Buffers Flashcards
What is a buffer solution?
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.
What does a buffer consist of?
A buffer can consist of weak acid - conjugate base or weak base - conjugate acid
How does aqueous ethanoic acid and sodium ethanoate form a buffer?
Ethanoic acid is a weak acid and partially ionises in solution to form a relatively low concentration of ethanoate ions.
Sodium ethanoate (CH₃COONa) is a salt which fully ionises in solution.
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.
CH3COOH ⇌ CH3COO- + H+
How does buffer solution react when H+ ions are added?
When hydrogen ions are added to the solution the pH of the solution would decrease
However, the ethanoate (CH3COO-) ions in the buffer solution react with the hydrogen ions to prevent this and keep the pH constant.
CH3COO- + H+ ⇌ CH3COOH
How does buffer solution react when OH- ions are added?
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.
CH3COOH ⇌ CH3COO- + H+
Why is there a large reverve supply of both CH3COOH and CH3COO- in a buffer?
The concentration of CH3COOH/CH3COO- in solution doesn’t change much when CH3COOH dissociates to form more H+ ions/CH3COO- combines with H+ ions to form CH3COOH
How do you calculate the pH of a buffer solution?
The pH of a buffer solution can be calculated using:
- The Ka of the weak acid
- The equilibrium concentration of the weak acid and its conjugate base (salt)
[H+] = Ka x [CH3COOH] / [CH3COO-]
pH = -log10[H+]
How is pH controlled in the blood?
In humans, HCO3- ions act as a buffer to keep the blood pH between 7.35 and 7.45.
Body cells produce CO2 during aerobic respiration.
This CO2 will combine with water in blood to form a solution containing H+ ions.
CO2 (g) + H2O (l) ⇌ H+ (aq) + HCO3- (aq)
If there is an increase in H+ ions
The equilibrium position shifts to the left until equilibrium is restored
H+ (aq) + HCO3- (aq) ⇌ CO2 (g) + H2O (l)
If there is a decrease in H+ ions
The equilibrium position shifts to the right until equilibrium is restored
CO2 (g) + H2O (l) ⇌ H+ (aq) + HCO3- (aq)
Why can a strong acid not form a buffer solution?
Dissociates completely into ions - not a high enough conc of [HA] left.
Why can a weak acid not form a buffer solution alone?
Not high enough conc of conjugate base [A-]
What must a buffer solution contain?
A buffer solution must contain a high concentration of undissociated acid, [HA] and a high concentration of that acid’s conjugate base , [A-]
