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Chemistry A Level > Chapter 21 - Acids, bases and buffers 1 > Flashcards

Chapter 21 - Acids, bases and buffers 1 Flashcards

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1
Q

What are the 3 types of acids

A

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

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2
Q

What is the definition of an acid

A

it is a proton donor (hydrogen ion donor)

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3
Q

What is the definition of a base

A

It is a proton acceptor (hydrogen ion acceptor)

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4
Q

What is the definition of an acid base reaction

A

It is a reaction which involves the transfer of protons

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5
Q

Is water an acid or a base

A

It is amphoteric so both

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6
Q

What is a strong acid

A

It is one that fully dissociates in water

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7
Q

What is pH

A

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

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8
Q

What is the definition of pH

A

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

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9
Q

What is the equation to find concentration of H+ ions

A

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

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10
Q

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

A

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

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11
Q

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

A

C1V1 = C2V2

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12
Q

What equilibrium is established in water and aqueous solution

A

H20 <-> H+ + OH-

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13
Q

What is the Kc equation for water

A

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

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14
Q

What is Kw

A

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

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15
Q

What happens to Kw as temperature rises

A

As temperature rises so does Kw

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16
Q

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

A

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

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17
Q

Whats the equation of a strong acid dissociating

A

HX -> H+ + X-

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18
Q

Whats the equation for a weak acid dissociating

A

HX <-> H+ + X-

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19
Q

What is Ka

A

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

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20
Q

What happens to Ka as acidity increases

A

it increases

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21
Q

What happens to Ka as temperature increases

A

It increases

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22
Q

What is the equation for pKa

A

= - log (Ka)

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23
Q

Whats the equation for Ka from pKa

A

Ka = 10 -pKa

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24
Q

What happens to pKa as acidity increases

A

It decreases

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25
Q

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

A

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

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26
Q

What equation do we use for weak acids with water

A

Ka = [H+]2 / [HA]

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27
Q

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

A

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

28
Q

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

A

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

29
Q

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

A

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

30
Q

What is a half neutralisation of a weak acid strong base

A

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

31
Q

What is the equivalence point

A

When the same moles of acid and base are mixed together

32
Q

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

A

7

33
Q

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

A

approximately 7

34
Q

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

A

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

35
Q

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

A

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

36
Q

What are indicators

A

They are water soluble weak acids

37
Q

What is the general formula of a indicator

A

Hln

38
Q

What makes a indicator work

A

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

39
Q

What is the universal indicator

A

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

40
Q

What is the equivalence point

A

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

41
Q

What makes an indicator suitable for a specific reaction

A

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

42
Q

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

A

Phenolphthalein or methyl orange

43
Q

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

A

Methyl orange

44
Q

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

A

Phenolphthalein

45
Q

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

A

No suitable indicator

46
Q

What is a buffer solution

A

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

47
Q

What is an acidic buffer

A

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

48
Q

How can you make an acidic buffer

A

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

49
Q

How does an acidic buffer work

A

The weak acid partially dissociates
The salt fully dissociates when it dissolves

HA <-> A- + H+

50
Q

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

A

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

51
Q

What is a basic buffer

A

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

52
Q

How can basic buffers be made

A

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

53
Q

How do basic buffers work

A

The weak base reacts partially with water
The salt fully dissociates when it dissolves

HA +H2O <-> A- + OH-

54
Q

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

A

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

55
Q

When does an acidic buffer work best

A

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

56
Q

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

A

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

57
Q

What steps do you do for acidic buffer calculations

A

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

58
Q

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

A

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

59
Q

What does a gradual pH change mean for the end point

A

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

60
Q

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

A

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

61
Q

What is different about diprotic acid titration curves

A

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

62
Q

Why is [H2O] not shown in the Kw equation

A

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

63
Q

Why is a pH probe washed with distilled water between measurements

A

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

64
Q

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

A

To avoid missing the end point

65
Q

When is an indicator suitable from a titration curve

A

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

Chemistry A Level (20 decks)

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