12. Acid Base Equilibria

Question 1

What is the Bronsted Lowry definition of an acid

Answer 1

a proton donor

Question 2

What is the Bronsted Lowry definition of a base

Answer 2

a proton acceptor

Question 3

What is a conjugate base

Answer 3

the species that is formed when an acid loses a proton

Question 4

What is a conjugate acid

Answer 4

the species that is formed when a base gains a proton

Question 5

What is [H+] in a diprotic acid

Answer 5

we estimate that [H+] = 2[acid]
in reality dissociation occurs in two steps

Question 6

Give the equation for the ionic product of water

Answer 6

Kw = [H+][OH-]

Question 7

Why doesn’t water become more acidic as temperature increase

Answer 7

water dissociates to hydrogen and hydroxide ions
[H+] always equals [OH-] so the solution is always neutral

Question 8

Give the equation for calculating the pH of a strong alkali

Answer 8

pH = pKw - pOH
pKw is usually 14

Question 9

What does the Ka value determine about the strength of a weak acid

Answer 9

larger Ka = stronger acid
larger pKa = weaker acid

Question 10

Give two assumptions when calculating the pH of a weak acid from Ka and concentration

Answer 10

1. [H+] = [A-] -> we ignore H+ from the dissociation of water
2. [HA]eq = [HA]initial -> because dissociation of weak acid is so low

Question 11

Why does the pH rise as much as we expect when diluting a weak acid

Answer 11

HA + H2o -> H3O+ + A-
dilution increases the concentration of water, so equilibrium shifts to the right
dissociation increases and the concentration of hydrogen ions is higher than expected

Question 12

Give two ways of following an acid-base titration

Answer 12

1. using indicators
2. using a pH probe

Question 13

How are indicators used in acid-base titrations

Answer 13

they determine the end point of a titration
not necessarily when [H+] = [OH-]

Question 14

How are pH probes used in an acid-base titration

Answer 14

determines equivalence point when [H+] = [OH-]

at pH = 7 for strong acids
slightly below pH7 for weak alkalis
slightly above pH7 for weak acids

Question 15

Why will the equivalence point be slightly below pH7 for weak alkalis (NH3)

Answer 15

NH3 + HCl -> NH4+ + Cl-
ammonium ions are slightly acidic

Question 16

Why will the equivalence point be slightly below pH7 for weak acids (carboxylic acids)

Answer 16

CH3COOH + NaOH -> CH3COO-Na+ + H2O
CH3COO- + H2O -> CH3COOH + OH-
the hydroxide ions will make the solution slightly alkaline

Question 17

Give the general method for determining the equivalence point in a titration

Answer 17

1. calibrate the pH probe using a pH4 and pH7 buffer solution
2. pipette 25cm3 acid into a conical flask
3. put the pH probe in the flask and measure the pH
4. add the alkali 5cm3 at a time and measure the pH after every addition
5. as the pH starts to rise, measure the pH after every 1cm3 addition until it starts to level off

Question 18

Give the half-volume method for calculating the Ka of an acid

Answer 18

1. calibrate pH probe using pH 4 and pH 7 buffer solutions
2. pipette 25cm3 of weak acid into a conical flask
3. add phenolphthalein indicator
4. titrate against sodium hydroxide solution until the end point is reached
5. add another 25cm3 of acid to the flask
6. measure the pH using the pH probe
   - this pH = pKa

Question 19

What is an indicator

Answer 19

indicators are weak acids or bases that change colour when they lose or gain protons

Question 20

What is the colour change of the litmus indicator

Answer 20

red in acidic solutions
blue in alkaline solutions

Question 21

What is the colour change of the methyl orange indicator

Answer 21

red in acidic solutions
yellow in alkaline solutions

Question 22

What is the colour change of the phenolphthalein indicator

Answer 22

colourless in acidic solutions
pink in alkaline solutions

Question 23

Describe how to select an appropriate indicator for a titration

Answer 23

choose an indicator that has its pH range within the vertical section of the pH curve

strong acid = methyl orange
strong alkali = phenolphthalein