Topic 12: Acid-Base Equilibria

Question 1

What is a Bronsted-Lowry acid

Answer 1

Proton donor

Question 2

What is a Bronsted-Lowry base

Answer 2

A proton Acceptor

Question 3

What does an acid-base reaction involve?

Answer 3

The transfer of protons

Question 4

What is the conjugate base of H2SO4

Answer 4

HSO4^-

Question 5

What is the conjugate acid of NH3

Answer 5

NH4^+

Question 6

What is pH

Answer 6

-log[H+]

Question 7

How do you calculate [H+] from pH

Answer 7

10^(-pH)

Question 8

What is the difference between a strong acid and a weak acid?

Answer 8

A strong acid fully dissociates in solution whereas a weak acid only partially dissociates.

Question 9

What is the pH of a monoprotic strong acid?

Answer 9

pH = -log[H+]
[H+]=[HA]

Question 10

What is the expression for Ka?

Answer 10

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

Question 11

What are the assumptions that you (usually) have to make when doing calculations using Ka?

Answer 11

[H+]=[A-]
[HA] initial = [HA] equilibrium
(usually also that diprotic weak acids dont significantly dissociate twice)

Question 12

What is Kw?

Answer 12

Kw = [H+][OH-]

Question 13

pOH + pH = ?

at room temp

Answer 13

14

Question 14

what is pKa?

Answer 14

-log(Ka)

Question 15

What is pKw?

Answer 15

-log(Kw)

Question 16

What is the relationship between pKa and pH?

Answer 16

pKa = pH at half-way neutralisation

Question 17

Which indicator should you use for a titration between a weak acid and a weak base?

Answer 17

Trick question! there is no vertical region so you cannot use any indicator for a titration between weak acid and weak base

Question 18

What is the vertical region for strong acid strong base?

Answer 18

3-11

Question 19

What is the vertical region for strong acid + weak base?

Answer 19

3-7

Question 20

What is the vertical region for weak acid + strong base?

Answer 20

7-11

Question 21

What is the vertical region for weak acid + weak base?

Answer 21

N/A there isn’t one

Question 22

What is a buffer solution?

Answer 22

A buffer solution/mixture which minimises the change in pH even if a small amount of acid or base is added. Consists of a mixture of roughly equal amounts weak acid and conjugate base.

Question 23

What is the buffer region on a titration curve?

Answer 23

The horizontal region on a weak acid titration curve where the conjugate base and weak acid are present in roughly equal concentrations.

Question 24

Why is the enthalpy of neutralisation of weak acids less exothermic than for strong acids?

Answer 24

More endothermic because energy needed to break the A-H bond as not all the acid dissociated.

Question 25

What is the significance of carbonic acid?

Answer 25

Carbonic acid and hydrogencarbonate ions act as a buffer solution in the blood, resisting changes in pH.

Question 26

What happens when you add a small amount of H+ to a buffer solution?

Answer 26

the conjugate base “soaks up” H+, forming the weak acid. The position of equilibrium shifts left.

Question 27

What happens if you add a small amount of base to a buffer solution?

Answer 27

Base reacts with H+ so the position of equilibrium shifts to the right so that more weak acid dissociates to replace the H+.

Question 28

Devise an experiment to determine the Ka for a solution of ethanoic acid of unknown concentration

Give both methods :)

Answer 28

1. Titrate the ethanoic acid with sodium hydroxide (or a strong base)
2. Method 1: measure pH at regular intervals and plot pH against volume, then use graph to find pH at half equivalence point
3. Method 2: use phenolphthalein indicator to find end-point, then add the same volume of acid to the mixture at the end point, and measure the pH of the resultant mixture
4. Determine the Ka -> pH = pKa so Ka = 10^-pH

Only one of method 1 or 2 is needed