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Chemistry Year 13 > 3.1.12 Acids and bases > Flashcards

3.1.12 Acids and bases Flashcards

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1
Q

What is a Bronsted - Lowry acid?

A
  • Proton donors
  • When we mix acids with water , hydrogen ions (H+) are released
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2
Q

What are Bronsted- Lowry bases

A
  • Proton acceptors
  • When we mix bases with water they react with H+ ions to form hydroxide ions (OH-)
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3
Q

What do strong bases do

A

Dissociate (ionise ) almost completely

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4
Q

What do weak bases do

A

Dissociate poorly

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5
Q

what do strong acids do

A

Dissociate (ionise) almost completely

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6
Q

What do weak acids do

A

Dissociate poorly

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7
Q

What is an example of a weak acid and some information about it

A
  • Ethanoic acid and other carboxylic acids
  • Backwards reaction favoured so not many H+ (protons) produced
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8
Q

What are some examples of a strong acid and some information about it

A
  • HCl (hydrochloric acid)
  • H2SO4 ( Sulfuric acid)
  • HNO3 ( nitric acid)
    Forwards reaction favoured strongly . Lots of H+ produced
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9
Q

What are examples of a strong base and some information about it

A

NaOH ( sodium hydroxide )
KOH ( potassium hydroxide)
Forward reaction favoured strongly . Lots of OH- ions produced

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10
Q

What is an example of a weak base and some information about it

A

NH3 ( ammonia)
Backwards reaction favoured so not many OH- ions produced

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11
Q

What happens when acids and bases react with each other

A

Protons are exchanged

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12
Q

What does water do when acid is added to it

A

Acts as a base (accepts a proton)

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13
Q

What is the Kc equation to represent water

A

[H+][OH-] / [H2O]

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14
Q

What can water dissociate into and what is the equation to show this

A

Water dissociates into OH- ions and H+ ions
H2O (reversible reaction) H+ + OH-

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15
Q

What is Kw?

A

Ionic product of water

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16
Q

What are the units for Kw?

A

mol^2dm-6

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17
Q

what is the expression for Kw

A

[H+][OH-]

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18
Q

What is the value of Kw

A

1.00 x10^-14 mol^2dm^-6

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19
Q

When does the value of Kw change

A

When temperature changes
- Value of Kw is the same in a solution at a given temperature

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20
Q

What concentration of OH- ions and H+ ions does pure water have

A

The same concentration of H+ and OH- ions.

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21
Q

What is the expression for pure water

A

Kw = [H+]^2

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22
Q

What is pH

A

Is a logarithmic scale that measures the concentration of H+ ions in solution

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23
Q

What is the pH equation

A

pH = -log10[H+]

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24
Q

What is the equation to work out H+ ions from pH?

A

[H+] = 10^-pH
(press shift then log for the 10)

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25
Q

What are examples of monoprotic acids

A

HCl
HNO3

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26
Q

What are monoprotic acids

A

They are acids which dissociate to produce one H+ ion for every acid molecule. This means the he concentration of the acid = the concentration of H+ ions

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27
Q

What are diprotic acids

A

They dissociate to produce two H+ ions for every acid molecule. This means the concentration of the acid = 2 x the concentration of the H+
ions

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28
Q

What is an example of a diprotic acid

A

H2SO4 - sulfuric acid

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29
Q

What is the equation to work out the pH of a diprotic acid from H+ ions

A

2 [H+] = [Acid]
pH = -log10[H+]

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30
Q

What do strong bases do

A

Dissociate to produce one OH- ion for every base molecule . This means the concentration of the base = the concentration of the OH- ions

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31
Q

How do we calculate the pH of a base

A

We need the [H+] . To get this we need to use the Kw expression.
To work out [H+] we need to know Kw and [OH-] at a specific temperature . Once we know [H+] we can work out the pH

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32
Q

what decimal place should pH always be in

A

To 2 decimal places

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33
Q

what is the equation to work out [H+] of diluted solution once water is added

A

[H+] of original acid solution x old volume / new volume

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34
Q

why do we have to use another constant for weak acids

A

weak acids only dissociate slightly in aqueous solutions so we have to use another constant to help work out their pH values

