Chapter 15 - Acid-Base Equilibria

Question 1

What is a conjugate acid-base pair?

Answer 1

A conjugate acid-base pair constitutes two species which differ from each other by a proton.

Question 2

What is ionic product of water (Kw)?

Answer 2

Kw = [H+][OH-]

Question 3

How do we calculate pH of strong acids/bases? (2)

Answer 3

1) normal case: [H+] = [acid]
2) when concentration of acid/base is 10^-7 moldm3 or lower, the contribution of H+/OH- from water becomes significant. add 10^-7 from water (cos water is pH 7)

Question 4

How can the value of Ka/Kb be used to determine strength of weak acids/bases?

Answer 4

at the same temparature,

1) larger Ka = smaller pKa = stronger acid.
2) larger Kb = smaller pKb = stronger base

Question 5

What does the value of Ka/Kb indicate?

Answer 5

It indicates the extent to which the weak acid/base dissociates in water at the specified temperature.

Question 6

How can we find Ka/Kb with only one of the values?

Answer 6

for any conjugate acid-base pair,

Ka x Kb = Kw

Question 7

What is degree of dissociation?

Answer 7

It is the fraction (ratio) of molecules which is ionised into ions in water.

Question 8

Why is Ka the best indicator of the strength of a weak acid?

Answer 8

Ka does not vary with concentration and is constant at constant temperature.

Question 9

What is salt hydrolysis?

Answer 9

It is a reaction in which ions of a salt react with water to form OH- or H3O+ ions.

Question 10

What are 5 cases of salt hydrolysis?

Answer 10

1) strong acid + strong base
2) weak acid + strong base
3) strong acid + weak base
4) weak acid + weak base
5) cations with high charge density

Question 11

How does strong acid + strong base undergo salt hydrolysis? What is the resulting pH?

Answer 11

They do not go through salt hydrolysis and thus the resulting pH is 7.

Question 12

How do acid-base reactions of SA/WB or WA/SB undergo salt hydrolysis? What is the resulting pH?

Answer 12

The weaker ion will react with water and form the “opposite” ion (weak acid forms OH-, weak base forms H+)

Question 13

How does weak acid + weak base undergo salt hydrolysis? What is the resulting pH?

Answer 13

Both cation and anion hydrolyse and the pH of the solution depends on Ka&b of acid&base.
Ka > Kb = salt solution acidic
Kb > Ka = salt solution basic

Question 14

How does cations with high charge density undergo salt hydrolysis? What is the resulting pH?

Answer 14

The small and highly charged cation withdraws sufficient electron density from O-H bonds, weakening the bonds and once the bond breaks, H+ is released (this is hydrolysis). pH < 7.

Question 15

How do we determine pH of salt solutions? (2 steps)

Answer 15

1) determine whether or not salts undergo salt hydrolysis

2) calculate [H+]/[OH-} using Ka/Kb of the ion that undergoes hydrolysis.

Question 16

Define a buffer solution.

Answer 16

A buffer solution is one that is able to resist pH changes upon addition of a small amount of acid/base.

Question 17

What does a buffer solution usually contain?

Answer 17

two species, in large amount, which make up a conjugate acid-base pair of a weak acid/weak base.

Question 18

How can we prepare buffers?

Answer 18

1) mixing weak acid and its weak conjugate base
2) mixing weak base and its conjugate acid
3) partial neutralization (leave a bit unneutralized so there is a mixture of acid and base)

Question 19

How does the buffer consisting large reservoirs of acid and its conjugate base resist pH changes on addition of small amounts of acid?

Answer 19

1) when a small amount of H+ is added, the H+ ions react with the large reservoir of the conjugate base.
2) added H+ are removed and concentration of acid increases slightly, while concentration of conjugate base decreases slightly.
3) since the original amounts of the acid and base are large compared to the amount of H+ ions added, the ratio of acid and base remains almost constant.
4) since Ka = [H+][cb]/[acid], and Ka is constant, [H+] remains almost constant and thus pH.

Question 20

How does the buffer consisting large reservoirs of acid and its conjugate base resist pH changes on addition of small amounts of base?

Answer 20

1) when a small amount of OH- if added, the OH- ions react with the large reservoir of unreacted acid.
2) added OH- are removed and concentration of cb increases slightly, while concentration of acid decreases slightly.
3) since the original amounts of the acid and base are large compared to OH- added, the ratio of acid and base remains almost constant.
4) since since Ka = [H+][cb]/[acid], and Ka is constant, [H+] remains almost constant and thus pH.

Question 21

What is buffer capacity?

Answer 21

It is the quantity of H3O+ or OH- it can remove before pH changes drastically.

Question 22

What are 2 factors that buffer capacity depend on?

Answer 22

1) The more concentrated the components of the buffer, the greater the buffer capacity.
2) ratio of concentration of weak acid to conjugate base. if there is much more acid, it will be more effective in resisting changes of pH on addition of OH- and vice versa.

Question 23

What does it mean when a buffer is effective?

Answer 23

A buffer is effective when the ratio of acid to base is 1, thus pH = pKa (acidic buffer) or pOH = pKb (alkaline buffer). It thus has maximum buffer capacity and can most effectively resist a change in pH in either direction.

Question 24

What is the effective buffer range?

Answer 24

0.1 to 10 (when one of the components is ten times the concentration of another)