] in solution. Strength is a fixed property of the acid itself, while concentration can be changed simply by adding or removing water.
| Property (same conc.) | Strong acid | Weak acid |
|---|---|---|
| Degree of ionisation | Complete | Partial |
| [H+] | High | Low |
| pH | Lower | Higher |
| Rate with Mg | Fast | Slow |
Worked example
Two beakers each hold 0.1 mol dm−3 acid — one HCl, one CH3COOH. Which has the lower pH?
Answer: HCl. It ionises completely so its [H+] is higher; higher [H+] means a lower pH.
Remember
- Strength = degree of ionisation; concentration = amount of solute per volume.
- Strong ⇒ full ionisation ⇒ single arrow (→).
- Weak ⇒ partial ionisation ⇒ reversible arrow (⇌).