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35
Q

what is the equilibrium constant for weak acids

A

Acid dissociation constant Ka

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36
Q

what’s the equilibrium for weak acids

A

HA (reversible reaction sign) H+ + A-

37
Q

What are the two assumptions of weak acids

A

1) Only a small amount of the weak acid dissociates so we can assume that the concentration of the acid at equilibrium is approximately equal to the acid at the start
Ka = [H+] [A-] / [HA]
• the dissociation of acid is greater than the dissociation of water present in the solution . We can assume all the H* ions come from the acid
Ka = [H+] ^2 / [HA]

38
Q

what are the units for Ka

A

moldm-3

39
Q

what’s the expression for Ka

A

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

40
Q

what can the Ka expression be simplified to

A

Ka = [ H+] ^2 / [HA]

41
Q

What is pKa

A

Another way of measuring the strength of an acid similar to pH. The lower the value , the stronger the acid

42
Q

What’s the expression for pKa

A

pKa = -log10Ka

43
Q

What’s the expression for pKa

A

pKa = -log10Ka

44
Q

What’s the expression for Ka from pKa

A

Ka = 10 ^-pKa

45
Q

what’s the expression if HA = OH ( half a neutralisation reaction )

A

pH = pKa

46
Q

EQ : Ionic product of water , Kw = [H+] [OH-]
Explain why H2O is not shown in the expression

A

H20 is constant

47
Q

EQ : Explain why water is neutral at 50 degrees ( example ) (1)

A

[H+] = [OH-]

48
Q

EQ: For pure water at 40 degrees , pH = 6.67 . A student thought that the water was acidic . Explain why the student was incorrect (1)

A

[H+] = [OH-]

49
Q

How are titration practicals performed

A
  • have an acid or base in burette with known concentrations
  • add the chemical in the burette to the conical flask until the indicator changes colour. This is known as the end point
  • Add drop by drop near end point
  • Have an acid or base with an unknown concentration but known volume in conical flask , add a few drops off indicator too
50
Q

what is the titration curve of strong acid and. strong base

A

S shaped
- graph starts at pH 1 as there is excess strong acid
- Ends at around pH 13 as we now have excess strong base

51
Q

what is the titration curve of strong acid / weak base

A

smaller s shape
- graph starts at pH 1 as there is excess strong acid
- Ends at around pH 9 as we now have excess weak base

52
Q

What is the pH curve of weak acid / strong base

A

S shaped at the top
- graph starts at around PH 5 as there is excess weak acid
- Ends at around pH 13 as we now have excess strong base

53
Q

what is the titration curve for weak acid and weak base

A

not a prominent S shaped curve ( quite straight )
- graph starts at around pH 5 as there is excess weak acid
- Ends at around pH as we now have excess weak base

54
Q

what is the equivalence point

A

At this point the acid has been neutralised fully by the base . The sharp vertical rise shows a rapid change in pH

55
Q

when is the change in pH the smallest

A

When using a weak acid and weak base together in a titration

56
Q

what is the half neutralisation point

A

this is the point half way between zero and the equivalence point . It can be used to calculate pKA of a weak acid by taking the pH at this point

57
Q

How can we work out pKa using a titration curve

A

[HA] = [A-]
Therefore Ka = [H+]
-logKa = -log[H+]
so pka = pH

58
Q

what are the two most common types of indicators

A

methyl orange
phenolphthalein

59
Q

what is the colour change for methyl orange and when do you use it

A

red at low pH and yellow at high pH
It can be used for :
- strong acid / strong base
- strong acid / weak base

60
Q

what is phenolphthalein used in and what is the colour change

A

Colourless at low pH and pink at high pH
It can be used for :
- Weak acid / strong base

61
Q

why can’t you use a pH indicator for weak acids and weak base reactions

A

They have no sharp pH change . No indicator is suitable so we have to use a pH meter

62
Q

why do diprotic acids have 2 pH curves

A
  • when reacting with a base it’s neutralised in 2 steps
  • This is because protons are released from acid molecules separately
    • first equivalence point shows how OH- ion is reacting with the first proton
    • second equivalence point shows how OH- ion is reacting with the second proton
63
Q

What is a buffer

A

A chemical that resists the change in pH when small amounts of acid or base are added

64
Q

what are the 2 types of buffer

A
  • acidic
  • basic
65
Q

what is an acidic buffer

A

they resist the change in pH in order to keep the solution below pH 7. They are made from a weak acid and its salt

66
Q

In an acidic buffer , where does the equilibrium lie in the weak acid

A
  • 2 equilibrium equations co-exist in the same beaker
    • Weak acids dissociate weakly so equilibrium lies well over to the left
67
Q

