Strong and Weak Acids and Bases
Quick Notes
- Strong acids: completely ionize in water.
- Examples: HCl, HBr, HI, HNO₃, H₂SO₄
- Weak acids: only partially ionize.
- Example: CH₃COOH (ethanoic acid)
- Strong bases: completely dissociate.
- Examples: Group 1 hydroxides like NaOH, KOH
- Weak bases: partially dissociate.
- Example: NH₃ (ammonia)
- Acid–base equilibria favour formation of the weaker conjugate acid–base pair.
- Strength = extent of ionization.
Concentration = amount of solute per unit volume.
You can have a dilute solution of a strong acid and a concentrated solution of a weak acid.
Full Notes
Strong and Weak Acids
Strong acids and bases ionize or dissociate completely in solution. Weak acids and bases only partially ionize or dissociate.
Ionization vs. Dissociation: These terms are often used interchangeably, but they technically have distinct meanings.
- Ionization refers to the formation of ions from molecular compounds, such as acids forming H⁺ in solution.
HCl(g) → H⁺(aq) + Cl⁻(aq) - Dissociation refers to the separation of pre-existing ions in an ionic compound when it dissolves in water.
NaOH(s) → Na⁺(aq) + OH⁻(aq)
Acids and bases can do either:
- Strong acids like HCl ionize completely in water.
- Strong bases like NaOH dissociate completely in water.
Common Examples
- Strong acids:
- Hydrochloric acid: HCl
- Hydrobromic acid: HBr
- Hydroiodic acid: HI
- Nitric acid: HNO₃
- Sulfuric acid: H₂SO₄ (only the first proton fully ionizes)
- Strong bases:
- Sodium hydroxide: NaOH
- Potassium hydroxide: KOH
- All group 1 metal hydroxides
- Weak acids and bases:
- Ethanoic acid (CH₃COOH) → weak acid
- Ammonia (NH₃) → weak base
Equilibrium and Conjugate Pairs
In acid–base reactions, the equilibrium lies toward the weaker conjugate pair.
Strong acids have weak conjugate bases, and vice versa.
Example HCl and CH₃COOH
- HCl (strong acid) → H⁺ + Cl⁻
Cl⁻ is a very weak base and does not accept H⁺ back. - CH₃COOH (weak acid) ⇌ H⁺ + CH₃COO⁻
CH₃COO⁻ is a stronger base than for a strong acid and can accept H⁺, reforming CH₃COOH.
Strength vs. Concentration
Strength refers to how completely a substance ionizes or dissociates.
Concentration refers to how much of it is present in a given volume.
Example Comparing strength and concentration
- A dilute solution of HCl is still a strong acid.
- A concentrated solution of CH₃COOH is still a weak acid.
Summary
- Strong acids and bases ionize or dissociate completely, weak ones only partially.
- Strong acids have weak conjugate bases; weak acids have stronger conjugate bases.
- Strength refers to ionization, concentration refers to solute amount.
- Strong vs weak acids can be distinguished by pH, conductivity, and reaction rate.
Linked Course Questions
How would you expect the equilibrium constants of strong and weak acids to compare?
Because strong acids ionize almost completely in water, the equilibrium for their ionization lies far to the right. This means they have a very large equilibrium constant (Ka).
Weak acids only partially ionize, so the equilibrium lies further to the left. As a result, they have a much smaller Ka.
In summary:
- Strong acids give high [H⁺] at equilibrium = large Ka
- Weak acids give lower [H⁺] at equilibrium = small Ka
Why does the acid strength of the hydrogen halides increase down group 17?
As you move down Group 17 (from HF to HI), the H–X bond becomes longer and weaker because the halogen atoms get larger. Weaker H–X bonds are easier to break in solution, so protons (H⁺) are more readily released.
- HF is a weak acid (strong H–F bond, harder to ionize)
- HCl, HBr, and HI are strong acids (weaker bonds, easier to ionize)
So, acid strength increases down the group because bond strength decreases, allowing more complete ionization.
What physical and chemical properties can be observed to distinguish between weak and strong acids or bases of the same concentration?
At the same concentration, strong acids and bases ionize completely, while weak acids and bases ionize only partially. This leads to observable differences such as:
- pH: Strong acids have a lower pH than weak acids; strong bases have a higher pH than weak bases.
- Electrical conductivity: Strong acids/bases conduct electricity better due to a higher concentration of ions.
- Rate of reaction: Strong acids react more quickly with reactive metals or carbonates due to higher [H⁺].