Buffer Solutions - Action
- A buffer solution is a solution which resists changes in pH when small amounts of acids or alkalis are added
- A buffer solution is used to keep the pH almost constant
- A buffer can consists of weak acid - conjugate base or weak base - conjugate acid
Ethanoic acid & sodium ethanoate as a buffer
- A common buffer solution is an aqueous mixture of ethanoic acid and sodium ethanoate
- Ethanoic acid is a weak acid and partially ionises in solution to form a relatively low concentration of ethanoate ions
- Sodium ethanoate is a salt which fully ionises in solution
- There are reserve supplies of the acid (CH3COOH) and its conjugate base (CH3COO-)
- The buffer solution contains relatively high concentrations of CH3COOH (due to partial ionisation of ethanoic acid) and CH3COO- (due to full ionisation of sodium ethanoate)
- In the buffer solution, the ethanoic acid is in equilibrium with hydrogen and ethanoate ions
- When H+ ions are added:
- The equilibrium position shifts to the left as H+ ions react with CH3COO- ions to form more CH3COOH until equilibrium is re-established
- As there is a large reserve supply of CH3COO- the concentration of CH3COO- in solution doesn’t change much as it reacts with the added H+ ions
- As there is a large reserve supply of CH3COOH the concentration of CH3COOH in solution doesn’t change much as CH3COOH is formed from the reaction of CH3COO- with H+
- As a result, the pH remains reasonably constant
When hydrogen ions are added to the solution the pH of the solution would decrease; However, the ethanoate ions in the buffer solution react with the hydrogen ions to prevent this and keep the pH constant
- When OH- ions are added:
- The OH- reacts with H+ to form water
OH- (aq) + H+ (aq) → H2O (l)
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- The H+ concentration decreases
- The equilibrium position shifts to the right and more CH3COOH molecules ionise to form more H+and CH3COO- until equilibrium is re-established
CH3COOH (aq) → H+ (aq) + CH3COO- (aq)
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- As there is a large reserve supply of CH3COOH the concentration of CH3COOH in solution doesn’t change much when CH3COOH dissociates to form more H+ ions
- As there is a large reserve supply of CH3COO- the concentration of CH3COO- in solution doesn’t change much
- As a result, the pH remains reasonably constant
When hydroxide ions are added to the solution, the hydrogen ions react with them to form water; The decrease in hydrogen ions would mean that the pH would increase however the equilibrium moves to the right to replace the removed hydrogen ions and keep the pH constant
Worked example
Which of these mixtures would form a buffer solution with a pH below 7?
☐ | A | NaOH (aq) and excess HCl (aq) | |
☐ | B | NaOH (aq) and excess CH3COOH (aq) | |
☐ | C | excess NaOH (aq) and HCl (aq) | |
☐ | D | excess NaOH (aq) and CH3COOH (aq) |
Answer:
- B / NaOH (aq) and excess CH3COOH (aq)
This is because:
- A buffer with a pH lower than 7 can be made from a weak acid and its salt or by partial neutralisation of a weak acid
- B contains a weak acid that is in excess and NaOH, so the weak acid will be partially neutralised leaving a solution that contains a weak acid and its conjugate base - this is a buffer solution
- All of the other options are incorrect because the solutions would not form a buffer