Chapter 21- Acids, bases and buffers

Question 1

What is a Bronsted-Lowry acid?

Answer 1

A substance that donates protons

Question 2

What is a Bronsted-Lowry base?

Answer 2

A substance that accepts protons

Question 3

What happens when a Bronsted-Lowry acid is added to water?

Answer 3

Release H+ ions which react with water
HA + H2O –> H3O+ + A-

Question 4

What happens when a Bronsted-Lowry base is added to water?

Answer 4

Bond to H+ ions in water molecules
B + H2O –> BH+ + OH-

Question 5

What is a strong acid?

Answer 5

An acid that completely dissociates in solution

Question 6

What is a weak acid?

Answer 6

An acid that only slightly dissociates when in solution so only a small number of H+ ions are released

Question 7

What is a strong base?

Answer 7

A base that completely ionises in water

Question 8

What is a weak base?

Answer 8

A base that only slightly ionises in water

Question 9

What is pH?

Answer 9

A measure of the acidity or alkalinity of a solution

Question 10

How do you find pH given [H+]?

Answer 10

pH = -log[H+]

Question 11

How do you find [H+] given pH?

Answer 11

[H+] = 10^-pH

Question 12

How does the pH scale work?

Answer 12

A logarithmic scale from 0-14 which is a measure of [H+] in a solution, showing their acidity or alkalinity
pH<7 is acidic, pH>7 is alkaline

Question 13

How many decimal places is a pH given to?

Answer 13

2 dp

Question 14

What is Kw?

Answer 14

The ionic product of water, which is the equilibrium constant for the dissociation of water

Question 15

What is the equation and value for Kw?

Answer 15

Kw= [H+][OH-]
At 298K, Kw = 1 x 10^-14

Question 16

Where does Kw come from?

Answer 16

H2O ⇌ H+ + OH-
Kc = [H+][OH-]/[H2O]
As H2O barely dissociates, [H2O] is very large so approximation means Kc (Kw) = [H+][OH-]

Question 17

How does Kw change with temperature?

Answer 17

Forward reaction in equilibrium of water is endothermic so as temp increases, forward reaction is favoured so [H+] increases so becomes more acidic

Question 18

What is Ka and how does it show the strength of an acid?

Answer 18

The dissociation constant for a weak acid. If Ka is large, equilibrium lies to the right so [H+] is greater meaning the larger the Ka, the stronger the acid

Question 19

What is the equation for Ka and what is important when it is used for weak acids?

Answer 19

HA ⇌ H+ + A-
Ka = [H+][A-]/[HA]
important: [A-] = [H+] when dissociation occurs
[HA] assumed to be the same before and after dissociation as the change is so small

Question 20

What are some equations involving Ka?

Answer 20

pKa = -logKa
Ka = 10^-pKa

Question 21

What is special when [HA] = [A-] for a weak acid?

Answer 21

Ka = [H+]
so pKa = pH

Question 22

What is a pH curve?

Answer 22

A graph of pH against volume of acid/base added to a solution

Question 23

What are some key features of a pH curve?

Answer 23

Start pH, end pH
End point pH
Equivalence point

Question 24

What is the equivalence point?

Answer 24

The point where full neutralisation occurs, the point where [H+] = [OH-]

Question 25

What is the end point?

Answer 25

The point during titration where the indicator changes colour. A suitable indicator should change colour near the equivalence point

Question 26

What is the end point pH for a strong acid v strong base titration?

Answer 26

pH= 7

Question 27

What is the end point pH for a strong acid v weak base titration?

Answer 27

pH<7 (more acidic)

Question 28

What is the end point pH for a weak acid v strong base titration?

Answer 28

pH>7 (more alkaline)

Question 29

What is the end point pH for a weak acid v weak base titration?

Answer 29

normally pH=7 but hard to determine

Question 30

What is an indicator?

Answer 30

A chemical solution that will change colour when there is a change in pH, they are weak organic acids

Question 31

How does an indicator work?

Answer 31

HIn ⇌ H+ + In-
HIn: colour A (acidic conditions), in acid equ shifts to left to decrease [H+] producing colour A
In-: colour B (alkaline conditions), conjugate base, in alkaline equ shifts to right to increase [H+] as they react with OH-, producing colour B

Question 32

What is the pH for colour change and what is it for methyl orange?

Answer 32

Colour change red to yellow at pH 3-5

Question 33

What is the pH for colour change and what is it for phenolphthalein?

Answer 33

Colour change colourless to pink at pH 8-10

Question 34

What is the pH for colour change and what is it for bromothymol blue?

Answer 34

Colour change yellow to blue at pH 6-8

Question 35

How do you find the point at which an indicator changes colour?

Answer 35

Using Ka, Ka = [H+][In-]/ [HIn], for change point [In-]=[HIn] so Ka = [H+] so change pH is pKa=pH

Question 36

What is a buffer solution?

Answer 36

A solution which resists pH change when small amounts of a strong acid/base are added

Question 37

What is an acidic buffer?

Answer 37

A buffer containing a weak acid and salt of the weak acid (conjugate base)

Question 38

What is an alkaline buffer?

Answer 38

A buffer containing a weak base and salt of the weak base (conjugate acid)

Question 39

How do you find pH of a buffer?

Answer 39

HA ⇌ H+ + A-
Use Ka to find [H+] then find pH

Question 40

What are the proportions of reactants in an acid buffer?

Answer 40

Lots of weak acid, lots of conjugate base (due to it from salt as well) and little H+