Acids & Bases

Question 1

Define a Bronsted-Lowry:

a) acid

b) base

Answer 1

a) An acid is a proton donor

b) A base is a proton acceptor

Question 2

What does amphoteric mean, as applied to water?

Answer 2

Water can act as both an acid and a base.

Question 3

What is the ionic product of water?
In aqueous solution, 1dm3 has a mass of 1000g.
1000g/18gmol- = 55.6 moldm-3

Answer 3

Kw = [H+][OH-] = 1.00x10-14

Question 4

What is a diprotic acid? Explain why the second dissociation involves a weak acid, giving an example.

Answer 4

An acid that dissociates to produce two H+ ions (e.g. H2SO4). H+ ions from the first dissociation suppress the second ionisation. Equilibrium lies to the left.
e.g. 1. H2SO4 -> HSO4- + H+ 2. HSO4- -> SO42- + H+

Question 5

Why does the dilution by a factor of 10 increase the pH of a weak acid by 0.5, while a strong acid increases by 1?

Answer 5

Dilution shifts equilibrium to the right. The fall in H+ ion concentration is less for weak acids than strong acids.
As a result, the H+ ion concentration is greater than expected for weak acids.

Question 6

What assumptions do you make when calculating the pH for a weak acid?

Answer 6

1. [H+] = [A-] (H+ from water ionisation is small & suppressed by H+ from HA ionisation, so it is ignored)
2. [HA] equilibrium = [HA] initial

Question 7

What is the difference between the endpoint and the equivalence point in a titration?

Answer 7

Endpoint: when the indicator causes the solution to change colour.
Equivalence point: when equal no. of moles of acid and alkali have been added. pH not necessarily 7 (salt present)

Question 8

At the half-equivalence point, what assumption can you make?

Answer 8

[HA] and [A-] cancel, so Ka = [H+] so pKa = pH

Question 9

What is an indicator and what does it consist of?

Answer 9

It shows when the equivalence point has been reached.
Consists of a weak acid and a conjugate base (different colour to weak acid).
HInd >

Question 10

Define ‘buffer solution’.

Answer 10

One which resists large changes in pH upon the addition of small amounts of acid or alkali.

Question 11

How does a buffer work, using the reaction HA >

Answer 11

There is a large reservoir of HA and A-.
Adding acid: H+ ion are removed by large reservoir of A-. H+ + A- > HA. Equilibrium shifts to the left.
Adding alkali: added OH- ions are removed by HA reservoir. HA + OH- > H2O + A-. Equilibrium lies to the right.
Overall the ratio of [HA] to [A-] hardly changes, so the pH hardly changes.

Question 12

How do you calculate the pH of a mixture of acid and alkali?

Answer 12

1. Calculate no. of moles of acid
2. Calculate no. of moles of alkali
3. Find excess moles after neutralisation
4. Calculate the concentration of H+ and then the pH

Question 13

a) What is the enthalpy of ‘neutralisation’?

b) Why do strong and weak acids have similar pH values?

Answer 13

a) When an acid and base react to form 1 mol of water under standard conditions (298K, 1 atm and 1 moldm-3)
b) OH- ions remove H+ ions from solution. Equilibrium shifts to the right so all the acid reacts.

Question 14

What does the dissociation constant, Ka measure? What does it indicate about the strength of an acid?

Answer 14

It provides a direct measure of the strength of acid as it measures the extent of dissociation (the larger the Ka, the greater the dissociation).
The stronger the acid, the larger the Ka.

Question 15

Define an 'acid' and a 'base' using:
a) The 19th and 20th century theory
b) Arrhenius' theory
c) Lavoisier's theory
State the problems with each theory.

Answer 15

a) An acid has a sour taste, bases have a soapy feel: H2SO4 and NaOH are harmful
b) An acid is an electrolyte that produces H+ ions when dissolved in water; bases produce OH- ions: H+ ions do not exist independently in solution but as the oxonium ion H3O+
c) Acids contained oxygen

Question 16

What does the term ‘buffer capacity’ mean in regards to food spoilage? What factor affects this and why?

Answer 16

The amount of acid or base required to change the pH of food significantly.
The more protein in food, the higher the buffer capacity because amino acids have both acidic and basic properties.

Question 17

Explain why a solution of propanoic acid alone is not a buffer solution.

Answer 17

Propanoic acid is a weak acid so a solution has significant CH3CH2COOH but few CH3CH2COO- ions.
When OH- ions are added, they react with HA to produce A- + H2O. The [CH3CH2COO-] rises significantly.
When H+ ions are added, they remove A-, so the [CH3CH2COO-] drops significantly.
Overall the ratio of [acid]:[salt] alters and the pH changes.