Topic 12: Acid-base Equilibria 1️⃣

Question 1

Define a bronsted lowry acid

Answer 1

Proton donor

Question 2

Define a bronsted lowry base

Answer 2

Proton acceptor

Question 3

Define a bronsted lowry conjugate acid

Answer 3

Species formed when a base accepts a proton

Question 4

Define a bronsted lowry conjugate base

Answer 4

The species formed when a proton is removed from a base

Question 5

Define strong acids

Answer 5

An acid that almost completely dissociates into ions when in aqueous solution

Question 6

Define a weak acid

Answer 6

An acid that only partially dissociates when in aqueous solution

Question 7

Give the equation to calculate pH

Answer 7

pH = -log[H+]

Question 8

Give the equation to calculate conc of H+ ions using pH

Answer 8

[H+] = 10^-pH

Question 9

Give the equation to calculate the [H+] in weak acids

Answer 9

[H+] [A-]
Ka = ————-
[HA]

Question 10

Give the equation to calculate pKa

Answer 10

pKa = -logKa

Question 11

How do you calculate Ka from pKa

Answer 11

Ka = 10^-pKa

Question 12

What does a low pKa value indicate

Answer 12

Strong acid

Question 13

What is the effect of diluting strong acids

Answer 13

If you dilute a strong acid 10 times its pH will increase by one unit
Diluting it 100 times would therefore increase the pH by two units

Question 14

How do weak acids behave upon dilution

Answer 14

Weak acids are not fully dissociated in solution so diluting them causes the equilibrium to shift to oppose the change.
This means a 10 times dilution of a weak acid would increase the pH by less than one unit

Question 15

What is the equilibrium constant for Kw

Answer 15

Kw = [H+][OH-]

Question 16

How does temp affect the value of Kw

Answer 16

The forward reaction in the equilibrium of water is endothermic and is therefore favoured when the temp of water is increased.
As a result as temp increases, more H+ ions are produced making water more acidic

Question 17

Give the equation for pKw

Answer 17

pKw = -log Kw

Question 18

Give the equation for Kw from pKw

Answer 18

Kw = 10^-pKw

Question 19

What is the equivalence point

Answer 19

When the acid and base have reacted together in the same proportions as dictated by the stoichiometric equation

Question 20

What does the end point refer to

Answer 20

When the colour of the indicator just changes colour

Question 21

What does the pH at the equivalence point depend on ?

Answer 21

The combination of acid and base used
Eg strong acid strong base, weak acid strong base etc.

Question 22

Describe the pH titration curve for a strong acid strong base titration

Answer 22

• starts at approx pH 14 finishes at approx pH 0.5 (uses full scale)
• pH only falls by a very small amount until large steep section quite near the equivalence point

Question 23

What is plotted on the x and y axis of a pH titration curve ?

Answer 23

x axis = volume of acid added
y axis = pH

Question 24

Describe the pH titration curve for a weak acid strong base titration

Answer 24

• starts at approx pH 14 finishes at approx pH 3 (uses 3/4 of scale)
• curve like that of strong acid strong base pH titration up to equivalence point
• past the equivalence point the mixture acts as a buffer resisting any fluctuations in pH upon further addition of acid

Question 25

Describe the pH titration curve for a strong acid weak base titration

Answer 25

- curve starts at approx pH 11 finishes at approx pH 0.5 (uses 3/4 of scale) - when acid is first added curve starts to fall but then levels out because buffer solution has been formed - steep section to meet equivalence point

Question 26

Describe the pH titration curve for a weak acid weak base titration

Answer 26

- starts at approx pH 11 finishes at approx pH 3 - no steep section to curve , instead ‘point of inflexion’ - why it is difficult to do titration of weak acid weak base with indicator

Question 27

An acid-base indicator is either…

Answer 27

A weak acid or weak base Most are weak acids (HIn)

Question 28

How is the dissociation of an indicator that is a weak acid in aq solution represented

Answer 28

HIn (aq) ⇌ H+ (aq) + In- (aq) HIn and In- have different colours in aqueous solution

Question 29

What does the equilibrium constant KIn give us

Answer 29

There will be a stage where [HIn] = [In-] indicator in the case of methyl orange will appear orange The exact pH when this is reached can be determined using KIn

Question 30

How can we determine a suitable indicator for an acid-base titration using the pH titration curve

Answer 30

Each indicator has an exact pH range So long as the exact pH range of the indicator falls within the steep section of the curve it is a suitable indicator

Question 31

Give the Henderson-Hasselbalch equation used to calculate the pH of a buffer solution

Answer 31

pH = pKa + log [salt] / [acid]

Question 32

What is a buffer solution

Answer 32

A solution that minimises the change in pH when a **small** amount of either acid or alkali is added

Question 33

What mixture of buffers is used to control the pH of blood

Answer 33

Carbonic acid-hydrogencarbonate buffer mixture

Question 34

Give the equilibrium equation for the control of blood pH Identify the weak acid and conjugate base

Answer 34

H2CO3 (aq) ⇌ HCO3- (aq) + H+ (aq) H2CO3 = weak acid HCO3- = conjugate base

Question 35

Under normal circumstances give the ratio of H2CO3 : HCO3- ion present in blood plasma

Answer 35

1:20 Due to fact normal metabolism produces more acids than bases which is consistent with needs of the body

Question 36

What is the effect of any increase in H+ ions in the blood? (due to lactic acid for example)

Answer 36

H+ ions will react with HCO3- ions shifting equilibria to the **left**

Question 37

What is the carbonic acid-hydrogencarbonate buffer mixture coupled with ?

Answer 37

The respiratory system of the body H2CO3 (aq) ⇌ CO2 (aq) + H2O (aq) aqueous CO2 is also in equilibrium with gaseous CO2 The 2 equations can be combined