5.3 - Acids, bases and buffers

Question 1

Define Bronsted Lowry acid

Answer 1

proton donor

Question 2

Define Bronsted Lowry base

Answer 2

proton acceptor

Question 3

Define conjugate base pair

Answer 3

2 species interconnected by transfer of a proton

Question 4

Write an equation for the neutralisation of an alkali

Answer 4

H3O+ + OH- → 2H2O
or H+ + OH- → H2O

Question 5

Define buffer solution

Answer 5

• system that minimises pH change on addition of small amounts of an acid or base
• formed from weak acid and its salt or from excess weak acid and strong alkali

Question 6

Define monobasic acid

Answer 6

acid that can donate 1 H atom per molecule of acid

Question 7

Define dibasic acid

Answer 7

acid that can donate 2 H atoms per molecule of the acid

Question 8

Define tribasic acid

Answer 8

acid that can donate 3 H atoms per molecule of acid

Question 9

Define end point

Answer 9

the point during a titration when the indicator changes colour

Question 10

Define equivalence point

Answer 10

point during a titration when the amount of acid is exactly equal to the amount of base

Question 11

What is ±1 pH equal to in [H+]?

Answer 11

x10 [H+]

Question 12

Which is the only factor to change Ka?

Answer 12

temperature

Question 13

What does a stronger acid have?

Answer 13

• larger Ka
• smaller pKa
• equilibrium right

Question 14

What does a weaker acid have?

Answer 14

• smaller Ka
• larger pKa
• equilibrium left

Question 15

How do dicarboxylic acids and alcohols dissociate?

Answer 15

carboxylic acids: one [H+] at a time
alcohol groups do not dissociate

Question 16

How can Ka be determined experimentally?

Answer 16

prepare a standard solution of weak acid of known concentration, then measuring pH using pH meter

Question 17

When is the dissociation of water significant?

Answer 17

when pH>6

Question 18

Equation for whole number pH and [H+] and [OH-] concentrations

Answer 18

p[H+] + p[OH-] = 14

Question 19

Describe the two ways a buffer solution can be prepared

Answer 19

First way:
- acid added to water and partially dissociates
- salt provides conjugate base and completely dissolves when added to water

Second way:
- weak acid partially neutralised by alkali -> conjugate base
- some weak acid left so mixture contains weak acid and its salt

Question 20

How does adding acid affect position of equilibrium?

Answer 20

• [H+] increases
• H+ reacts with conjugate base A-
• equilibrium shifts left
• removes most H+ ions

Question 21

How does adding alkali affect position of equilibrium?

Answer 21

• [OH-] increases
• H+ + OH- -> H2O
• HA dissociates
• equilibrium shifts right
• [H+] ions restored and alkaline removed

Question 22

When is a buffer most effective?

Answer 22

when [HA] = [A-]

Question 23

Write an equation for the dissociation of carbonic acid

Answer 23

H2CO3 ⇌ H+ + HCO3-

Question 24

Define indicator

Answer 24

weak acid that changes colour with changing pH due to an equilibrium shift between HA and A-

Question 25

Describe recrystallisation method

Answer 25

• dissolve in min. quantity of hot solvent
• cool, filter and dry

Question 26

Effect of temperature on Kw

Answer 26

increases = Kw increases
(and converse)

Question 27

Kw for pure water

Answer 27

[H+]²

Question 28

pH of pure water at temperatures above 298K

Answer 28

<7

Question 29

What does an equilibrium shift left mean for acid strength?

Answer 29

• further left: weaker acid
• so less it dissociates
• so stronger its conjugate base

Question 30

Assumptions for weak acid calculations

Answer 30

• [H+] = [A-] and ignore any H+ from H2O
• equilibrium [HA] = initial [HA]
• [HA] much greater than equilibrium [H+] (only if Ka<0.001 or not dilute solution)

Question 31

Why might a measured pH not be true value?

Answer 31

• no concordant results
• pH probe not calibrated
• measuring cylinder used instead of graduated pipette and filler

Question 32

When is pH = pKa?

Answer 32

half way to end point
(more accurate for one experiment)