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AQA A Level Physical Chemistry > 12. Acids and Bases > Flashcards

12. Acids and Bases Flashcards

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1
Q

What is a Bronsted-Lowry acid?

A

A proton donor

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2
Q

What is a Bronsted-Lowry base?

A

A proton acceptor

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3
Q

When can protons only be given up by acids?

A

When there is a base that accept them

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4
Q

What do reactions between acids and bases involve?

A

Proton transfer

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5
Q

Where do most common acid-base reactions take place?

A

In aqueous solution

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6
Q

How are acids and bases generally defined? Why is this?

A

By the way they react with water - as most acid-base reactions take place in aqueous solution

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7
Q

What does how strong an acid/base is depend on?

A

How much they dissociate in water

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8
Q

How much do strong acids dissociate in water?

A

Almost completely dissociate

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9
Q

What is another phrase for ‘dissociates’?

A

Ionises

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10
Q

General equation for strong acids/bases dissociating in water?

A

HA ⇋ H⁺ + A⁻

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11
Q

How much do weak acids dissociate in water?

A

Only slightly dissociate

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12
Q

Example of a strong acid dissociating in water?

A

HCl ⇋ H⁺ + Cl⁻

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13
Q

Example of a strong base dissociating in water?

A

NaOH ⇋ Na⁺ + OH⁻

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14
Q

Example of a weak acid dissociating in water?

A

CH₃COOH ⇋ CH₃COO⁻ + H⁺

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15
Q

Example of a weak base dissociating in water?

A

NH₃ + H₂O ⇋ NH₄⁺ + OH⁻

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16
Q

Is the strength of an acid/base different to its concentration?

A

Yes

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17
Q

What does the acidity of a solution depend on?

A

The concentration of H⁺ (aq) ions

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18
Q

What is pH defined as?

A

-log₁₀[H⁺]

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19
Q

What type of scale is the pH scale? Why is this?

A

Logarithmic, as the concentration of hydrogen ions in aqueous solution covers a wide range

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20
Q

When there is a smaller pH, what will be true of the concentration of H⁺ ions?

A

Higher concentration of H⁺

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21
Q

What types of acid are there?

A
  • monoprotic

* diprotic

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22
Q

What is a monoprotic acid?

A

One that releases one H⁺ ion per molecule

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23
Q

What is a diprotic acid?

A

One that releases two H⁺ ions per molecule

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24
Q

Is HCl monoprotic or diprotic?

A

Monoprotic

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25
Q

Is H₂SO₄ monoprotic or diprotic?

A

Diprotic

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26
Q

In strong acids, what is the moles of a monoprotic acid equal to? Why?

A

The moles of H⁺, therefore [H⁺] = conc. of acid, because strong acids fully dissociate

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27
Q

In strong diprotic acids, what are the moles of H⁺ equal to?

A

Double the moles of acid, therefore [H⁺] = double conc. of acid

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28
Q

What effect do diprotic acids have on pH compared to monoprotic?

A

Results in lower pH (stronger acid)

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29
Q

How many decimal places is pH given to?

A

2

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30
Q

How can pH be used to calculate the concentration of H⁺?

A

[H⁺] = 10⁻ᵖᴴ

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31
Q

If pH is used to find [H⁺] of a monoprotic acid, what is the concentration of the acid?

A

Conc. of H⁺ = conc. of acid

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32
Q

If pH is used to find [H⁺] of a diprotic acid, what is the concentration of the acid?

A

Conc. of H⁺ must be halved to determine conc. of acid

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33
Q

What effect will diluting a strong acid have?

A
  • reduce its concentration
  • hence concentration of H⁺ ions
  • increasing pH slightly
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34
Q

How would you calculate the pH of the solution when 100 cm³ water is added to 50 cm³ of 0.1 moldm⁻³ HNO₃

A

[H⁺] original solution = 0.1

[H⁺] diluted solution = 0.1 x (original volume/total volume)

= 0.1 x 50/150 = 0.0333

pH = -log(0.0333) = 1.48

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35
Q

How would you calculate the pH when an original solution is diluted with water?

A

[H⁺] of diluted solution = [H⁺] of original solution x (original volume/total volume)

pH = -log(ANS)

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36
Q

What does it mean, that water is slight dissociated?

A

It exists not just as molecules but in equilibrium

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37
Q

What is the equation for dissociation of water?

A

H₂O ⇋ H⁺ + OH⁻

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38
Q

Do H⁺ ions exist in water alone?

A

No

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39
Q

What do H⁺ ions exist as in water?