In an acidic buffer , where does equilibrium lie in salts

A
  • 2 equilibrium equations co-exist in the same beaker
  • Salts dissociate fully so equilibrium lies well over to the right
68
Q

what are basic buffers

A

Resist the change in pH in order to keep the solution above pH 7.
They are made from a weak base and its salt

69
Q

in a basic buffer , where does equilibrium lie with its weak base

A

weak based produce very little OH- ions so equilibrium lies well over to the left

70
Q

In a basic buffer where does equilibrium lie in its salts

A

Salts dissociate strongly so equilibrium lies well over to the right

71
Q

what do you need to know to calculate the pH of a buffer

A

The Ka value and the concentration of the weak acid and its salt

72
Q

what can we assume in calculations of butters

A

Assume salts dissociate fully and weak acids dissociate poorly so
salt = [A-]
and
[HA start] = [HA equilibrium]

73
Q

what are the uses of buffers in shampoo

A

Shampoo are acidic with a pH if 5.6
Hair becomes dry and damaged if exposed to alkaline conditions
The buffer helps to resist changes in the pH to keep hair soft and strong

74
Q

what are the uses of buffers in washing powder

A

Biological washing powders contain enzymes which can only work efficiently at a specific pH
Buffers help to resist changes in pH to allow them to work in their optimum pH

75
Q

what are the uses of buffers in blood

A

It’s vital to make sure blood pH is maintained a close to pH 7.4 as possible
Our body systems rely on this so a buffer is present in our blood to help. CO2 plays a big role here

76
Q

EQ : what is meant by the term strong when describing an acid (1)

A

Completely ionises to give H+ ions in water

77
Q

what is the end point

A

stage in a titration where there’s a colour change

78
Q

EQ : state why calibrating a pH meter just before it is used improved the accuracy of the pH measurement (1)

A

Over time meter does not give accurate readings

79
Q

EQ : Describe how you would obtain the pH curve for the titration (5)

A
  • measuring starting pH of acid
  • Add alkali from burette in 1cm3 portions ( 1-2cm3)
  • Stir or swirl flask
  • Measure pH after each addition
  • Repeat until alkali in excess
  • add alkali in smaller increments, 0.1cm3 as you near the end point
80
Q

EQ : Suggest a suitable piece of apparatus that could be used to measure out the sodium hydroxide solution . Explain why this apparatus is more suitable than a pipette for this purpose (2)

A

Burette
Can deliver variable volumes

81
Q

EQ : Explain why chloroethanoic acid is a stronger acid than ethanoic acid (2)

A
  • Chlorine is more electronegative so withdraws electrons
  • Weakens O-H bond
82
Q

EQ : Explain why data books do not usually contain values of Ka for strong acids (2)

A
  • Strong acids completely dissociated .
  • Ka value for strong acids is very large
83
Q

EQ : Explain why the expression for Kw does not include the conc for water (2)

A
  • Conc of H2O is very high / very few H+ and OH- ions
  • Conc of H2O is constant
84
Q

EQ : Explain why the value of Kw increases as the temp increases (2)

A
  • Breaking bonds is endothermic
  • Equilibrium shifts to the right to oppose increase in temp
85
Q

EQ : HCO3- (reverse sign) CO3 2- + H+
Explain how a solution containing sodium hydrogencarbonate and sodium carbonate can act as a buffer when small amounts of acid or small amounts of alkali are added . (3)

A

acid :
• Increase in concentration of H+ ions , equilibrium moves to the left .
alkali :
• OH- reacts with H+ ions , equilibrium moves to the right to replace the H+ ions
- Concentration of H+ remains constant

86
Q

EQ : explain why [H2O] is not shown in the Kw expression

A

[H2O] is constant

87
Q

EQ : Suggest why the pH prove is washed with distilled water between each of the calibration measurements (1)

A

Different solutions must not contaminate each other

88
Q

EQ : The calibrated pH meter is used to monitor the pH using a titration of HCl and NaOH.
Explain why the volume of NaOH solution added between each pH measurement is smaller as the end point of the titration is approached (1)

A

To avoid missing the end point

89
Q

EQ : A buffer solution has a constant pH even when diluted . Use a mathematical expression to explain this . (1)

A

[HX] / [X-]

Chemistry Year 13 (6 decks)

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