12: Buffers, Titrations and pH Curves

Question 1

What is a buffer?

Answer 1

A solution that resists changes in pH when small amounts of acid or alkali are added.

Question 2

Describe the pH of an acidic buffer.

Answer 2

Have a pH of less than 7.

Question 3

What is contained in an acidic buffer?

Answer 3

A mixture of a weak acid and a salt of that weak acid.

Question 4

What is contained in a basic buffer?

Answer 4

A mixture of a weak base and a salt of that base.

Question 5

Give an equation for the dissociation of ethanoic acid (CH3COOH) .

Answer 5

CH3COOH (reversible) CH3COO- + H+

Question 6

Give an example of a salt used in a buffer made of ethanoic acid.

Answer 6

CH3COONa

Question 7

What happens when H+ is added to an acidic buffer?

Answer 7

Extra H+ ions combine with the CH3COO- ions.
This reduces the H+ ion concentration closely back to its original level causing pH to remain constant.

Question 8

What happens when -OH is added to an acidic buffer?

Answer 8

Extra -OH ions combine with the CH3COO- ions and H+ to make water, so the -OH concentration is reduced back to its original level and pH remains constant.

Question 9

If ammonia is used as a weak base, name the salt that would be present in the buffer.

Answer 9

NH4Cl

Question 10

What does the salt do when in the solution of ammonia?

Answer 10

Dissociates fully.

Question 11

Give an ionic equation for the dissociation of NH4Cl.

Answer 11

NH4Cl -> NH4+ + Cl-

Question 12

Give an equation for the ammonia molecules NH3 reacting with water molecules.

Answer 12

NH3 + H2O (reversible) NH4+ + OH-

Question 13

What happens when a small amount of acid is added to a basic buffer? (What forms?)
What happens to the equilibrium position?

Answer 13

The H+ concentration increases, increasing the acidity of the solution.
Some H+ ions react with the -OH ions to make H2O.
Equilibrium moves to the right to replace -OH ions that have been used up and remove the extra H+ added, keeping pH constant.

Question 14

What happens when a small amount of base is added to a solution?
The additional -OH ions react with which part of the buffer solution to produce water?
What happens to position of equilibrium?

Answer 14

-OH concentration increases making the solution more alkaline.
NH4+ ions react with the extra -OH to form NH3 and H2O.
Equilibrium shifts left to remove -OH ions from the solution, preventing change in pH.

Question 15

What happens when you dilute a buffer with water?

Answer 15

The water slightly dissociates so the extra H+ and -OH ions push the equilibrium the same amount in both directions leaving it unchanged.

Question 16

Explain why biological washing powders use buffers.

Answer 16

They contain enzymes, so buffers help to maintain the optimum pH for the enzymes so they don’t denature.

Question 17

Why are buffers used in shampoos?

Answer 17

They increase the roughness of individual hairs surfaces to keep hair smooth and shiny.

Question 18

Give an example of why buffers are used in the body.

Answer 18

An optimum pH is needed in the blood to ensure cells can work correctly.

Question 19

Describe the shape of a pH curve with a strong acid, strong base.

Answer 19

Curve begins low and rises to 14

Question 20

Describe the shape of a pH curve with a strong acid, weak base.

Answer 20

Curve begins low and ends near pH 8/9

Question 21

Describe the shape of a pH curve with a weak acid, strong base.

Answer 21

Curve begins around pH 5 and ends around pH 14.

Question 22

Describe the shape of a pH curve with a weak acid, weak base.

Answer 22

pH starts around 5 and ends around pH 9.

Question 23

Describe the shape of a pH curve with a strong base, strong acid.

Answer 23

Curve starts at pH 14 and ends at pH 0.

Question 24

Describe the shape of a pH curve with a strong base, weak acid.

Answer 24

Curve starts at pH 14 and ends at pH 5.

Question 25

Describe the shape of a pH curve with a weak base, strong acid.

Answer 25

Curve starts around pH 8 and ends around pH 1.

Question 26

Describe the shape of a pH curve with a weak base, weak acid.

Answer 26

Curve begins around pH 9 and ends at pH 5.

Question 27

What is the midpoint on the vertical section of the pH curve called?

Answer 27

Equivalence point.

Question 28

What is the equivalence point?

Answer 28

The point when the base or acid completely neutralises the solution.

Question 29

How do you select the correct indicator needed for a titration?

Answer 29

The indicators pH range must fall within the vertical section on a pH curve.