Buffer Action

Question 1

What is a buffer?

Answer 1

A buffer is a solution that resists changes in pH when small amounts of acid or base are added, or when it’s diluted

Question 2

What are the two types of buffer?

Answer 2

1. Acidic buffers

2. Basic buffers

Question 3

What are acidic buffers?

Answer 3

Acidic buffers have a pH of less than 7 - they’re made by mixing a weak acid with one of its salts

Question 4

Give an example of an acidic buffer

Answer 4

Ethanoic acid and sodium ethanoate. The ethanoic acid is a weak acid so it only dissociates slightly:

CH3COOH —– H+ + CH3COO-

But the salt fully dissociates into its ions when it dissolves:

CH3COONa + water —– CH3COO- + Na+

So in the solution you have a lot of undissociated ethanoic acid molecules, and lots of ethanoate ions from the salts

Question 5

What happens when you alter the concentration of H+ ions in the acidic buffer solution of ethanoic acid and sodium ethanoate

Answer 5

CH3COOH —– H+ + CH3COO-

If a small amount of acid is added, the H+ concentration increases. Most of the extra H+ ions combine with the CH3COO- ions to form CH3COOH. This shifts the equilibrium to the left, reducing the H+ concentration close to its original value, so the pH doesn’t change

Question 6

What happens when you alter the concentration of OH- ions in the acidic buffer solution of ethanoic acid and sodium ethanoate

Answer 6

CH3COOH —– H+ + CH3COO-

If a small amount of base is added, the OH- concentration increases. Most of the OH- ions react with the H+ ions to form water - removing H+ ions from the solution. This causes more CH3COOH to dissociate to form H+ ions - shifting the equilibrium to the right. The H+ concentration increases until it’s close to its original value, so the pH doesn’t change

Question 7

What are basic buffers?

Answer 7

Basic buffers have a pH greater than 7 - they’re made by mixing a weak base with one of its salts

Question 8

Give an example of a basic buffer

Answer 8

Ammonia and ammonium chloride. The salt fully dissociates in solution:

NH4Cl —– NH4+ + Cl-

Some of the ammonia molecules will react with the water molecules:

NH3 + water —– NH4+ + OH-

So the solution will contain loads of ammonium ions and lots of ammonia molecules

Question 9

What happens if you alter the concentration of OH- ions in the basic buffer solution of ammonia and ammonium chloride?

Answer 9

NH3 + H20 —– NH4+ + OH-

If a small amount of base is added, the OH- concentration increases. Most of the extra OH-ions will react with the NH4+ ions, to form NH3 and H2O. So the equilibrium will shift to the left, removing OH- ions from the solution, stopping the pH from changing

Question 10

What happens if you alter the concentration of H+ ions in the basic buffer solution of ammonia and ammonium chloride?

Answer 10

NH3 + H2O —– NH4+ + OH-

If a small amount of acid is added, the H+ concentration increases. Some of the H+ ions react with the OH- to make H2O. When this happens the equilibrium moves to the right to replace the OH- ions that have been used up. Some of the H+ ions react with NH3 molecules to form NH4+. These reactions remove most of the extra H+ ions so the pH won’t change

Question 11

Give 3 examples of real life uses of buffers

Answer 11

1. Most shampoos contain a pH 5.5 buffer - human hair becomes rougher if it’s exposed to alkaline conditions so the buffer in the shampoo stops this
2. Biological washing powers contain buffers - they keep the pH at the right level for the enzymes to work
3. There are lots of biological buffers in our bodies to keep our tissues at the right pH - blood contains a buffer solution keeping it at pH 7.4