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A2 Chemistry 1 > 1.5 Acid-Base Equilibria > Flashcards

1.5 Acid-Base Equilibria Flashcards

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1
Q

acid + metal ->

A

salt + hydrogen

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2
Q

acid + base ->

A

salt + water

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3
Q

acid + alkali ->

A

salt + water

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4
Q

acid + carbonate ->

A

salt + water + carbon dioxide

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5
Q

define Brønsted-Lowry acid

A

a proton (H+) donor

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6
Q

define Brønsted-Lowry base

A

a proton (H+) acceptor

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7
Q

What is a monobasic acid?

A

donates one proton per molecule

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8
Q

Name a dibasic acid

A

H2SO4

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9
Q

Name a tribasic acid

A

H3PO4

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10
Q

How does an acid relate to its conjugate base?

A

The acid has one more hydrogen ion than its conjugate base

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11
Q

How does a base relate to its conjugate acid?

A

The base has one less hydrogen ion than its conjugate acid

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12
Q

What is the conjugate base for HCl?

A

Cl^-

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13
Q

What is the conjugate acid for H2O?

A

H3O^+

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14
Q

What is the acid for the conjugate base NO3^-?

A

HNO3

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15
Q

What is the base for the conjugate acid H2SO4?

A

HSO4^-

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16
Q

What is an acid-base pair?

A

a pair of two species that transform into each other by gain or loss of a proton

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17
Q

What will a H+ ion do in an aqueous solution?

A

In an aqueous solution a H+ ion is too small to exist on its own and therefore will combine with H2O to form H3O+ ion (hydroxonium ion).

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18
Q

What does the pH of a solution depend on?

A

the concentration of H^+(aq)

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19
Q

How can pH be calculated?

A

pH = -log10[H+]

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20
Q

How can [H+] be calculated?

A

[H+] = 10^-pH

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21
Q

Name the transformation of a log equation used in the pH equation

A

logaB = C -> a^c = B

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22
Q

What is the relationship between pH and [H^+(aq)]

A

negative - low pH means a high [H^+(aq)]

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23
Q

Describe the strength of an acid HA

A

the extent of its dissociation into H^+ and A^- ions

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24
Q

What is a strong acid?

A

An acid that completely dissociates in solution

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25
Q

Name some strong acids

A

HCl, H2SO4, H3PO4, HNO3

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26
Q

Name some weak acids

A

CH3COOH, HCN, H2SO3

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27
Q

What is a weak acid?

A

An acid that partially dissociates in solution. Equilibrium lies well over to the left

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28
Q

What is the symbol of the acid dissociation constant?

A

Ka

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29
Q

What is the equation for the acid dissociation constant?

A

Ka = [H+(aq][A-(aq)] / [HA(aq)]

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30
Q

What does the value of the acid dissociation constant represent?

A

the extent of acid dissociation

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31
Q

What does a large Ka value indicate?

A

a large extent of dissociation (equilibrium to the right) - a strong acid (pH 3-4)

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32
Q

What does a small Ka value indicate?

A

a small extent of dissociation (equilibrium to the left) - a weak acid (pH 5-6)

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33
Q

How is pKa calculated?

A

pKa = -log10Ka

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34
Q

How does pKa value relate to acid strength?

A

The smaller the pKa value, the stronger the acid

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35
Q

How does pKa value relate to Ka value?

A

Large Ka = small pKa (‘stronger’ weak acid)

36
Q

Why does full dissociation not occur in very concentrated solutions?

A

Not enough water molecules to aid dissociation

37
Q

Give the general equation for the dissociation of an acid

A

HA(aq) ⇌ H+(aq) + A-(aq)

38
Q

How can the equilibrium equation be modified for weak acids?

A

Ka = [H+(aq)]^2 / [HA(aq)]

Because [H+] = [A-]

39
Q

Give the equilibrium equation for the ionisation of water

A

Kc = [H+(aq)][OH-(aq)] / [H2O(l)]

40
Q

What is the concentration of water?

A

55.6moldm^-3

41
Q

How is ionic product of water (Kw) derived?

A

Kc x [H2O(l)] = [H+(aq)][OH-(aq)]

Kc and H2O are both constants - combine to form ionic product of water (Kw)

42
Q

What is the equation for ionic product of water?

A

Kw = [H+][OH-]

43
Q

What is the pH of water at 25º?

A

7

44
Q

What is the value of ionic product of water?

A

Kw = (1 x 10^-7)^2 = 1 x 10^-14 mol^2dm^-6

45
Q

What is the equation for pKw?

A

pKw = -log10Kw

46
Q

What is the strength of a base a measure of?

A

its dissociation in solution to generate OH- ions.

47
Q

NH3(aq) + aq ⇌

A

NH4+(aq) + OH-(aq)

48
Q

Define buffer solution

A

a solution which resists changes in pH on addition of small amounts of acid or alkali

49
Q

What is a buffer solution a mixture of?

A

A weak acid, HA

Its conjugate base, A-

50
Q

How can a buffer solution be made and give an example

A

a weak acid and a salt of the weak acid, eg. Ethanoic acid and sodium ethanoate.

51
Q

Describe how ethanoic acid and sodium ethanoate forms a buffer system

A
  • Weak acid dissociates partially: CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
  • Salt dissociates completely: CH3COONa(aq) -> CH3COO-(aq) + Na+(aq)
  • Resulting buffer solution contains large reservoirs of the weak acid and its conjugate base
52
Q

How does a buffer solution react when a small amount of acid is added?

