1.12 Acids and Bases

Question 1

Brønsted-Lowry acid

Answer 1

Proton (H+) donor

Question 2

Brønsted-Lowry base

Answer 2

Proton (H+) acceptor

Question 3

Brønsted-Lowry acid-base reaction

Answer 3

Reaction involving the transfer of a proton

Question 4

Monoprotic acid

Answer 4

Acid that releases one H+ ion per molecule e.g. HCl

Question 5

Diprotic acid

Answer 5

Acid that releases two H+ ions per molecule e.g. H2SO4

Question 6

Definition of pH, and useful rearrangement

Answer 6

pH = – log [H+]

[H+] = 10^-pH

Question 7

Ionic Product of Water (Kw)

Answer 7

(Kw) H2O ⇌ H+ + OH-
ΔH = endothermic

Kc = [H+] [OH-]/[H2O]
So Kc [H2O] = [H+] [OH-]

so Kc [H2O] = a constant = Kw
Kw = [H+][OH-] where Kw = 1x10^-14

Question 8

The effect of temperature on the pH of water and the neutrality of water

Answer 8

As the temperature increases, the equilibrium moves right to oppose the increase in temperature

Therefore [H+] and [OH-] increase
So Kw increases and therefore pH decreases

However, the water is still neutral as [H+] = [OH-]

Question 9

Calculating the pH of water

Answer 9

In pure water, [H+] = [OH-]
So Kw = [H+]^2
Therefore [H+] = √Kw

Question 10

pH of a Strong Base

Answer 10

Find [OH-] and then substitute into: [H+] = Kw/[OH-]

Question 11

Mixtures of Strong Acids and Strong Bases

Answer 11

1) Calculate moles H+
2) Calculate moles OH-
3) Calculate moles XS H+ or OH-
4) Calculate XS [H+] or XS [OH-]
5) Calculate pH

Question 12

Weak Acids

Answer 12

HA ⇌ H+ + A-

Question 13

The acid dissociation constant, Ka

Answer 13

Ka = [H+] [A-] / [HA]
pKa = -log Ka
Ka = 10^-pKa
These expressions hold for weak acids at all times

Question 14

The acid dissociation constant, Ka, notes

Answer 14

• Ka – has units mol dm-3
• Ka – the bigger the value, the stronger the acid
• pKa – the smaller the value, the stronger the acid

Question 15

In a solution of a weak acid in water, with nothing else added:

Answer 15

a) [H+] = [A-]
b) [HA] ~ [HA]initial

Ka = [H+]^2 / [HA]
This expression ONLY holds for weak acids in aqueous solution with nothing else added

Question 16

Reactions between weak acids and strong bases

Answer 16

When a weak acid reacts with a strong base, for every mole of OH- added, one mole of HA is used up and one mole of A- is formed.

Question 17

Finding the pH in reactions between weak acids and strong bases

Answer 17

1) Calculate moles HA (it is still HA and not H+ as it is a weak acid)
2) Calculate moles OH-
3) Calculate moles XS HA or OH-

If XS HA
4) Calculate moles HA left and A- formed
5) Calculate [HA] leftover and [A-] formed
6) Use Ka to find [H+]
7) Find pH

OR

If XS OH-
4) Calculate [OH-]
5) Use Kw to find [H+]
6) Find pH

OR

If mol HA = OH-
4) pH = pKa of weak acid

Question 18

Titration Calculations

Answer 18

These ionic equations which help a great deal in titration calculations. H+ + OH- ➔ H2O
2H+ + CO3^2- ➔ H2O + CO2
H+ + HCO3^- ➔H2O + CO2
H+ + NH3 ➔ NH4^+

Question 19

pH Curves and Indicators
What are indicators and how do they work?

Answer 19

• Indicators are weak acids where HA and A- are different colours. HA ⇌ H+ + A-
• At low pH, HA is the main species present. At high pH, A- is the main species present.
• The pH at which the colour changes varies from one indicator to another.
• Note that universal indicator is a mixture of indicators and so shows many colours at different pHs.

Question 20

What is a buffer solution?

Answer 20

• Buffer solution = solution that resists changes in pH when small amounts of acid or alkali are added.
• Note – the pH does change, just not by much.
• Acidic buffer solutions have a pH less than 7.
• Basic buffer solutions have a pH less than 7.

Question 21

Acidic buffers

Answer 21

• Acidic buffer solutions are made from a mixture of a weak acid and one of its salts (i.e. HA and A-) (e.g. ethanoic acid & sodium ethanoate).
• An acidic buffer solution can also be made by mixing an excess of a weak acid with a strong alkali as it results in a mixture of HA and A-.
• The key in an acidic buffer solution is that the [acid] and [salt] are much higher than [H+].

Question 22

Basic buffers

Answer 22

• Basic buffer solutions are made from a mixture of a weak alkali and one of its salts (e.g. ammonia & ammonium chloride).
• A basic buffer solution can also be made by mixing an excess of a weak alkali with a strong acid
• The key in a basic buffer solution is that the [base] and [salt] are much higher than [OH-].

Question 23

To work out the pH of an acidic buffer

Answer 23

Use: [H+] = Ka x [HA] / [A-]

Question 24

To work out the change in pH of an acidic buffer when acid is added

Answer 24

Use: HA (aq) ⇋ A- (aq) + H+ (aq)
Where: H+ added shifts the equilibrium to the left so A- decreases and HA increases
Calculate the original moles and then adjust these values by the moles of the acid added as above.

Question 25

To work out the change in pH of an acidic buffer when alkali is added

Answer 25

Use: HA (aq) + OH- ⇋ A- (aq) + H2O(aq) Where: OH- added reacts with HA which will decrease and produces more A- which will increase. Calculate the original moles and then adjust these values by the moles of alkali added as above.