Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 14.17–14.18 Buffer solutions and buffer action-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 14.17–14.18 Buffer solutions and buffer action- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 14.17–14.18 Buffer solutions and buffer action- 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
14.17 Buffer Solution
A buffer solution is a system that resists changes in pH when small amounts of acid or base are added. Buffers are essential in maintaining stable pH conditions in chemical and biological systems.
Definition
A buffer solution is a solution that minimises changes in pH when small amounts of acid or alkali are added.
Composition of a Buffer
- Weak acid + its conjugate base (e.g. \( \mathrm{CH_3COOH / CH_3COO^-} \))
- Weak base + its conjugate acid (e.g. \( \mathrm{NH_3 / NH_4^+} \))
How a Buffer Works
When acid is added:
- The conjugate base reacts with added \( \mathrm{H^+} \).
- Removes excess \( \mathrm{H^+} \), limiting pH change.
\( \mathrm{A^- + H^+ \rightarrow HA} \)
When base is added:
- The weak acid reacts with \( \mathrm{OH^-} \).
- Neutralises added base.
\( \mathrm{HA + OH^- \rightarrow A^- + H_2O} \)
Key Features
- Contains both weak acid/base and its conjugate.
- Resists pH changes.
- Works best when concentrations of both components are similar.
Limitations
- Only effective for small additions of acid/base.
- Large additions will overwhelm the buffer.
Example 1:
Explain why a mixture of \( \mathrm{CH_3COOH} \) and \( \mathrm{CH_3COONa} \) acts as a buffer solution.
▶️ Answer/Explanation
It contains a weak acid (\( \mathrm{CH_3COOH} \)) and its conjugate base (\( \mathrm{CH_3COO^-} \)).
The conjugate base reacts with added \( \mathrm{H^+} \), while the acid reacts with \( \mathrm{OH^-} \).
This resists changes in pH.
Example 2:
Explain how a buffer resists a change in pH when a small amount of \( \mathrm{NaOH} \) is added.
▶️ Answer/Explanation
The added \( \mathrm{OH^-} \) reacts with the weak acid component of the buffer.
This forms the conjugate base and water.
As a result, the increase in pH is minimal.
14.18 Action of a Buffer Solution
The action of a buffer solution refers to how it resists changes in pH when small amounts of acid or base are added. This is explained using equilibrium principles and Brønsted–Lowry theory.
Composition of a Buffer
- Weak acid and its conjugate base: \( \mathrm{HA / A^-} \)
- Example: \( \mathrm{CH_3COOH / CH_3COO^-} \)
Equilibrium present:
\( \mathrm{HA \rightleftharpoons H^+ + A^-} \)
Action When Acid is Added
- Added \( \mathrm{H^+} \) increases \( \mathrm{[H^+]} \).
- Equilibrium shifts to the left (Le Chatelier’s Principle).
- Conjugate base removes \( \mathrm{H^+} \):
\( \mathrm{A^- + H^+ \rightarrow HA} \)
Result:
- Most added \( \mathrm{H^+} \) is neutralised.
- Small change in pH.
Action When Base is Added
- Added \( \mathrm{OH^-} \) reacts with \( \mathrm{H^+} \):
\( \mathrm{H^+ + OH^- \rightarrow H_2O} \)
- This lowers \( \mathrm{[H^+]} \).
- Equilibrium shifts to the right.
- Weak acid dissociates to replace \( \mathrm{H^+} \):
\( \mathrm{HA \rightarrow H^+ + A^-} \)
Result:
- \( \mathrm{H^+} \) concentration is restored.
- pH changes only slightly.
Key Explanation
- Buffer action is explained by equilibrium shift.
- Conjugate base removes added acid.
- Weak acid replaces removed \( \mathrm{H^+} \) when base is added.
Important Conditions
- Both components must be present in significant amounts.
- Works only for small additions of acid/base.
Key Features
- Based on reversible equilibrium.
- Uses conjugate acid–base pair.
- Minimises pH change by neutralisation and equilibrium shift.
Example 1:
Explain how a buffer solution resists a decrease in pH when a small amount of acid is added.
▶️ Answer/Explanation
The added \( \mathrm{H^+} \) is removed by reaction with the conjugate base.
This forms more weak acid.
As a result, the increase in \( \mathrm{[H^+]} \) is small, so pH changes only slightly.
Example 2:
Explain the role of equilibrium in buffer action when alkali is added.
▶️ Answer/Explanation
Added \( \mathrm{OH^-} \) removes \( \mathrm{H^+} \), decreasing its concentration.
The equilibrium shifts to the right to produce more \( \mathrm{H^+} \).
This restores the pH and limits the change.