Chapter 21 - Acids, bases and buffers 1

Question 1

What are the 3 types of acids

Answer 1

Monoprotic - 1 hydrogen ion
Diprotic - 2 hydrogen ions
Triprotic - 3 hydrogen ions

Question 2

What is the definition of an acid

Answer 2

it is a proton donor (hydrogen ion donor)

Question 3

What is the definition of a base

Answer 3

It is a proton acceptor (hydrogen ion acceptor)

Question 4

What is the definition of an acid base reaction

Answer 4

It is a reaction which involves the transfer of protons

Question 5

Is water an acid or a base

Answer 5

It is amphoteric so both

Question 6

What is a strong acid

Answer 6

It is one that fully dissociates in water

Question 7

What is pH

Answer 7

It is a measure of the concentration of H+ ions in a solution

Question 8

What is the definition of pH

Answer 8

pH = -log (H+)
Give to 2 decimal places

Question 9

What is the equation to find concentration of H+ ions

Answer 9

(H+) = 10 -pH
Give to 2 decimal places

Question 10

What happens when water is added to solution of a strong acid

Answer 10

The number of moles stays the same but the volume and concentration change

Question 11

What equation do you use for a dilution of a strong acid

Answer 11

C1V1 = C2V2

Question 12

What equilibrium is established in water and aqueous solution

Answer 12

H20 <-> H+ + OH-

Question 13

What is the Kc equation for water

Answer 13

Kc = [H+][OH-] / [H2O]
Kc x [H2O] = [H+][OH-]

Question 14

What is Kw

Answer 14

It is equivalent of Kc x [H2O]
Kw = [H+][OH-]
Kw = 1x10-14 at 298K

Question 15

What happens to Kw as temperature rises

Answer 15

As temperature rises so does Kw

Question 16

What are the steps needed to calculate the pH of a strong acid and strong base

Answer 16

Moles H+
Moles OH-
XS moles of H+ or OH-
XS concentration of H+ or OH-
pH

Question 17

Whats the equation of a strong acid dissociating

Answer 17

HX -> H+ + X-

Question 18

Whats the equation for a weak acid dissociating

Answer 18

HX <-> H+ + X-

Question 19

What is Ka

Answer 19

The acid dissociation constant
Used for weak acids
Ka = [H+][A-] / [HA]

Question 20

What happens to Ka as acidity increases

Answer 20

it increases

Question 21

What happens to Ka as temperature increases

Answer 21

It increases

Question 22

What is the equation for pKa

Answer 22

= - log (Ka)

Question 23

Whats the equation for Ka from pKa

Answer 23

Ka = 10 -pKa

Question 24

What happens to pKa as acidity increases

Answer 24

It decreases

Question 25

What is assumed for calculations of a monoprotic weak acid in water

Answer 25

the concentration of H+ is the same as the concentration of A-

Question 26

What equation do we use for weak acids with water

Answer 26

Ka = [H+]2 / [HA]

Question 27

What can't be used for a weak acid strong base calculation

Answer 27

That the concentration of H+ is the same as A-

Question 28

What steps do you use for a weak acid strong base calculation if HA is in XS

Answer 28

Moles HA Moles OH- XS HA Moles of HA left and A- formed Concentration of HA and A- Use Ka to find [H+] pH

Question 29

What steps do you use for a weak acid strong base calculation if OH- is in XS

Answer 29

Moles HA Moles OH- XS OH- Use Kw to find [H+] pH

Question 30

What is a half neutralisation of a weak acid strong base

Answer 30

It is when the moles of HA = A- This means Ka = [H+] pKa = pH

Question 31

What is the equivalence point

Answer 31

When the same moles of acid and base are mixed together

Question 32

What is the pH at equivalence point for a strong base strong acid mixture

Answer 32

7

Question 33

What is the pH at equivalence point for a weak acid weak base mixture

Answer 33

approximately 7

Question 34

What is the pH at equivalence point for a weak base strong acid mixture

Answer 34

Below 7 Because a salt of a weak base which is acidic is formed

Question 35

What is the pH at equivalence point for a weak acid strong base mixture

Answer 35

Above 7 Because a salt of a weak acid which is basic

Question 36

What are indicators

Answer 36

They are water soluble weak acids

Question 37

What is the general formula of a indicator

Answer 37

Hln

Question 38

What makes a indicator work

Answer 38

Hln and ln- have different colours The rapid change in pH at equivalence causes the equilibrium to shift from one side to the other

