6.1.16Additional lessons
Buffer Solutions (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 130 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4What a Buffer Is
Buffers are used wherever a steady pH is vital, such as in blood and in many laboratory experiments.
Definition
- Definition A buffer is a solution that resists changes in pH when small amounts of acid or base are added.
Acidic buffer
- Acidic buffer This contains a weak acid and its conjugate base, for example CH₃COOH with CH₃COONa.
Basic buffer
- Basic buffer This contains a weak base and its conjugate acid, for example NH₃ with NH₄Cl.
Preparation
- Preparation A buffer can be made by mixing a weak acid with its salt, or by partly neutralising a weak acid with a strong base.
Section 2 of 4How a Buffer Works
How a Buffer Works
Adding acid
- Adding acid Added H⁺ is removed by the conjugate base: A⁻ + H⁺ → HA, so the pH changes very little.
Adding alkali
- Adding alkali Added OH⁻ is removed by the weak acid: HA + OH⁻ → A⁻ + H₂O, again keeping the pH nearly constant.
Equilibrium shift
- Equilibrium shift The HA ⇌ H⁺ + A⁻ equilibrium shifts, but the ratio [A⁻]/[HA] changes only slightly.
Blood
- Blood The H₂CO₃ / HCO₃⁻ pair keeps blood close to pH 7.4.
| Buffer | Weak component | Conjugate component |
|---|---|---|
| Acidic | CH₃COOH | CH₃COO⁻ from CH₃COONa |
| Basic | NH₃ | NH₄⁺ from NH₄Cl |
| Blood | H₂CO₃ | HCO₃⁻ |
Section 3 of 4Limits and Capacity
1
A buffer works well only while both components are still present in significant amounts.
2
If too much acid is added, the conjugate base is used up and the pH falls sharply.
3
If too much alkali is added, the weak acid is used up and the pH rises quickly.
4
Higher concentrations of the two components give a larger buffer capacity.
5
Dilution with water changes neither the ratio nor the pH much, but it does reduce the capacity.
Section 4 of 4Exam Checklist & Tips
6.1.16 – Key Points for the Exam
- Buffer solution A solution that resists changes in pH on the addition of small amounts of acid or base.
- Acidic buffer A mixture of a weak acid and its conjugate base.
- Basic buffer A mixture of a weak base and its conjugate acid.
- Buffer capacity The amount of acid or base a buffer can absorb before the pH changes significantly.
Common Mistakes
- Saying a buffer prevents any change in pH.
- Making a buffer from a strong acid and its salt.
- Thinking that dilution changes the buffer pH greatly.
- Forgetting that both components must be present in large amounts.
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