1.5 Acid-Base Equilibria Flashcards
1
Q
acid + metal ->
A
salt + hydrogen
2
Q
acid + base ->
A
salt + water
3
Q
acid + alkali ->
A
salt + water
4
Q
acid + carbonate ->
A
salt + water + carbon dioxide
5
Q
define Brønsted-Lowry acid
A
a proton (H+) donor
6
Q
define Brønsted-Lowry base
A
a proton (H+) acceptor
7
Q
What is a monobasic acid?
A
donates one proton per molecule
8
Q
Name a dibasic acid
A
H2SO4
9
Q
Name a tribasic acid
A
H3PO4
10
Q
How does an acid relate to its conjugate base?
A
The acid has one more hydrogen ion than its conjugate base
11
Q
How does a base relate to its conjugate acid?
A
The base has one less hydrogen ion than its conjugate acid
12
Q
What is the conjugate base for HCl?
A
Cl^-
13
Q
What is the conjugate acid for H2O?
A
H3O^+
14
Q
What is the acid for the conjugate base NO3^-?
A
HNO3
15
Q
What is the base for the conjugate acid H2SO4?
A
HSO4^-
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
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).
18
Q
What does the pH of a solution depend on?
A
the concentration of H^+(aq)
19
Q
How can pH be calculated?
A
pH = -log10[H+]
20
Q
How can [H+] be calculated?
A
[H+] = 10^-pH
21
Q
Name the transformation of a log equation used in the pH equation
A
logaB = C -> a^c = B
22
Q
What is the relationship between pH and [H^+(aq)]
A
negative - low pH means a high [H^+(aq)]
23
Q
Describe the strength of an acid HA
A
the extent of its dissociation into H^+ and A^- ions
24
Q
What is a strong acid?
A
An acid that completely dissociates in solution
25
Name some strong acids
HCl, H2SO4, H3PO4, HNO3
26
Name some weak acids
CH3COOH, HCN, H2SO3
27
What is a weak acid?
An acid that partially dissociates in solution. Equilibrium lies well over to the left
28
What is the symbol of the acid dissociation constant?
Ka
29
What is the equation for the acid dissociation constant?
Ka = [H+(aq][A-(aq)] / [HA(aq)]
30
What does the value of the acid dissociation constant represent?
the extent of acid dissociation
31
What does a large Ka value indicate?
a large extent of dissociation (equilibrium to the right) - a strong acid (pH 3-4)
32
What does a small Ka value indicate?
a small extent of dissociation (equilibrium to the left) - a weak acid (pH 5-6)
33
How is pKa calculated?
pKa = -log10Ka
34
How does pKa value relate to acid strength?
The smaller the pKa value, the stronger the acid
35
How does pKa value relate to Ka value?
Large Ka = small pKa ('stronger' weak acid)
36
Why does full dissociation not occur in very concentrated solutions?
Not enough water molecules to aid dissociation
37
Give the general equation for the dissociation of an acid
HA(aq) ⇌ H+(aq) + A-(aq)
38
How can the equilibrium equation be modified for weak acids?
Ka = [H+(aq)]^2 / [HA(aq)]
Because [H+] = [A-]
39
Give the equilibrium equation for the ionisation of water
Kc = [H+(aq)][OH-(aq)] / [H2O(l)]
40
What is the concentration of water?
55.6moldm^-3
41
How is ionic product of water (Kw) derived?
Kc x [H2O(l)] = [H+(aq)][OH-(aq)]
Kc and H2O are both constants - combine to form ionic product of water (Kw)
42
What is the equation for ionic product of water?
Kw = [H+][OH-]
43
What is the pH of water at 25º?
7
44
What is the value of ionic product of water?
Kw = (1 x 10^-7)^2 = 1 x 10^-14 mol^2dm^-6
45
What is the equation for pKw?
pKw = -log10Kw
46
What is the strength of a base a measure of?
its dissociation in solution to generate OH- ions.
