12 - Acid-base equilibria

Question 1

Bronsted-lowry acid

Answer 1

A proton donor

Question 2

Bronsted-lowry base

Answer 2

A proton acceptor

Question 3

What do acid-base reactions involve?

Answer 3

Transfer of protons.

Question 4

What is pH?

Answer 4

pH = -log10[H+]

Define pH using this equation.

Question 5

How do you calculate [H+] when given pH?

Answer 5

[H+} = 10^-pH

Question 6

What is a conjugate acid-base pair?

Answer 6

It consists of either a base and its conjugate acid, or an acid and its conjugate base.

Question 7

How is a conjugate acid formed?

Answer 7

When a base accepts a proton, the species formed is the conjugate acid of the base.

Question 8

How is a conjugate base formed?

Answer 8

When an acid donates a proton, the species formed is the conjugate base of the acid.

Question 9

What is an atmospheric substance?

Answer 9

A substance that can act both as an acid and as a base.

Question 10

What is a strong acid?

Answer 10

Strong acids dissociate almost completely in aqueous solution.

Question 11

What is a weak acid?

Answer 11

Weak bases dissociate only partially (less than 10%) in aqueous solution.

Question 12

How do you calculate pH of a STRONG acid?

Answer 12

• Strong acids almost completely dissociate in aqueous solution.
• monoprotic acids: same number of moles of H+ as original acid.
• Diprotic acid: 2x number of moles of H+ as original acid.
• Then use pH = -log10[H+] to find pH.

Question 13

What does a low pH mean?

Answer 13

More acidic, higher [H+].

Question 14

What does a high pH mean?

Answer 14

More basic, lower [H+].

Question 15

What is Ka (acid dissociation constant)?

Answer 15

Ka expresses how easily an acid releases a proton (strength as an acid).

• Large Ka = large extent of dissociation.
• Small Ka = small extent of dissociation.

Question 16

Expression for Ka?

Answer 16

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

units are moldm^-3.

Question 17

How do you calculate the pH of a WEAK acid?

Answer 17

Use the equation:
Ka = ([H+][A-])/([HA]), to find [H+].
Then use this to find pH.

Question 18

What are the ASSUMPTIONS when calculating the pH of a weak acid?

Answer 18

1. [acid]initial = [acid]equilibrium.

2. [H+] = [A-]

Question 19

Define the ionic product of water, Kw?

Answer 19

Kw = [H+][OH-]

(aqueous for both)

units are mol^2dm^-6

Question 20

What is the value of Kw at 298K?

Answer 20

1.00x10^-14 mol^2dm^-6

Question 21

How do you calculate pH of a STRONG base from its concentration?

Answer 21

Strong bases almost completely fully dissociate in aqueous solution, so [OH-] = [strong base].
Then use Kw = [H+][OH-], with Kw = 1.00x10^-14 (at 298K) to calculate OH-. Then with found [H+], find pH.

Question 22

Calculate pH between strong acids and strong bases?

Answer 22

• Calculate moles of unreacted H+ (if acid in excess), or unreacted OH- (if base in excess).
• Calculate excess concentration of the unreacted H+ or OH-.
• Calculate pH using pH = -log10[H+] if we have [H+], or use Kw = [H+][OH-] first if we have [OH-].

Question 23

Calculate pH of a solution of weak acid and strong base (excess base)?

Answer 23

• Calculate moles of each
• Check which is in excess.
• If base is in excess, substract moles of H+ from moles of OH- to know moles of excess OH-.
• Calculate excess [OH-].
  Use Kw to find pH of a strong base as there is no acid left.

^^ For every mole of strong base added, 1 mole of weak acid dissociates.

Not the same case as a strong acid, where it completely dissolves straight away in aqueous solution.

Question 24

Calculate pH of a solution of weak acid and strong base (excess acid)?

