Notes for R3.1.10 Acid and base strength and constants (Higher Level Only) - IB | RevisionDojo
R3.1.10 Acid and base strength and constants (Higher Level Only) Notes
Equilibrium Constants and Acid/Base Strength
Acid Ionization Constant ($K_a$)
When a weak acid dissolves in water, it establishes an equilibrium between the undissociated acid and its ions: $$ HA(aq) \rightleftharpoons H^+(aq) + A^-(aq) $$
The equilibrium constant for this reaction is called the acid ionization constant ($K_a$): $$ K_a = \frac{[H^+][A^-]}{[HA]} $$
[H⁺]: Concentration of hydrogen ions (or hydronium ions, $H_3O^+$).
[A⁻]: Concentration of the conjugate base.
[HA]: Concentration of the undissociated acid.
Hint
A larger $K_a$ indicates a stronger acid because more $HA$ dissociates, producing more $H^+$ ions.
Conversely, a smaller $K_a$ indicates a weaker acid.
Example
Acetic acid ($CH_3COOH$) has a $K_a$ of $1.8 \times 10^{-5}$.
This small value shows that acetic acid is a weak acid, as only a small fraction of its molecules dissociate in water.
This is why vinegar, which contains acetic acid, has a sour but not overpowering taste.
Base Ionization Constant ($K_b$)
For weak bases, the dissociation in water can be represented as: $$ BOH(aq) \rightleftharpoons B^+(aq) + OH^-(aq) $$
The base ionization constant ($K_b$) is defined as: $$ K_b = \frac{[OH^-][B^+]}{[BOH]} $$
[OH⁻]: Concentration of hydroxide ions.
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Recap question
Base X has Kb=1.0×10−3 and base Y has Kb=1.0×10−6. Which statement is correct about their strengths?
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What does a larger Ka value indicate?
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Note
Introduction to Acid and Base Strength
Acids and bases are substances that can donate or accept protons (H+ ions) in chemical reactions.
The strength of an acid or base refers to its ability to dissociate (break apart) into ions in solution.
Strong acids/bases dissociate completely, while weak acids/bases only partially dissociate.
AnalogyThink of a strong acid like a strict teacher who always gives away all their notes (protons), while a weak acid is like a cautious teacher who only shares a few notes.
ExampleHydrochloric acid (HCl) is a strong acid, while acetic acid (CH₃COOH) is a weak acid. This is why vinegar (which contains acetic acid) is safe to consume, while concentrated HCl is not.
DefinitionDissociation: The process by which molecules split into smaller particles, such as ions, when dissolved in a solvent.
NoteNot all acids and bases are equal in strength - this is why lemon juice (citric acid) tastes sour but doesn't burn like battery acid (sulfuric acid).
