Reactivity 3.1.6 – Strong and Weak Acids and Bases

Introduction to Ionization in Acids and Bases

• Acids and bases are classified as strong or weak depending on the extent to which they ionize (dissociate) in aqueous solution.
• Ionization refers to the process where an acid or base releases ions into solution, forming an equilibrium between the undissociated species and its ions.
• Strong acids and bases ionize almost completely; weak acids and bases ionize only partially, reaching a dynamic equilibrium.      

Strong Acids

• A strong acid completely ionizes in water — nearly all acid molecules donate protons (H⁺) to water molecules.
• This means the concentration of hydrogen ions \( [\text{H}^+] \) is nearly equal to the concentration of the acid itself.
• The reaction goes essentially to completion with negligible reverse reaction:
• \( \text{HA}(aq) + \text{H}_2\text{O}(l) \rightarrow \text{H}_3\text{O}^+(aq) + \text{A}^-(aq) \)

Examples of strong acids:

• • Hydrochloric acid – HCl
  • Hydrobromic acid – HBr
  • Hydroiodic acid – HI
  • Nitric acid – HNO₃
  • Sulfuric acid – H₂SO₄ (only the first proton is fully ionized)

Strong Bases

• Strong bases also fully dissociate in aqueous solution, releasing hydroxide ions \( \text{OH}^- \).
• These substances readily accept protons and shift the equilibrium far toward products.
• Group 1 hydroxides are the most common examples of strong bases in IB Chemistry.

Examples:

• • Sodium hydroxide – NaOH
  • Potassium hydroxide – KOH
  • Rubidium hydroxide – RbOH
  • Cesium hydroxide – CsOH

Weak Acids

• Weak acids partially ionize in solution and establish an equilibrium between the ionized and non-ionized forms.
• This means only a small fraction of the acid molecules donate H⁺ ions.
• The equilibrium lies to the left (towards the undissociated acid).
• \( \text{HA}(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{A}^-(aq) \)

Examples of weak acids:

• • Ethanoic acid – CH₃COOH
  • Carbonic acid – H₂CO₃
  • Phosphoric acid – H₃PO₄
  • Hydrofluoric acid – HF

Weak Bases

• Weak bases only partially accept protons in aqueous solution and also reach a state of equilibrium.
• This results in a relatively low concentration of OH⁻ in solution.

Examples:

• • Ammonia – NH₃
  • Methylamine – CH₃NH₂
  • Pyridine – C₅H₅N

Key Differences Between Strong and Weak Acids/Bases

Important IB Distinction: Strength vs. Concentration

• Strength refers to the degree of ionization of an acid or base.
• Concentration refers to the amount of solute (acid/base) per unit volume of solution (mol·dm⁻³).
• It is possible to have:
  • A concentrated weak acid (high amount, but only partial ionization)
  • A dilute strong acid (low amount, but fully ionized)
• This distinction is frequently tested in IB questions — do not confuse them!

Acid–Base Equilibria and Conjugate Strengths

Understanding Conjugate Acid–Base Pairs

• According to the Brønsted–Lowry theory, acids are proton (H⁺) donors and bases are proton acceptors.
• Every acid–base reaction involves two conjugate pairs:
  • The acid and its conjugate base
  • The base and its conjugate acid

Example:
\( \text{HA}(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{A}^-(aq) \)
Here:

• • HA = acid, A⁻ = conjugate base
  • H₂O = base, H₃O⁺ = conjugate acid

Equilibrium Favors the Weaker Conjugate

• Acid–base reactions are equilibria that lie toward the weaker acid and weaker base.
• Stronger acid/base → reacts more readily → equilibrium favors the weaker side
• If the acid is strong, its conjugate base will be weak.
• Conversely, a weak acid has a stronger conjugate base because the forward donation of a proton is less favored.
• Therefore: The direction of equilibrium is determined by comparing the relative strength of the acid and base on both sides of the reaction.

Relative Strengths of Conjugates

Implications for IB Chemistry

• This principle helps predict the extent of proton transfer in acid–base reactions.
• It is also crucial for understanding buffer behavior, titration curves, and pKa/pKb concepts (at HL).
• Key point: If you know which species are strong and weak, you can predict the direction of the equilibrium.

Comparisons of Acids and Bases with respect to Equilibrium 

Acid and Equilibrium Comparison Table

Base and Equilibrium Comparison Table

Comparison of Strong and Weak Acids

Distinguishing Between Strong and Weak Acids

• Extent of Ionization:
  • Strong acids ionize completely in aqueous solution.
  • Weak acids only partially ionize in water.

• Position of Equilibrium:
  • Strong acids: equilibrium lies far to the right.
  • Weak acids: equilibrium lies to the left.

• pH and [H⁺] Concentration:
  • Strong acids: low pH (typically below 3)
  • Weak acids: higher pH (around 4–6)

• Ka (Acid Dissociation Constant):
  • Strong acids: very large Ka (Ka → ∞)
  • Weak acids: small Ka

• Electrical Conductivity:
  • Strong acids: high conductivity
  • Weak acids: lower conductivity

• Reaction Rates (with metals/carbonates):
  • Strong acids: fast and vigorous
  • Weak acids: slower

• Experimental Distinctions:
  • pH Measurement: Strong acids give a lower pH.
  • Conductivity Test: Strong acids show higher conductivity.
  • Rate of Gas Evolution: Strong acids bubble more rapidly with metals/carbonates.

Comparison of Strong and Weak Bases