Answer 24

• Calculate moles of each
• Check which is in excess (this case acid).
• Calculate moles of excess weak acid and moles of A- formed in reaction (EXCESS HA AND NOT H+).
• Calculate concentrations of HA and A-.
• Use Ka = ([H+][A-])/([HA]) to find [H+].
• Find pH.

Question 25

Define pKa?

Answer 25

pKa = -log10Ka. Ka = 10^-pKa

Question 26

Define pKw?

Answer 26

pKw = -log10Kw At 298K, Kw = 1.00x10^-14 mol^2dm^-6, pKw = 14.00

Question 27

Strong and weak acids pH meaning?

Answer 27

Higher the pH, weaker the acid.

Question 28

Strong and weak bases pH meaning?

Answer 28

Higher the pH, stronger the base.

Question 29

pH of salt solutions?

Answer 29

Salt of a strong acid and strong base, pH = 7 (neutral). Salt of a weak acid and a strong base, pH > 7 (alkaline). Salt of a strong acid and a strong base, pH < 7 (acidic). Salt of a weak acid and a weak base (a for acid, b for base): if Ka = Kb, pH = 7. if Ka > Kb, pH < 7. if Ka < Kb, pH > 7.

Question 30

How does pH change as you dilute aqueous solutions of acids?

Answer 30

Strong acid: pH increases by 1 when diluted by a factor of 10. Weak acid: pH increase by 0.5 when diluted by a factor of 10.

Question 31

What is a buffer solution?

Answer 31

A buffer solution is a solution that minimises the change in pH when a small amount of either acid or base is added.

Question 32

How to make a buffer solution?

Answer 32

- mix weak acid with its conjugate base. | mix weak base with its conjugate acid.

Question 33

Important things to remember when finding pH of a buffer solution?

Answer 33

- initial HA concentration = equilibrium HA concentration, because we assume that the dissociation of the acid is negligible. - The salt (conjugate base of acid) dissociates fully in aqueous solution, and since we assume that the dissociation of the acid is negligible, then we can say the A- is entirely made up from the salt. So concentration of salt = A- concentration.

Question 34

Henderson-Hasselbalch equation (to calculate pH of a buffer solution)?

Answer 34

pH = pKa + log10([base]/[acid]).

Question 35

How to make a buffer solution with a required pH?

Answer 35

Overall you work out: [acid]/[salt]. You basically find a ratio of the concentrations needed of the acid and salt, using the same volumes.

Question 36

How do you know which indicator to use when looking at a titration curve?

Answer 36

The pH range over which the indicator changes colour has to be within the vertical region (end point) of the titration curve.

Question 37

On titration curve, strong acid pH?

Answer 37

1 ish

Question 38

On titration curve, weak acid pH?

Answer 38

3 ish

Question 39

On titration curve, strong base pH?

Answer 39

13 ish

Question 40

On titration curve, weak base pH?

Answer 40

11 ish

Question 41

Explain the straight horizontal part of a titration curve?

Answer 41

buffer action.

Question 42

What is the equivalence point?

Answer 42

The volume at this point is the minimum volume of base required to completely react with all of the acid.

Question 43

What is so special at the half-equivalence point?

Answer 43

pH = Pka of weak acid. Ka can be easily calculated from this.

Question 44

Why is the enthalpy change of neutralisation less for a weak acid than a strong acid?

Answer 44

Weak acids only partially dissociate. Energy is released when the molecules dissociate, but less thermal energy would be released compared to strong acids, since strong acids fully dissociate and weak acids only partially dissociate. Therefore less exothermic.

Question 45

Explain why HF, in spite of being a weak acid has an enthalpy of neutralisation which is higher than -57KJmol-1.

Answer 45

The enthalpy of hydration of the ions in HF is very exothermic and releases more heat energy than the energy taken in during dissociation of the molecules. Therefore the thermal energy released compensates the energy taken in. Hence the overall effect is that the standard enthalpy change of neutralisation of HF by NaOH is greater than -57KJmol-1.

Question 46

How many decimal places is pH?

Answer 46

2dp ALWAYS