A

Hydroxonium ions (H₃O⁺)

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40
Q

Equation for hydroxonium ions?

A

H₃O⁺

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41
Q

What equation can be used to show how water acts as a base?

A

2H₂O ⇋ H₃O⁺ + OH⁻

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42
Q

What is the equilibrium expression for the dissociation of water?

A

Kc = [H⁺] [OH⁻] / [H₂O]

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43
Q

Why is the Kc of water the same as Kw?

A

The concentration of water is constant

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44
Q

What is the equation for Kw?

A

Kw = [H⁺(aq)] [OH⁻(aq)]

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45
Q

What is Kw also called?

A

The ionic product of water

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46
Q

What is the value of Kw at 298K?

A

1 x 10⁻¹⁴

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47
Q

What is true of the concentration of H⁺ and OH⁻ in pure water?

A

They must be equal

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48
Q

What does it mean for the Kw equation, that H⁺ and OH⁻ concentrations are equal?

A

Expression simplified to Kw = [H⁺]²

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49
Q

What type of reaction is the dissociation of water?

A

Equilibrium

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50
Q

What would happen to the pH of water if the temperature was increased?

H₂O ⇋ H⁺ + OH⁻ (ΔH = 57.3 kJmol⁻¹)

A
  • equilibrium shifts right in endothermic reaction to lower temperature
  • which increases concentration of H⁺ ions
  • so decreases pH
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51
Q

Why is water always neutral?

A

The number of H⁺ and OH⁻ ions are always equal, despite pH varying

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52
Q

Is water always neutral?

A

Yes

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53
Q

Is neutral always at pH 7?

A

No

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54
Q

What is the pH of water dependent on?

A

Temperature

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55
Q

What happens if a base is dibasic, when calculating the pH of a strong base?

A

[OH⁻] is twice the concentration of the base

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56
Q

When calculating the concentration of a strong base given the pH, what is the concentration of OH⁻ ions equal to?

A
  • the concentration of the monobasic base

* dibasic base - concentration of OH⁻ needs to be halved to determine concentration of base

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57
Q

What are weak acids and bases?

A

Those that are only partially dissociated when dissolved in water

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58
Q

What does it mean, that weak acids and bases only partially dissociate in water?

A

Not all of the molecules split up into ions

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59
Q

Is the extent of dissociation of a substance linked to concentration?

A

No

60
Q

Why is it possible to have a high concentration of a weak acid/base?

A

The extent of dissociation is not linked to concentration

61
Q

What is the equilibrium equation for when a weak acid dissociates?

A

HA ⇋ H⁺ + A⁻

62
Q

For a weak acid, where will equilibrium lie?

A

To the left

63
Q

What is the equilibrium constant for a weak acid called?

A

The acid dissociation constant (Ka)

64
Q

What is Ka?

A

The acid dissociation constant

65
Q

What is the acid dissociation constant (Ka)?

A

The equilibrium constant for a weak acid

66
Q

How is Ka written?

A

[H⁺] [A⁻] / [HA]

or

[H⁺]² / [HA]

67
Q

How are the units for Ka worked out?

A

The same as other equilibrium constants - cancelling using the equation

68
Q

What will a stronger acid mean for Ka?

A

Larger value of Ka

69
Q

What will a larger value of Ka mean for the strength of an acid?

A

Stronger acid

70
Q

Why is an acid stronger with a larger value of Ka?

A

More H⁺ is dissociated and equilibrium lies further to the right

71
Q

In weak acids, is the concentration of acid equal to the [H⁺]?

A

No

72
Q

What does it mean, that in weak acids concentration of acid ≠ [H⁺]?

A

[H⁺] must be calculated

73
Q

For weak acids, how is [H⁺] calculated?

A
  • looking closer at Ka, for every mole H⁺ dissociated, one mole A⁻ is also dissociated
  • so equation becomes Ka = [H⁺] / [HA]
74
Q

Why does the equation for Ka become [H⁺] / [HA]?

A

In a weak acid, for every one mole H⁺ dissociated, one mole of A⁻ is also dissociated

75
Q

What type of acid is ethanoic acid?

A

Weak acid

76
Q

What can be assumed about the concentration of ethanoic acid initially and at equilibrium?

A

As it is a weak acid, it can be assumed that concentrated at equilibrium = initial concentration

77
Q

When is pKa more convenient to use?

A

When Ka is very small

78
Q

How is pKa calculated from Ka?

A

pKa = -log₁₀Ka

79
Q

What can pKa measure?

A

How strong or weak an acid is

80
Q

What does a smaller value of pKa mean for the strength of an acid?

