Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 14.6–14.7 Strong acids and pH of strong acids-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 14.6–14.7 Strong acids and pH of strong acids- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 14.6–14.7 Strong acids and pH of strong acids- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
14.6 Strong vs Weak Acids (Degree of Dissociation)
The difference between strong and weak acids is based on how completely they dissociate (ionise) in aqueous solution. This is described by the degree of dissociation.
Degree of Dissociation
The degree of dissociation is the fraction of acid molecules that ionise to produce \( \mathrm{H^+} \) ions in solution.
(i) Strong Acids
A strong acid completely dissociates in aqueous solution.
General Equation
\( \mathrm{HA(aq) \rightarrow H^+(aq) + A^-(aq)} \)
Key Features
- Degree of dissociation ≈ 100%
- Essentially all acid molecules ionise.
- High concentration of \( \mathrm{H^+} \).
- Reaction goes to completion (irreversible arrow often used).
Examples
- \( \mathrm{HCl} \), \( \mathrm{HNO_3} \), \( \mathrm{H_2SO_4} \) (first dissociation)
(ii) Weak Acids
A weak acid partially dissociates in aqueous solution.
General Equation
\( \mathrm{HA(aq) \rightleftharpoons H^+(aq) + A^-(aq)} \)
Key Features
- Degree of dissociation is small (typically < 5%).
- Most acid molecules remain undissociated.
- Lower concentration of \( \mathrm{H^+} \).
- Establishes a dynamic equilibrium.
Examples
- \( \mathrm{CH_3COOH} \), \( \mathrm{H_2CO_3} \)
Comparison
- Strong acids: complete dissociation, high \( \mathrm{[H^+]} \).
- Weak acids: partial dissociation, lower \( \mathrm{[H^+]} \).
- Strength depends on extent of ionisation, not concentration.
Important Distinction
- Strength ≠ Concentration
- A dilute strong acid can have lower \( \mathrm{[H^+]} \) than a concentrated weak acid.
Key Features
- Strong acids fully dissociate; weak acids partially dissociate.
- Weak acids establish equilibrium.
- Degree of dissociation determines acid strength.
Example 1:
Explain why \( \mathrm{HCl} \) is a strong acid but \( \mathrm{CH_3COOH} \) is a weak acid.
▶️ Answer/Explanation
\( \mathrm{HCl} \) completely dissociates in water, producing a high concentration of \( \mathrm{H^+} \) ions.
\( \mathrm{CH_3COOH} \) only partially dissociates, so most molecules remain undissociated.
Therefore, \( \mathrm{HCl} \) has a much higher degree of dissociation and is stronger.
Example 2:
A 0.10 mol \( \mathrm{dm^{-3}} \) strong acid and a 0.10 mol \( \mathrm{dm^{-3}} \) weak acid are compared. Explain the difference in pH.
▶️ Answer/Explanation
The strong acid fully dissociates, so \( \mathrm{[H^+] = 0.10} \).
The weak acid only partially dissociates, so \( \mathrm{[H^+]} \) is much lower.
Therefore, the strong acid has a lower pH.
14.7 Calculating the pH of a Strong Acid
The pH of a strong acid can be calculated directly because strong acids completely dissociate in aqueous solution. This means the hydrogen ion concentration can be determined directly from the acid concentration.
Key Principle
For a strong acid: \( \mathrm{[H^+] = \text{acid concentration} \times \text{number of } H^+ \text{ released per molecule}} \)
Step-by-Step Method
- Identify the strong acid and its concentration.
- Determine how many \( \mathrm{H^+} \) ions are released per molecule.
- Calculate \( \mathrm{[H^+]} \).
- Use:
\( \mathrm{pH = -\log_{10}[H^+]} \)
Case 1: Monoprotic Strong Acids
(e.g. \( \mathrm{HCl}, \mathrm{HNO_3} \))
- Release 1 mole of \( \mathrm{H^+} \) per mole of acid.
- \( \mathrm{[H^+] = \text{acid concentration}} \)
Case 2: Polyprotic Strong Acids
(e.g. \( \mathrm{H_2SO_4} \), first dissociation strong)
- May release more than one \( \mathrm{H^+} \).
- Multiply concentration accordingly.
Key Features
- Complete dissociation simplifies calculation.
- No equilibrium considerations needed.
- Must account for number of protons released.
Example 1:
Calculate the pH of \( \mathrm{0.010\ mol\ dm^{-3}} \) \( \mathrm{HCl} \).
▶️ Answer/Explanation
\( \mathrm{HCl} \) is a strong acid and fully dissociates.
\( \mathrm{[H^+] = 0.010} \)
\( \mathrm{pH = -\log_{10}(0.010)} \)
\( \mathrm{pH = 2.00} \)
Example 2:
Calculate the pH of \( \mathrm{0.050\ mol\ dm^{-3}} \) \( \mathrm{H_2SO_4} \) (assume full dissociation of both protons).
▶️ Answer/Explanation
Each molecule produces 2 \( \mathrm{H^+} \).
\( \mathrm{[H^+] = 2 \times 0.050 = 0.100} \)
\( \mathrm{pH = -\log_{10}(0.100)} \)
\( \mathrm{pH = 1.00} \)