Question 39

What is the universal indicator

Answer 39

It is a mixture of several indicators so shows lots of colours

Question 40

What is the equivalence point

Answer 40

The point at which equimolar amounts of acid and alkali are present

Question 41

What makes an indicator suitable for a specific reaction

Answer 41

Have an end point that coincides with the rapid change of pH at the equivalence point Change colour quickly upon the addition of a drop of acid or alkali Give a distinct colour change Range of pH values matches up with equivalence point range

Question 42

What is the best indicator for a strong acid strong base reaction

Answer 42

Phenolphthalein or methyl orange

Question 43

What is the best indicator for a strong acid weak base reaction

Answer 43

Methyl orange

Question 44

What is the best indicator for a weak acid strong base reaction

Answer 44

Phenolphthalein

Question 45

What is the best indicator for a weak acid weak base reaction

Answer 45

No suitable indicator

Question 46

What is a buffer solution

Answer 46

It is one that resists change in pH upon addition of small amounts of acid and alkali

Question 47

What is an acidic buffer

Answer 47

It is a solution with a pH less than 7 and contains a weak acid and salt of the weak acid

Question 48

How can you make an acidic buffer

Answer 48

Mixing a weak acid and its salt Half / partially neutralising a weak acid with a strong base

Question 49

How does an acidic buffer work

Answer 49

The weak acid partially dissociates The salt fully dissociates when it dissolves HA <-> A- + H+

Question 50

What happens when you add H+ and OH- ions to a acidic buffer

Answer 50

H+ ions move the equilibrium left as there are XS H+ ions OH- ions move the equilibrium right as they react with the H+ ions

Question 51

What is a basic buffer

Answer 51

A solution with a pH greater than 7 and contains a weak base and the salt of that weak base

Question 52

How can basic buffers be made

Answer 52

Weak base and its salt Half/Partially neutralise a weak base with a strong acid

Question 53

How do basic buffers work

Answer 53

The weak base reacts partially with water The salt fully dissociates when it dissolves HA +H2O <-> A- + OH-

Question 54

What happens when you add H+ and OH- to a basic buffer

Answer 54

H+ ions cause the equilibrium to shift right as it reacts with OH_ ions OH- ions cause the equilibrium to shift left as there is a higher concentration of OH- ions

Question 55

When does an acidic buffer work best

Answer 55

When its pH is equal to the pKa value of the weak acid as it will be equally good at resisting change in pH when acid and alkali are added

Question 56

What equation is used to find the pH of acidic buffer solutions

Answer 56

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

Question 57

What steps do you do for acidic buffer calculations

Answer 57

mol acid/HA and salt/A- Concentration acid and salt Concentration of H+ pH

Question 58

What steps do you do for acidic buffer calculations when acid or base's are added

Answer 58

mol acid/HA and salt/A- Concentration acid and salt Concentration of H+ pH of buffer mol of added acid or base ICE table - moles of HA, A- or OH- Concentration of HA, A- or OH- Concentration of H+ pH

Question 59

What does a gradual pH change mean for the end point

Answer 59

The indicator would change over a range of pH so end point is hard to determine

Question 60

What equation explains why buffers pH doesn't change when diluted

Answer 60

ratio of [HX] / [X-] remains almost constant

Question 61

What is different about diprotic acid titration curves

Answer 61

They have 2 different vertical sections As the second H+ is only dissociated once all the first H+ ions are reacted

Question 62

Why is [H2O] not shown in the Kw equation

Answer 62

[H2O] is almost constant [H2O] is incorporated into Kw

Question 63

Why is a pH probe washed with distilled water between measurements

Answer 63

To wash away and remaining solution which could interfere with the reading

Question 64

Why is the volume of base added smaller nearer the end point

Answer 64

To avoid missing the end point

Question 65

When is an indicator suitable from a titration curve

Answer 65

When it has a pH range within the steep part of the titration curve