Blood plasma is maintained at a pH of approximately 7.4 by a carbonic acid / hydrogencarbonate buffer system. The relevant equilibrium is: H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq). Explain how this buffer system resists a change in pH when a small amount of hydrochloric acid is added to blood plasma.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Blood plasma is maintained at a pH of approximately 7.4 by a carbonic acid / hydrogencarbonate buffer system. The relevant equilibrium is: H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq).
Model answer (5 marks)
Hydrochloric acid dissociates completely, giving H⁺ ions.
The added H⁺ ions react with the bicarbonate ions (HCO₃⁻) to form carbonic acid.
This consumes H⁺ and shifts the equilibrium to the left.
The concentration of free H⁺ ions therefore remains almost unchanged, so the pH stays close to 7.4.
The system can only buffer small amounts of added acid; large quantities overwhelm the HCO₃⁻ supply and pH falls.
The added H⁺ ions react with the bicarbonate ions (HCO₃⁻) to form carbonic acid.
This consumes H⁺ and shifts the equilibrium to the left.
The concentration of free H⁺ ions therefore remains almost unchanged, so the pH stays close to 7.4.
The system can only buffer small amounts of added acid; large quantities overwhelm the HCO₃⁻ supply and pH falls.
Examiner tips
- Show the dissociation of HCl and the reaction H⁺+HCO₃⁻→H₂CO₃. Explain the shift of the equilibrium. State that the pH stays ~7.4 and note the limited capacity.
Common mistakes
- Forgetting that HCl fully dissociates. Not mentioning the shift to the left. Claiming the buffer works for any amount of acid.
Mark scheme (5 marks)
- Hydrochloric acid fully dissociates / the added acid provides extra H⁺ ions
- The hydrogencarbonate ions (HCO₃⁻) react with / remove the added H⁺ ions
- The equilibrium shifts to the left
- The concentration of H⁺ ions remains approximately constant / the pH remains approximately 7.4
- The buffer only works for small / limited amounts of added acid, not large quantities
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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