A

Stronger acid

81
Q

Are there units for pKa?

A

No

82
Q

How is Ka calculated from pKa?

A

Ka = 10⁻ᵖᴷᵃ

83
Q

Are pH calculations involving weak bases required at A-level?

A

No

84
Q

Are carboxylic acids strong or weak?

A

Weak

85
Q

Examples of strong acids?

A

HCl, HNO₃ and H₂SO₄

86
Q

Example of a weak base?

A

NH₃

87
Q

Examples of strong bases?

A

NaOH, KOH and Ba(OH)₂

88
Q

In an acid-base titration, what is the acid added from?

A

A burette

89
Q

What is added to what in an acid-base titration?

A

Acid is added from a burette to a measured volume of alkali using a pipette

90
Q

Steps to calculate the pH of a mixture of strong acids and strong bases?

A
  1. Calculate moles H⁺
  2. Calculate moles OH⁻
  3. Determine which in XS (biggest moles)
  4. Calculate moles H⁺/OH⁻ in XS
  5. Calculate [H⁺] or [OH⁻] in XS
  6. Calculate pH
91
Q

When a weak acid and strong base react, what is true for every mole of OH⁻ added?

A

For every mole OH⁻ added, one mole HA is used up and one mole A⁻ is formed

92
Q

Steps to calculate pH of a mixture of weak acids with strong bases, when HA is in excess?

A
  1. Calculate moles HA (it is still HA not H⁺ as it’s weak)
  2. Calculate moles OH⁻
  3. Determine which in XS
  4. Calculate moles HA/OH⁻ in XS
  5. Calculate moles HA left and A⁻ formed
  6. Calculate leftover [HA] and [A⁻] fomed
  7. Use Ka to find [H⁺]
  8. Find pH
93
Q

Steps to calculate pH of a mixture of weak acids with strong bases, when OH⁻ is in excess?

A
  1. Calculate moles HA (it is still HA not H⁺ as it’s weak)
  2. Calculate moles OH⁻
  3. Determine which in XS
  4. Calculate moles HA/OH⁻ in XS
  5. Calculate [OH⁻]
  6. Use Kw to find [H⁺]
  7. Find pH
94
Q

When a weak acid reacts with a strong base, when does [HA] = [A⁻]?

A

When half of the HA molecules have reacted with OH⁻

95
Q

What is the half equivalence point?

A

When a weak acid and strong base react, and half the HA molecules react with OH⁻ so that [HA] = [A⁻]

96
Q

What does Ka equal at the half equivalence point?

A

Ka = [H⁺]

97
Q

What does it mean, that at the half equivalence point Ka = [H⁺]?

A

pH at half equivalence = pKa of the weak acid

98
Q

What can a pH meter be used to measure?

A

Changes in pH at points before and after the end point in a titration

99
Q

What can be done after a pH meter measures the change in pH at points before and after the endpoint?

A

A graph of pH against volume of solution can be plotted

100
Q

Is the shape for a pH curve of a titration linear?

A

No

101
Q

Suitable indicator for strong acid - strong base reaction?

A

Phenolphthalein

102
Q

Suitable indicator for strong acid - weak base reaction?

A

Methyl orange

103
Q

Suitable indicator for weak acid - strong base reaction?

A

Phenolphthalein

104
Q

Suitable indicator for weak acid - weak base reaction?

A

Phenolphthalein

105
Q

What is the equivalence point?

A

When sufficient base has been added to just neutralise the acid

106
Q

What does the end-point refer to?

A

The indicator

107
Q

What is the end-point?

A

The volume at which there are exactly the same moles of H⁺ ions as OH⁻ ions

108
Q

Is the pH at the end-point exactly 7?

A

No

109
Q

In terms of equivalence points, what do diprotic acids result in? Why is this?

A

They have two equivalence points, due to released two protons but at different times

110
Q

What would an inappropriate indicator result in?

A

An end point being observed when equivalence has not yet been reached - or comes after equivalence

111
Q

When is the indicator for a particular titration classed as suitable?

A
  • when there’s a sharp colour change at end point
  • when end point of titration is provided by colour change of indicator
  • when indicator gives a distinct colour change
112
Q

What are indicators?

A

Weak acids where HA and A⁻ are different colours

HA ⇋ H⁺ + A⁻

113
Q

What colour is HA in methyl orange?

A

Red

114
Q

What is the pH range of colour change for methyl orange?

A

3.2 - 4.4

115
Q

What colour is A⁻ in methyl orange?

A

Yellow

116
Q

What colour is HA in phenolphthalein?