47
NH3(aq) + aq ⇌
NH4+(aq) + OH-(aq)
48
Define buffer solution
a solution which resists changes in pH on addition of small amounts of acid or alkali
49
What is a buffer solution a mixture of?
A weak acid, HA
| Its conjugate base, A-
50
How can a buffer solution be made and give an example
a weak acid and a salt of the weak acid, eg. Ethanoic acid and sodium ethanoate.
51
Describe how ethanoic acid and sodium ethanoate forms a buffer system
- 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
How does a buffer solution react when a small amount of acid is added?
- [H+] increases
- Conjugate base (eg. CH3COO-) reacts with H+ ions
- Equilibrium shifts to the left (CH3COOH ⇌ H+ + CH3COO-)
53
How does a buffer solution react when a small amount of base is added?
- [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
What is the pH of healthy blood plasma?
7.35-7.45
55
What is the main buffer system in the blood?
carbonic acid-hydrogen carbonate buffer system
56
How does the carbonic acid-hydrogen carbonate buffer system react to a small increase in [H+]?
- 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
How does the carbonic acid-hydrogen carbonate buffer system react to a small increase in [OH-]?
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
Define equivalence point
the volume of one solution that reacts exactly with a known volume of another solution.
59
What is measured in titrations?
the equivalence point
60
Give two methods of plotting acid-base titration pH curves
- 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
What are the three sections to a titration curve?
1. A slight increase/decrease in pH
2. A sharp rise/decrease in pH
3. A slight increase/decrease in pH
62
What is an acid-base indicator made from, and what is its general formula representation?
A weak acid, represented as HIn
63
Give the general equation for dissociation of an acid-base indicator
HIn ⇌ H+ + In-
64
What colours are the components of methyl orange?
HIn = red (low pH)
In- = yellow (high pH)
end point = orange
65
What state is the acid-base indicator in when it is at its end point?
when [HIn] = [In-]
66
At what pH range does methyl orange change colour?
2.5-4.0
67
At what pH range does phenolphthalein change colour?
8.2-10.0
68
At what pH range does bromothymol blue change colour?
6.0-7.6
69
What colours are the components of phenolphthalein?
```
HIn = colourless (low pH)
In- = pink (high pH)
```
70
What colours are the components of bromothymol blue?
HIn = yellow (low pH)
In- = blue (high pH)
end point = green
71
What can be said about the colour at the end point of an acid-base indicator reaction?
midway between the colours of the acid and conjugate base forms
72
What should be considered when choosing an indicator for an acid-base titration?
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
What indicators are suitable for a strong acid-strong base reaction?
phenolphthalein
methyl orange
bromothymol blue
74
What indicators are suitable for a strong acid-weak base reaction?
best indicator is methyl orange
| bromothymol blue is also suitable
75
What indicators are suitable for a weak acid-strong base reaction?
best indicator is phenolphthalein
| bromothymol blue is also suitable
76
What indicators are suitable for a weak acid-weak base reaction?
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
Define salt
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
How can a salt be formed?
a combination of a strong or weak acid and a strong or weak base
79
Name some salts formed from a strong acid and a strong base
NaCl, K2SO4
80
Name some salts formed from a strong acid and a weak base
NH4NO3
81
Name some salts formed from a weak acid and a strong base
CH3COONa, HCOOK
82
What is the pH of a salt formed from a strong acid and a strong base?
neutral
83
What is the pH of a salt formed from a strong acid and a weak base and why?
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
What is the pH of a salt formed from a weak acid and a strong base and why?
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
give the equation for the dissolution of H2SO4 in solution.
H2SO4(l) + aq -> H+(aq) + HSO4-(aq)
86
define neutralisation
a reaction whereby an acid and a base react to form a salt and water.
87
Write the ionic equation for the reaction between aqueous hydrochloric acid
and aqueous sodium carbonate.
2H^+(aq) + CO3^2-(aq) -> H2O(l) + CO2(g)
