3.1.12 Acids and bases

Question 1

What is a Bronstead-Lowry acid and base?

Answer 1

Acid = proton donator
Base = proton acceptor

Question 2

Give examples of strong/weak acids and bases.

Answer 2

Strong acid: HCl, H2SO4, HNO3
Weak acid: CH3COOH, H2SO3, H3PO4

Strong base: NaOH, KOH, Ba(OH)2
Weak base: NH3, H2O

Question 3

What is meant by the term strong when describing an acid?

Answer 3

Fully dissociates in aqueous solution to give H+ ions

Question 4

Define pH

Answer 4

pH = -log[H+]

Question 5

What is the equation for Kw and its value at 298K?

Answer 5

Kw = [H+] x [OH-]

Question 6

Explain why the expression for Kw does not include [H2O].

Answer 6

• [H2O] is very high and acts as a constant

Question 7

How does a strong acid dissociate in aqueous solution?

Answer 7

Strong acid fully dissociates in aqueous solution

Question 8

Explain why the value of Kw increases as temeprature increases.

Answer 8

H2O ⇌ 2H+ + OH-
forward reaction is endothermic
- Equilibrium shifts to right in order to oppose the increase in temperature
- [H+] and [OH-] increase

Question 9

What is the expression for the weak acid dissociation?
HA ⇌ H+ + A-

Answer 9

Ka = ([H+] x [A-]) / [HA]

Question 10

How can Ka be simplified and reaaranged to find [H+]?

Answer 10

[H+]^2 = Ka x [HA]

Question 11

Explain why data books do not usually contain values of Ka for strong acids?

Answer 11

• Strong acids fully dissociate in aqueous solution
• Ka value for strong acids is very large

Question 12

How do you calculate pKa?

Answer 12

pKa = -log (Ka)

Question 13

How can you find the value for Ka?

think: practical

Answer 13

• Neutralise weak acid with strong base
• Find equivalence point of reaction from pH titration curve
• Halve this volume to find the half-equilvalence point
• Read pH at the half-equivalence point
• pKa = pH

Question 14

What is the equivalence point/stochiometric point?

Answer 14

When 2 solutions have reacted completely and there is an indicator change

Question 15

What are some suitable indicators for the following titrations:
a) strong acid + strong base
b) strong acid + weak base
c) weak acid + strong base
d) weak acid + weak base

Answer 15

a) phenolphthalein
b) methyl orange
c) phenolphthalein
d) none

Question 16

Write an equation for the dissociation of the weak acid-salt buffer, ethanoic acid + sodium ethanoate.

Answer 16

CH3COOH ⇌ CH3COO- + H+

Question 17

Write an expression for Ka with the ethanoic acid + sodium ethanoate buffer.

Answer 17

Ka = ([H+] x [CH3COO-]) / [CH3COOH]

Question 18

Explain the effect of adding the following to the ethanoic acid + sodium ethanoate buffer:
a) acid solution
b) alkali solution

Answer 18

a) H+ + CH3COO- → CH3COOH
- H+ reacts with ethanoate ions
- [H+] remains constant

b) OH- reacts with H+
- equilibrium shifts to the right to oppose the decrease in H+ ions
- [H+] remains almost constant

Question 19

What is the general equation for the following:
a) acidic buffer + acid
b) acidic buffer + base

Answer 19

a) H+ + X-→HX

b) HA + OH-→ A- + H2O