A

Colourless

117
Q

What is the pH range of colour change for phenolphthalein?

A

8.2 - 10.0

118
Q

What colour is A⁻ in phenolphthalein?

A

Pink

119
Q

In a titration, what happens when the last drop of acid/alkali is added?

A

The pH rapidly changes at equivalence

120
Q

What must be true for an indicator to change colour?

A

Moles of acid is equal to moles of base

121
Q

Why is universal indicator not suitable for a titration?

A

It is a mixture of indicators, so shows many colours at different pHs and gradual colour change

122
Q

What is a buffer solution?

A

A solution that maintains an approximately constant pH despite dilution or addition of small amounts of acid/base

123
Q

What are buffer solutions designed to do?

A

Keep the concentration of hydrogen and hydroxide ions in a solution almost unchanged

124
Q

What do buffer solutions rely on to do their job?

A

An equilibrium reaction - which will move in the direction to remove hydrogen and hydroxide ions added

125
Q

Does adding an acid/alkali in small amounts change the pH of a buffer solution?

A

Yes, but the pH will only change slightly

126
Q

What do basic buffers contain?

A

• a weak base and the salt of that weak base

or

• an excess of weak alkali with a strong acid

127
Q

What pH do basic buffers maintain?

A

A pH higher than 7

128
Q

Example of substances to form a basic buffer?

A

Ammonia (NH₃) and ammonium chloride (NH₄Cl)

129
Q

What happens when NH₃ and NH₄Cl are used as a basic buffer?

A
  • salt fully dissociates: NH₄Cl ⇋ NH₄⁺ + Cl⁻

* ammonia reacts with water: NH₃ + H₂O ⇋ NH₄⁺ + OH⁻

130
Q

What happens to the weak base and its salt in a basic buffer?

A
  • the salt fully dissociates

* the weak base reacts with water to form a weak alkaline solution

131
Q

What happens when alkali is added to an ammonia and ammonium chloride base?

[NH₃ + H₂O ⇋ NH₄⁺ + OH⁻]

A
  • increase concentration of OH⁻ ions
  • causing equilibrium to shift left to remove added OH⁻ ions
  • resulting in concentration of weak alkali (ammonia) to increase
132
Q

What happens when acid is added to an ammonia and ammonium chloride base?

[NH₃ + H₂O ⇋ NH₄⁺ + OH⁻]

A
  • it will react with OH⁻ ions

* so equilibrium shifts right to counteract the change

133
Q

Example of a buffer than contains an excess of weak alkali with a strong acid?

A

NH₃ and HCl

134
Q

Equation for an NH₃ and HCl buffer?

A

NH₃ + HCl ⇋ NH₄⁺ + Cl⁻

135
Q

What is the key in a basic buffer solution?

A

The concentration of the base and salt are much higher than the concentration of OH⁻ ions

136
Q

What do acidic buffers contain?

A

• a weak acid and the salt of that weak acid

or

• an excess of weak acid with strong alkali

137
Q

What pH do acidic buffers maintain?

A

One that is lower than 7

138
Q

What happens to a weak acid in solution?

A

Slightly dissociates:

HA ⇋ H⁺ + A⁻

139
Q

Overall, what will an acidic buffer contain?

A

A lot of HA (undissociated weak acid), a lot of A⁻ ions and very little H⁺ ions

140
Q

Example of an acidic buffer?

A

Ethanoic acid and sodium ethanoate

141
Q

What happens when alkali is added to an acidic buffer?

[HA ⇋ H⁺ + A⁻]

A
  • OH⁻ ions will react with H⁺ forming water
  • causing equilibrium to shift right to replace reacted H⁺
  • therefore decreasing the concentration of the undissociated acid
142
Q

What happens when acid is added to an acidic buffer?

[HA ⇋ H⁺ + A⁻]

A
  • equilibrium shifts left to remove acid

* therefore concentration of HA↑ and A⁻↓

143
Q

What is the function of the salt component of the acidic buffer?

A

To act as a source of A⁻ ions (-ve ion of the weak acid) which can remove any added H⁺ ions

144
Q

Why can an acidic buffer be formed from an excess of weak acid and strong alkali?

A

As the acid is in excess, the solution contains some of the salt of the weak acid

145
Q

What is the key in an acidic buffer solution?

A

The concentration of the acid and salt are much higher than the concentration of H⁺ ions

146
Q

If half the acid in an acidic buffer is neutralised, what will the resulting solution be?

A

A buffer with a pH equal to pKa

147
Q

What is the half equivalence point?

A

When half the acid is neutralised

AQA A Level Physical Chemistry (12 decks)

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