IB — chemistryHard2 min read

Acids and Bases

Master Brønsted-Lowry theory, pH, pOH, Ka, buffer solutions, and titration curves for IB Chemistry.

Tutor AI Team
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# Acids and Bases (IB)

The IB treatment of acids and bases covers Brønsted-Lowry theory, strong vs weak acids, pH calculations, buffer solutions, and titration curves. At HL, this includes Henderson-Hasselbalch, KaK_a/KbK_b calculations, and polyprotic acids.

1. Brønsted-Lowry Theory

  • Acid: proton (H⁺) donor
  • Base: proton acceptor
  • Amphiprotic: can act as both (e.g. H₂O, HCO₃⁻)
  • Conjugate pair: differ by one proton

2. pH Scale

pH=−log⁡[H+],pOH=−log⁡[OH−]\text{pH} = -\log[\text{H}^+], \quad \text{pOH} = -\log[\text{OH}^-] pH+pOH=14(at 25°C)\text{pH} + \text{pOH} = 14 \quad (\text{at } 25°\text{C}) Kw=[H+][OH−]=1.0×10−14K_w = [\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14}

Strong acids: [H+]=[acid][\text{H}^+] = [\text{acid}] (fully ionised)

Weak acids: Ka=[H+][A−][HA]K_a = \frac{[\text{H}^+][\text{A}^-]}{[\text{HA}]}; [H+]=Ka⋅c[\text{H}^+] = \sqrt{K_a \cdot c}

3. $K_a$ and $K_b$ (HL)

Ka×Kb=KwK_a \times K_b = K_w pKa+pKb=pKw=14pK_a + pK_b = pK_w = 14

Smaller pKapK_a = stronger acid. Larger KaK_a = stronger acid.

4. Buffer Solutions

Weak acid + conjugate base (e.g. CH₃COOH + CH₃COONa)

Henderson-Hasselbalch: pH=pKa+log⁡[A−][HA]\text{pH} = pK_a + \log\frac{[\text{A}^-]}{[\text{HA}]}

Buffers resist pH changes:

  • Add H⁺: reacts with A⁻ → HA
  • Add OH⁻: reacts with HA → A⁻ + H₂O

5. Titration Curves

Titration Equivalence pH Indicator
Strong acid + strong base 7 Any
Weak acid + strong base ~8–10 Phenolphthalein
Strong acid + weak base ~4–5 Methyl orange
Weak acid + weak base No sharp endpoint pH meter

Half-equivalence point: pH = pKapK_a (for weak acid titrations)

6. Practice Questions

    1. Calculate the pH of 0.025 mol/dm³ Ba(OH)₂.
    1. Calculate the pH of 0.10 mol/dm³ ethanoic acid (Ka=1.8×10−5K_a = 1.8 \times 10^{-5}).
    1. A buffer contains 0.15 mol/dm³ NH₃ and 0.20 mol/dm³ NH₄Cl. Calculate pH (pKb=4.74pK_b = 4.74).
    1. Sketch the titration curve for weak acid + strong base and mark the buffer region and half-equivalence point.
    1. Explain why methyl orange is unsuitable for a weak acid–strong base titration.

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Summary

  • pH = −log⁡[H+]-\log[\text{H}^+]; strong acids fully ionise; weak acids use KaK_a
  • Kw=[H+][OH−]=10−14K_w = [\text{H}^+][\text{OH}^-] = 10^{-14}
  • Buffers: Henderson-Hasselbalch equation
  • Titration curves: choose indicator matching equivalence point pH
Tags:
#ib#chemistry#acids-bases#pH#buffers

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