Edexcel A Level Chemistry:复习笔记5.2.7 Buffers

• 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)

• • 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

• 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)

   

• To determine the pH, the concentration of hydrogen ions is needed which can be found using the equilibrium expression

• To simplify the calculations, logarithms are used such that the expression becomes:

• Since -log10 [H+] = pH, the expression can also be rewritten as:

• This is known as the Hendersen-Hasselbalch equation

Answer

Ethanoic acid is a weak acid that ionises as follows:

CH3COOH (aq) ⇌ H+ (aq) + CH3COO- (aq)

Step 1: Write down the equilibrium expression to find Ka

Step 2: Rearrange the equation to find [H+]

Step 3: Substitute the values into the expression

   = 1.02 x 10-5 mol dm-3

Step 4: Calculate the pH

   pH = - log [H+]

   = -log 1.02 x 10-5

   = 4.99

How to make a buffer solution with a required pH

• To make a buffer solution with a pH of less than 7, you need to use a mixture of a weak acid and its conjugate base
• Conversely, you can make a buffer solution with a pH greater than 7 by using a mixture of a weak base and its conjugate acid
• Imagine we want to make a buffer solution with a pH of 5.00 at a temperature of 298K
• This would require a hydrogen ion concentration of

      [H+(aq)] = 1.00 x 10-5 mol dm-3

• The hydrogen ion concentration of a buffer solution of a weak acid and its conjugate base is calculated using the formula

• We will use ethanoic acid as our weak acid of choice

      Ka = 1.74 x 10-5 mol dm-3

• Substituting our known values into the equation we get

• This gives a value for the ratio of the concentrations of acid and base needed in our buffer solution

• Mixing an equal volume of ethanoic acid with a concentration of 0.575 mol dm-3 and a sodium ethanoate solution of 1.00 mol dm-3 would allow us to make this buffer solution
• This would give us a solution with an acid concentration of 0.2875 mol dm-3 and a salt concentration of 0.500 mol dm-3

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

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

• 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