Unit 2-Acid/Base Equilibria

Question 1

a) In water and aqueous solutions, there is an equilibrium between which substances?
b) What is the dissociation equation for the above equilibrium?
c) What is the equation for calculating the dissociation constant for the above equilibrium?

Answer 1

a) There is an equilibrium between the water molecules and hydronium (hydrogen) and hydroxide ions.
b) 2H₂O ⇌ H₃O⁺ + OH^-

H₂O ⇌ H⁺ + OH^-

c) K_w = [H⁺][OH^{<span>-</span>}]

In data booklet

Question 2

The data booklet states the value of the equilibrium constant/ionic product of water at 24 ºC is 10⁻¹⁴

What must the concentrations of hydrogen and hydroxide be at equilibrium?

Answer 2

K_^w = [H⁺][OH^-]

10^-14 = [H⁺][OH^-]

Both must have a concentration of 10^-7 moll^-1

Question 3

Which equation shows the relationship between pH and the hydrogen ion concentration?

Answer 3

pH = -log₁₀[H⁺]

[H⁺] = 10^-pH

Question 4

What are the Brønsted-Lowry definitions of acids and bases?

Answer 4

An acid is a proton donor

A base is a proton acceptor

Question 5

What is formed when an acid loses a proton?

What is formed when a base gains a proton?

Answer 5

Conjugate base

Conjugate acid

Question 6

What is the difference between strong and weak acids/bases?

Answer 6

Strong acids/bases are completely dissociated into ions in aqueous solutions

Weak acids/bases are partially dissociated into ions in aqueous solutions

Question 7

Give an example of a

(a) strong acid
(b) weak acid
(c) strong base
(d) weak base

Answer 7

(a) Hydrochloric acid, sulfuric acid or nitric acid
(b) Ethanoic acid, carbonic acid and sulfurous acid
(c) Solutions of metal hydroxides
(d) Ammonia and amines

Question 8

Compare equimolar weak and strong acids in terms of:

1. pH values
2. Conductivity
3. Reaction rates
4. Stoichiometry

Answer 8

1. Strong acids have lower pH (more hydrogen ions in solution)
2. Strong acids have higher conductivity (more ions in solution)
3. Strong acids react faster (more hydrogen ions in solution)
4. Both will react with the same number of molecules of base

Question 9

Explain why the volume of alkali required to neutralise a strong and a weak acid of the same concentration is the same.

Use ethanoic acid as the weak acid in this example.

Answer 9

CH₃COOH ⇌ H⁺ + CH₃COO^-

H⁺ + OH^- → H₂O

The hydroxide ions in the alkali react with all of the available hydrogen ions in solution. However, in a weak acid, this removes hydrogen ions from the equilibrium and causes the acid molecules to release more hydrogen ions. This continues until all the acid molecules have dissociated, i.e. until the acid is neutralised.

Question 10

The acid dissociation constant is represented by

K_a =?

Answer 10

Question 11

How can you convert K_a to pK_a and vice versa?

Answer 11

pK_a = -log₁₀K_a

Ka = 10^-pK_a

Question 12

How do you calculate the approximate pH of a weak acid?

Answer 12

pH = 1/2pK_a - 1/2log₁₀c

Question 13

A soluble salt of a strong acid and a strong base dissolves in water to produce what type of solution?

Answer 13

Neutral

Question 14

A soluble salt of a weak acid and a strong base dissolves in water to produce what type of solution?

Answer 14

Alkaline solution

Question 15

A soluble salt of a strong acid and a weak base dissolves in water to produce what type of solution?

Answer 15

Acidic solution

Question 16

Explain why a solution of ammonium chloride is acidic.

Answer 16

The solution will contain NH₄⁺ and Cl^- from the salt and H^{<span>+ </span>}and OH^{<span>-</span>} ions from the water.

Some of the NH₄⁺ and OH^{<span>- </span>}will react to form an equilibrium mixture of ammonium hydroxide.

This will leave an excess of H^{<span>+</span>} ions and therefore the solution will be acidic.

Question 17

Explain why a solution of sodium carbonate is basic.

Answer 17

The solution will contain Na⁺ and CO₃^2- from the salt and H^{<span>+ </span>}and OH^{<span>-</span>} ions from the water.

Some of the H⁺ and CO₃^{2-<span> </span>}will react to form an equilibrium mixture of carbonic acid.

This will leave an excess of OH^- ions and therefore the solution will be basic.

Question 18

What does a buffer solution do?

Answer 18

The pH of a buffer solution remains approximately constant when small amounts of acid, base or water are added.

Question 19

How is an acid buffer made?

Answer 19

A solution of a weak acid and one of its salts made from a strong base.

Question 20

How does an acid buffer work?

Answer 20

The weak acid provides the hydrogen ions when these are removed by the addition of a small amount of base.

If an acid (H⁺) is added the conjugate base will form the weak acid.

Question 21

How is a basic buffer made?

Answer 21

Solution of a weak base and one of its salts.

Question 22

How does a basic buffer work?

Answer 22

The weak base removes excess hydrogen ions,

The salt of this base provides the conjugate acid which combines with the hydroxide ions added.

Question 23

Which equation can be used to calculate the approximate pH of an acid buffer solution?

Answer 23

Question 24

What are indicators?

Answer 24

Weak acids which in aqueous solutions the colour of an acid indicator is distinctly different from that of its conjugate base.

Question 25

The expression for acid indicator dissociation constant K_in is

Answer 25

K_In = [H₃O⁺][In^-]/[HIn]

Question 26

The pH range over which a colour change for an indicator occurs can be estimated by the expression?

Answer 26

pH=pK_In±1

Question 27

What is meant by the equivalence point in a titration?

Answer 27

the equivalence point in a titration experiment is reached when the reaction between the titrant (added from the burette) and the titrate (in the flask) is just complete.

Question 28

Which of these indicators

phenolphthalein-pH range 8.2-10.0

bromothymol blue-pH range 6.0-7.6

methyl orange-pH range 3.0-4.4

would be suitable for

(a) strong acid and strong base titration
(b) strong acid and weak base titration
(c) weak acid and strong base titration

Answer 28

(a) bromothymol blue
(b) methyl orange or bromothymol blue
(c) phenolphthalein

Question 29

Why, in general, are no indicators suitable for the titration of a weak acid and weak base?

Answer 29

The pH change around the equivalence point is fairly gradual.

Such titrations have to be monitored using a pH meter.