A
  • [H+] increases
  • Conjugate base (eg. CH3COO-) reacts with H+ ions
  • Equilibrium shifts to the left (CH3COOH ⇌ H+ + CH3COO-)
53
Q

How does a buffer solution react when a small amount of base is added?

A
  • [OH-] increases
  • Small concentration of H+ ions react with OH- ions to form water
  • Weak acid (eg. CH3COOH) dissociates, shifting equilibrium to the right to restore most of the H+ ions that have reacted.
  • Eg. CH3COOH + OH- -> CH3COO- + H2O
54
Q

What is the pH of healthy blood plasma?

A

7.35-7.45

55
Q

What is the main buffer system in the blood?

A

carbonic acid-hydrogen carbonate buffer system

56
Q

How does the carbonic acid-hydrogen carbonate buffer system react to a small increase in [H+]?

A
  • An increase in H+ ions in the blood is removed by the conjugate base HCO3^- and the equilibrium shifts to the left, removing most of the H+ ions.
  • Carbonic acid can then be converted into aqueous CO2 through the action of an enzyme, and then released from the lungs as CO2 gas.
57
Q

How does the carbonic acid-hydrogen carbonate buffer system react to a small increase in [OH-]?

A

A small concentration of H+ ions react with the increase in OH- ions, causing the H2CO3 to dissociate to replace most of the H+ ions. This causes equilibrium to shift to the right hand side.
Overall: H2CO3(aq) + OH-(aq) ⇌ H2O(l) + HCO3-(aq)

58
Q

Define equivalence point

A

the volume of one solution that reacts exactly with a known volume of another solution.

59
Q

What is measured in titrations?

A

the equivalence point

60
Q

Give two methods of plotting acid-base titration pH curves

A
  • using a pH meter to measure the pH after the addition of small volumes of the burette solution
  • by continuous data logging in a computer
61
Q

What are the three sections to a titration curve?

A
  1. A slight increase/decrease in pH
  2. A sharp rise/decrease in pH
  3. A slight increase/decrease in pH
62
Q

What is an acid-base indicator made from, and what is its general formula representation?

A

A weak acid, represented as HIn

63
Q

Give the general equation for dissociation of an acid-base indicator

A

HIn ⇌ H+ + In-

64
Q

What colours are the components of methyl orange?

A

HIn = red (low pH)
In- = yellow (high pH)
end point = orange

65
Q

What state is the acid-base indicator in when it is at its end point?

A

when [HIn] = [In-]

66
Q

At what pH range does methyl orange change colour?

A

2.5-4.0

67
Q

At what pH range does phenolphthalein change colour?

A

8.2-10.0

68
Q

At what pH range does bromothymol blue change colour?

A

6.0-7.6

69
Q

What colours are the components of phenolphthalein?

A 
HIn = colourless (low pH)
In- = pink (high pH)
70
Q

What colours are the components of bromothymol blue?

A

HIn = yellow (low pH)
In- = blue (high pH)
end point = green

71
Q

What can be said about the colour at the end point of an acid-base indicator reaction?

A

midway between the colours of the acid and conjugate base forms

72
Q

What should be considered when choosing an indicator for an acid-base titration?

A

pH value of the end point is as close as possible to the pH value of the titration’s equivalence point. In practice, a suitable indicator changes colour in the vertical section of the titration curve.

73
Q

What indicators are suitable for a strong acid-strong base reaction?

A

phenolphthalein
methyl orange
bromothymol blue

74
Q

What indicators are suitable for a strong acid-weak base reaction?

A

best indicator is methyl orange

bromothymol blue is also suitable

75
Q

What indicators are suitable for a weak acid-strong base reaction?

A

best indicator is phenolphthalein

bromothymol blue is also suitable

76
Q

What indicators are suitable for a weak acid-weak base reaction?

A

There is no significant pH range in which the addition of small amounts of one reagent produces a sharp change
There are no indicators suitable for a titration of a weak acid and weak base

77
Q

Define salt

A

a compound formed when all or some of the H+ ions in an acid are replaced by positive ions (usually metal or ammonium ions)

78
Q

How can a salt be formed?

A

a combination of a strong or weak acid and a strong or weak base

79
Q

Name some salts formed from a strong acid and a strong base

A

NaCl, K2SO4

80
Q

Name some salts formed from a strong acid and a weak base

A

NH4NO3

81
Q

Name some salts formed from a weak acid and a strong base

A

CH3COONa, HCOOK

82
Q

What is the pH of a salt formed from a strong acid and a strong base?

A

neutral

83
Q

What is the pH of a salt formed from a strong acid and a weak base and why?

A

acidic
When you have a salt made from a weak base such as ammonia, the ammonium ion reacts with water to form hydroxonium ions, giving an acidic solution.
NH4+ + H2O ⇌ NH3 + H3O+

84
Q

What is the pH of a salt formed from a weak acid and a strong base and why?

A

alkaline
When you have a salt made from a weak acid such as methanol acid, the methanoate ion reacts with water to from hydroxide ions, giving an alkaline solution.
HCOO- + H2O ⇌ HCOOH + OH-

85
Q

give the equation for the dissolution of H2SO4 in solution.

A

H2SO4(l) + aq -> H+(aq) + HSO4-(aq)

86
Q

define neutralisation

A

a reaction whereby an acid and a base react to form a salt and water.

87
Q

Write the ionic equation for the reaction between aqueous hydrochloric acid
and aqueous sodium carbonate.

A

2H^+(aq) + CO3^2-(aq) -> H2O(l) + CO2(g)

A2 Chemistry 1 (10 decks)

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