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Acids and Bases - Strong and weak acids and bases

Grade 12IBChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Strong acids and bases are defined by their complete dissociation (ionization) in aqueous solution. For example, HCl(aq)→H+(aq)+Cl−(aq)HCl(aq) \rightarrow H^+(aq) + Cl^-(aq) and NaOH(aq)→Na+(aq)+OH−(aq)NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq).

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Weak acids and bases ionize only partially in aqueous solution, reaching a state of dynamic equilibrium. Common examples include ethanoic acid (CH3COOHCH_3COOH) and ammonia (NH3NH_3).

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The strength of an acid is measured by its acid dissociation constant, KaK_a. A higher KaK_a value (or a lower pKapK_a value) indicates a stronger acid that dissociates more effectively.

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For any conjugate acid-base pair at 298K298 K, the relationship between their constants is defined by Kw=Ka×Kb=1.0×10−14K_w = K_a \times K_b = 1.0 \times 10^{-14}.

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Strong and weak acids of the same concentration can be distinguished experimentally by measuring electrical conductivity (stronger acids have higher ion concentrations), pHpH (stronger acids have lower pHpH), or the rate of reaction with reactive metals/carbonates.

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A strong acid has a very weak conjugate base with negligible basic properties, whereas a weak acid has a relatively stronger conjugate base.

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Common strong acids include HClHCl, HNO3HNO_3, and H2SO4H_2SO_4. Common weak acids include H2CO3H_2CO_3, H3PO4H_3PO_4, and organic carboxylic acids.

📐Formulae

HA(aq)+H2O(l)⇌H3O+(aq)+A−(aq)HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq) units: mol dm−3mol\,dm^{-3}

Ka=[H3O+][A−][HA]K_a = \frac{[H_3O^+][A^-]}{[HA]}

pKa=−log⁡10KapK_a = -\log_{10} K_a

B(aq)+H2O(l)⇌BH+(aq)+OH−(aq)B(aq) + H_2O(l) \rightleftharpoons BH^+(aq) + OH^-(aq)

Kb=[BH+][OH−][B]K_b = \frac{[BH^+][OH^-]}{[B]}

pKw=pKa+pKb=14.00 (at 298K)pK_w = pK_a + pK_b = 14.00 \text{ (at } 298 K)

pH=−log⁡10[H+]pH = -\log_{10} [H^+]

💡Examples

Problem 1:

Calculate the pHpH of a 0.20 mol dm−30.20\,mol\,dm^{-3} solution of propanoic acid (C2H5COOHC_2H_5COOH) at 298 K298\,K, given that its Ka=1.3×10−5K_a = 1.3 \times 10^{-5}.

Solution:

  1. Set up the expression: Ka=[H+][A−][HA]K_a = \frac{[H^+][A^-]}{[HA]}. Assume [H+]≈[A−][H^+] \approx [A^-] and [HA]initial≈[HA]eq[HA]_{initial} \approx [HA]_{eq}.
  2. [H+]=Ka×[HA]=1.3×10−5×0.20=1.61×10−3 mol dm−3[H^+] = \sqrt{K_a \times [HA]} = \sqrt{1.3 \times 10^{-5} \times 0.20} = 1.61 \times 10^{-3}\,mol\,dm^{-3}.
  3. pH=−log⁡10(1.61×10−3)=2.79pH = -\log_{10}(1.61 \times 10^{-3}) = 2.79.

Explanation:

Because propanoic acid is a weak acid, it does not fully dissociate. We use the KaK_a value to determine the concentration of hydrogen ions at equilibrium before calculating the pHpH.

Problem 2:

Identify which solution has the highest electrical conductivity: 0.1 mol dm−3 HCl0.1\,mol\,dm^{-3}\,HCl, 0.1 mol dm−3 CH3COOH0.1\,mol\,dm^{-3}\,CH_3COOH, or 0.1 mol dm−3 NH30.1\,mol\,dm^{-3}\,NH_3.

Solution:

0.1 mol dm−3 HCl0.1\,mol\,dm^{-3}\,HCl has the highest conductivity.

Explanation:

Electrical conductivity depends on the concentration of mobile ions. HClHCl is a strong acid and dissociates 100%100\% into H+H^+ and Cl−Cl^-, providing the highest total ion concentration compared to the weak acid and weak base, which only partially ionize.

Strong and weak acids and bases - Revision Notes & Key Formulas | IB Grade 12 Chemistry