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4.5.3Unit 4. Rates, Equilibria and Further Organic Chemistry

Acid Strength — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 35 of 56 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Part 1 of 5Strong and Weak

Strength describes how far an acid dissociates in water and is not the same as concentration.

Strong acid
  • Strong acid A strong acid such as HCl or HNO₃ dissociates almost completely in water, producing a high concentration of H⁺ ions.
Weak acid
  • Weak acid A weak acid such as ethanoic acid dissociates only partly, so most molecules remain undissociated in the solution.
Concentration
  • Concentration Concentration says how much acid is dissolved;
    • a dilute strong acid can have a higher pH than a concentrated weak acid.
Equation
  • Equation A weak acid is shown with a reversible arrow: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq).

Part 2 of 5Acid Dissociation Constant

The equilibrium constant for dissociation measures acid strength numerically.

Acid Dissociation Constant
Ka expression
  • Ka expression For HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻] / [HA], with units mol dm⁻³.
Larger Ka
  • Larger Ka A larger Ka means more dissociation, so the acid is stronger.
pKa
  • pKa pKa = −log₁₀ Ka, so a smaller pKa means a stronger acid.
Convert back
  • Convert back Ka = 10⁻ᵖᴷᵃ, so pKa 4.76 corresponds to Ka of 1.74 × 10⁻⁵ mol dm⁻³.
AcidKa / mol dm⁻³pKa
HCOOH1.6 × 10⁻⁴3.80
CH₃COOH1.74 × 10⁻⁵4.76
HF5.6 × 10⁻⁴3.25

Part 3 of 5Comparing Acids

1
Write the Ka expression for each acid, making sure that the dissociation equation is correct.
2
Compare Ka values directly or convert to pKa; remember that a lower pKa means a stronger acid.
3
Rank the acids from strongest to weakest, using Ka or pKa values for the same temperature.
4
Explain that a stronger acid has a higher concentration of H⁺ ions than a weaker acid at the same concentration.

Part 4 of 5Worked Example

Question Calculate the pKa of methanoic acid, Ka = 1.6 × 10⁻⁴ mol dm⁻³, and decide whether it is stronger than ethanoic acid with pKa 4.76.

  1. Use pKa = −log₁₀ Ka.
  2. SubstitutepKa = −log₁₀(1.6 × 10⁻⁴) = 3.80.
  3. Compare3.80 is smaller than 4.76.

Answer pKa = 3.80, so methanoic acid is the stronger acid.

Part 5 of 5Exam Checklist & Tips

4.5.3 – Key Points for the Exam

  • Strong acid Acid that is almost completely dissociated in water.
  • Weak acid Acid that is only partially dissociated in water.
  • Ka Acid dissociation constant.
  • pKa The negative logarithm of Ka.

Common Mistakes

  • Mixing up strength with concentration.
  • Saying a higher pKa shows a stronger acid.
  • Writing Ka with the undissociated acid on top.
  • Giving Ka no units or the wrong units.

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