Buffers

Question 1

What are buffers?

Answer 1

• buffers are solutions that can resist changes of pH when small amounts of acid or alkali added to them
• pH still changes slightly but less
  
  • possible to saturate a buffer

Question 2

How do buffers work?

Answer 2

• keep conc of H+ and OH- unchanged
• bases on an eqm reaction which will move in the direction to remove additional H+ and OH- if these are added

Question 3

How do acidic buffers work?

Answer 3

• made from weak aacids and soluble salt of that acid
  
  • maintains pH of below 7
• dissociation of a weak acid is an equilibrium reaction
  *

Question 4

What happens when an alkali is added to an acidic buffer?

Answer 4

if alkali is added, it will react with HA to produce water and A-

this removes OH- so pH remains the same

Question 5

What happens when acid is added an acidic buffer?

Answer 5

• eqm shifts to left, H+ ions cobining with A- ions to produce HA
• since [A-] is small it will run out, so the soln is not a buffer
• however more A- can be added by adding a soluble salt of HA which fully ionises, e.g. Na+A-
• increases A- so more H+ can be used up

Question 6

What is the function of the salt and the weak acid?

Answer 6

• weak acid
  
  • act as a source of HA which can remove any added OH-
• function of salt
  
  • acts as a source A- ions which can remove any added H+

Question 7

Wat is another way of achieving a mixture of weak acid and tis salt?

Answer 7

• neutralising some of the weak acid with an alkali such as sodiu hydroxide
• if you neutralise half the acid, you end up with a buffer whose pH is equal to the pK_a of the acid as it has an equal supply of HA and A-
• at half-neutralisation : pH = pK_a
• useful buffer because it is equally efficient at resisting a change in pH whether acid or alkali is added

Question 8

What is a basic buffer?

Answer 8

• resist change but maintain a pH at above 7
• made from a mxiture of a weak base and a salt of that base

Question 9

Give an example of a basic buffer?

Answer 9

• aqueous ammonia removes added H⁺ to form NH₄⁺
• ammonium ion removes added OH^- to form ammonia and water

Question 10

How does blood act as a buffer?

Answer 10

• buffered to 7.4
  
  • 0.5 change can be fatal
• H⁺ (aq) + HCO₃^- (aq) ⇌ CO₂ (aq) + H₂O (l)
• addition of H+ moves eqm to right
• addition oh OH- removes H+ to form water
  
  • eqm moves to left producing more H+

Question 11

What happens if 0.1M of HCl added to buffer?

Answer 11

• [HA] increases by 0.1M
• [A-] decreases by 0.1M

Question 12

What happens when 0.1M of OH- is added?

Answer 12

• [HA] decrease by 0.1M
• [A-] increase by 0,1M