Buffers (Edexcel International A Level Chemistry)

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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

Buffers equation 1

  • Sodium ethanoate is a salt which fully ionises in solution

Buffers equation 2

  • 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

Buffers equation 3

  • 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

Equilibria - Effect of adding H+, downloadable AS & A Level Chemistry revision notes

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)

    • 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)

    • 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

Buffer Solutions - Applications

Controlling the pH of 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)

  • This equilibrium between CO2 and HCO3- is extremely important
  • If the concentration of H+ ions is not regulated, the blood pH would drop and cause ‘acidosis
    • Acidosis refers to a condition in which there is too much acid in the body fluids such as blood
    • This could cause body malfunctioning and eventually lead to coma

  • If there is an increase in H+ ions
  • The equilibrium position shifts to the left until equilibrium is restored

CO2 (g) + H2O (l) ⇌ H+ (aq) + HCO3- (aq)

  • This reduces the concentration of H+ and keeps the pH of the blood constant
  • 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)

  • This increases the concentration of H+ and keeps the pH of the blood constant

Buffers in food

  • A combination of various factors such as light, heat, oxygen and various kinds of microorganisms can spoil food
    • We try to reduce this by keeping food in airtight containers and/or in refrigerators
  • Microorganisms spoiling food depends on the pH value of the food.
  • Most microorganisms thrive when the pH of their surroundings is around neutral (pH 6.6 - 7.5)
  • Most bacteria can survive at pH values as low as 4.4 and as high as 9.0
  • An important factor in whether food spoils is its buffer capacity
    • The buffer capacity of food is a measure of the amount of acid or base required to significantly change the pH of the food
  • The more protein there is in the food, the higher the buffer capacity
    • This is because the amino acids present have both acidic and basic properties
    • This means it takes longer for the pH of the food to change enough for the bacteria to begin to multiply

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.