4.5.8Unit 4. Rates, Equilibria and Further Organic Chemistry
Buffers — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 40 of 56 Unit quiz
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.
Part 1 of 4What a Buffer Is
A buffer keeps pH almost constant when small amounts of acid or alkali are added.
Definition
- Definition A buffer solution resists changes in pH when small amounts of acid, base or water are added to it.
Acidic buffer
- Acidic buffer A weak acid and one of its salts, such as ethanoic acid with sodium ethanoate, gives a buffer below pH 7.
Basic buffer
- Basic buffer A weak base and its salt, such as ammonia with ammonium chloride, gives a buffer with pH above 7.
Two reservoirs
- Two reservoirs The mixture has a large supply of both the weak acid HA and its conjugate base A⁻ available to react.
Part 2 of 4How It Works
In CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq), the salt supplies lots of ethanoate ions.
1
When a little acid is added, extra H⁺ ions react with the large supply of ethanoate ions and form more ethanoic acid.
2
This pushes the position of equilibrium to the left, removing most of the added H⁺, so the pH hardly changes.
3
When a little alkali is added, OH⁻ ions react with H⁺ to form water, which removes H⁺ from the equilibrium.
4
The ethanoic acid then dissociates further, replacing the H⁺ that was removed, and the pH stays almost the same.
Part 3 of 4Making and Using Buffers
Making and Using Buffers
Partial neutralisation
- Partial neutralisation Adding less than the equivalent amount of NaOH to a weak acid produces a mixture of acid and salt.
Buffer capacity
- Buffer capacity A buffer can only cope with limited additions;
- once one component is used up, the pH changes rapidly.
Blood
- Blood Blood is buffered by carbonic acid and hydrogencarbonate ions to keep pH close to 7.4.
Uses
- Uses Buffers are used in shampoos, foods, biological experiments and calibration of pH meters.
Part 4 of 4Exam Checklist & Tips
4.5.8 – Key Points for the Exam
- Buffer Solution that resists changes in pH.
- Conjugate base The species formed when an acid loses a proton.
- Buffer capacity How much acid or base a buffer can absorb before its pH changes greatly.
- Salt Ionic compound formed from an acid and a base.
Common Mistakes
- Saying a buffer prevents any change in pH.
- Describing a buffer as made from a strong acid and its salt.
- Forgetting that the salt provides the large reservoir of conjugate base.
- Explaining the response to alkali without saying H⁺ is replaced.
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