Acids, Bases & pH

Question 1

Brønsted-Lowry acids

Answer 1

Proton donators

Strong - dissociate almost fully in water e.g. HCl
HA(aq) + H2O(l) —> H3O+(aq) + A-(aq)
Equilibrium far right

Weak - slightly dissociate in water e.g. ethanoic acid/citric acid
CH3COOH CH3COO- + H+
Equilibrium far left

Question 2

Brønsted-Lowry bases

Answer 2

Proton acceptors

Alkali = soluble base

Strong - ionise almost fully in water e.g. NaOH
B(aq) + H2O(l) —> BH+(aq) + OH-(aq)
Equilibrium far right

Weak - slightly ionise in water e.g. ammonia
NH3 + H2O NH4+ + OH-
Equilibrium far left

Question 3

Acid + base

Answer 3

Protons transferred

HA(aq) + B(aq) —> BH+(aq) + A-(aq)

Question 4

Acid + water

Answer 4

Water acts as base and accepts protons

HA(aq) + H2O(l) H3O(aq) + A-(aq)

Question 5

Ionic product of water

Answer 5

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

Kw = ionic product of water = [H+][OH-]

Pure water: [H+]=[OH-]
Kw = [H+]^2

Question 6

pH

Answer 6

Measures H+ ion conc on logarithmic scale

pH = -log10[H+]

[H+] = 10^-pH

Question 7

Ka

Answer 7

Acid dissociation constant

Weak acids: Ka = [H+]^2/[HA]

pKa = -log10Ka

Ka = 10^-pKa

Lower pKa = stronger acid

Question 8

Methyl orange pH range

Answer 8

3.1-4.4

Question 9

Phenolphthalein pH range

Answer 9

8.3-10

Question 10

Buffer

Answer 10

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

Question 11

Acidic buffer

Answer 11

Weak acid and one of its salts

E.g. ethanoic acid and sodium ethanoate

Question 12

Basic buffers

Answer 12

Weak base and one of its salts

E.g. ammonia and ammonium chloride

Question 13

Uses of buffers

Answer 13

• shampoo pH 5.5 buffer stops hair getting rough in alkaline conditions
• biological washing powders contain buffers for enzymes
• biological buffer systems e.g. blood stays at pH close to 7.4