12: Acid-base Equilibria

Question 1

What is a Brønsted–Lowry acid?

Answer 1

A proton donor

Question 2

What is a Brønsted-Lowry base?

Answer 2

A proton acceptor

Question 3

What is transferred in acid-base reactions?

Answer 3

Protons

Question 4

State the conjugagte base of the acid HA

Answer 4

A^-

Question 5

State the equation for pH

Answer 5

pH = -log[H⁺]

Question 6

State the equation to calculate the concentration of hydrogen ions in a solution from its pH.

Answer 6

[H⁺] = 10^-pH

Question 7

Describe the difference between a strong acid and a weak acid.

Answer 7

Strong acids fully dissociate,
Weak acids partially dissociate.

Question 8

Deduce the expression for the acid dissociation constant of the weak acid in this equation:
HA <=> H⁺ + A^-

Answer 8

K_a = [H⁺][A^-] / [HA]

Question 9

Explain 2 assumptions made when calculating the pH of a weak acid.

Answer 9

[H⁺] = [A^-], because H⁺ from H₂O is negligible,

[HA] at equilibrium = Initial [HA], because dissociation is very small

Question 10

State the equation for the ionic product of water

Answer 10

K_w = [H⁺][OH^-]

Question 11

State the equation for pK_a

Answer 11

pK_a = -log K_a

Question 12

State the equation for pK_w

Answer 12

pK_w = -log K_w

Question 13

Compare and explain the change in pH of a strong acid and a weak acid after dilution 10 times.

Answer 13

pH of a strong acid will increase by 1, because [H⁺] decreases by a factor of 10.
pH of a weak acid will increase by 0.5, because some of the undissociated acid molecules split up, so [H⁺] decreases slower.

Question 14

Descrive the titration curve for strong acid and strong base.

Answer 14

Question 15

Describe the titration curve for strong acid and weak base.

Answer 15

Question 16

Describe the titration curve for weak acid and strong base.

Answer 16

Question 17

Describe the titration curve for weak acid and weak base.

Answer 17

Question 18

State how you would select a suitable indicator using a titration curve.

Answer 18

Choose an indicator with the equivalence point of the curve within its pH range.

Question 19

Describe what is meant by the term ‘buffer solution’.

Answer 19

A system which resists dramatic changes in pH if small quantities of acid or alkali is added to it.
Contains a weak acid and one of its salts in high concentration.

Question 20

Explain the action of a buffer solution as a small quantity of acid is added.

Answer 20

HA <=> H⁺ + A^-

As H⁺ ions are added, they react with A^- ions from the salt in the buffer solution,
shifting equilbirium to the left,
so [H⁺] stays approximately constant.

Question 21

Explain the action of a buffer solution as a small quantity of alkali is added.

Answer 21

HA <=> H⁺ + A^-

As OH^- ions are added, they react with H⁺ ions from the acid in the buffer solution,
shifting equilbirium to the right,
so [H⁺] stays approximately constant.

Question 22

Explain why there is a difference in enthalpy changes of neutralisation values for strong and weak acids.

Answer 22

As weak acids are neutralised they dissociate more,
so energy is used up,
and the enthalpy change is less exothermic.

Question 23

Explain how the pH of blood is controlled.

Answer 23

A buffer solution of carbonic acid molecules and hydrogencarbonate ions.