Explain buffer action and bicarbonate in blood
Use equations to show how buffers respond to added acid or alkali and how bicarbonate helps control blood pH.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 25. Use the equilibrium expressions and data stated in the question; written practice is self-checked and human subject-teacher review remains pending.
Before you begin
- Use AS equilibrium expressions, mole calculations, logarithms, and Brønsted–Lowry acid–base ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which component of an HA/A⁻ buffer reacts with added H⁺?
What you will learn
- Explain acidic-buffer response to added H⁺
- Explain acidic-buffer response to added OH⁻
- Describe the H₂CO₃/HCO₃⁻ system's role in blood pH control
Conjugate components remove added acid or alkali
For HA/A⁻, added H⁺ is removed by A⁻ + H⁺ ⇌ HA. Added OH⁻ is removed by HA + OH⁻ → A⁻ + H₂O. These reactions reduce the change in free [H⁺], so the pH changes less than it would in water alone.
A buffer has finite capacity. If too much acid or alkali is added, one component is used up and pH can then change sharply. A buffer reduces the change; it does not keep pH exactly constant.
Hydrogencarbonate helps control blood pH
The relevant equilibrium can be represented as CO₂(aq) + H₂O(l) ⇌ H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq). Hydrogencarbonate can accept added H⁺ to form carbonic acid; the linked equilibria help limit changes in blood hydrogen-ion concentration.
When acid is added, HCO₃⁻ removes some H⁺. When alkali is added, the acid side of the pair supplies H⁺ that reacts with OH⁻ to form water. Keep the explanation tied to equilibrium and the buffer pair; do not claim that blood pH cannot change.
Pause and explain
What does a buffer do when a small amount of OH⁻ is added?
Worked example
Use equations to explain how an HA/A⁻ buffer responds to added HCl and added NaOH.
Show the worked solution
- HCl adds H⁺, which reacts with A⁻.
- NaOH adds OH⁻, which reacts with HA.
- Write both reactions and explain the pH effect.
Answer Added HCl: A⁻ + H⁺ → HA. Added NaOH: HA + OH⁻ → A⁻ + H₂O. Each reaction removes much of the added species, so the pH change is limited while both components remain.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the reaction that removes added H⁺ from an ethanoic acid/ethanoate buffer.
Give me a hint
Use the conjugate base as the proton acceptor.
Compare my reasoning
- Identify CH₃COO⁻ as the base.
- Add H⁺ to it.
- Form the conjugate acid.
CH₃COO⁻ + H⁺ → CH₃COOH.
Look for these in your work
- Conjugate base used
- Atoms and charge balanced
- Added H⁺ consumed
Explain how an ethanoic acid/ethanoate buffer limits a small addition of NaOH.
Give me a hint
The weak acid reacts with OH⁻.
Compare my reasoning
- Write CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O.
- Identify that OH⁻ is consumed.
- State why pH changes less while both components remain.
Ethanoic acid neutralises added OH⁻ to form ethanoate and water. This lowers the increase in free hydroxide and limits the pH rise until the acid component is depleted.
Look for these in your work
- Equation written
- Hydroxide consumption explained
- Finite buffer capacity acknowledged
Use the bicarbonate buffer pair to explain how added acid is partly removed from blood.
Give me a hint
Follow what HCO₃⁻ does when extra H⁺ is present.
Compare my reasoning
- Hydrogencarbonate accepts H⁺.
- H₂CO₃ forms and is linked to CO₂ and water by equilibrium.
- Connect H⁺ removal to a limited pH change.
HCO₃⁻ + H⁺ ⇌ H₂CO₃. The linked CO₂/H₂O/H₂CO₃ equilibrium helps remove some added H⁺, limiting the blood pH decrease while the buffer has capacity.
Look for these in your work
- HCO₃⁻ identified as proton acceptor
- Conjugate acid formed
- Role and limitation stated
Common mistakes
- Saying a buffer prevents every pH change
- Using the acid component to remove added H⁺
- Omitting equations when explaining buffer action
- Treating the blood bicarbonate system as a single irreversible reaction
What can you explain now?
Which component of an HA/A⁻ buffer reacts with added H⁺?
Compare with the explanation
A⁻
The conjugate base accepts H⁺ to form HA.
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Make it stick
How do the two members of a buffer pair remove small additions of H⁺ and OH⁻?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.