Theories of acids and bases Flashcards
(80 cards)
1
Q
What is an Arrhenius acid?
A
A compound that dissolves in water to yield hydrogen ions
2
Q
What is an Arrhenius base?
A
A compound that dissolves in water to yield hydroxide ions
3
Q
What is an example of Arrhenius acid?
A
HCl (g) → H⁺ (aq) + Cl⁻ (aq)
4
Q
What is an example of Arrhenius base?
A
NaOH (s) → Na⁺ (aq) + OH⁻ (aq)
5
Q
What is a Bronsted-lowry acid?
A
Proton donor
6
Q
What is a Bronsted-Lowry base?
A
Proton acceptor
7
Q
What is a Lewis acid?
A
electron-pair acceptor
8
Q
What is a Lewis base?
A
electron-pair donor
9
Q
What is a strong acid or base?
A
One that ionises completely in aqueous solutions
10
Q
What are examples of strong acids?
A
Mineral acids such as H₂SO₄, HCl, HNO₃
11
Q
What are some examples of strong bases?
A
- metal hydroxides such as NaOH, KOH, Ca(OH)₂, Ba(OH)₂
- Metal oxides such as Na₂O, K₂O, BaO
12
Q
What are weak acid or bases?
A
One that ionises partially in aqueous solution
13
Q
What are some examples of weak acids?
A
- most organic acids such as HCOOH
- some inorganic acids like HCN or HNO₂
14
Q
What are some examples of weak bases?
A
- NH₃
- Organic amines like CH₃NH₂
15
Q
How to measure the relative strength of acids or bases?
A
- degree of ionisation
- pH
- dissociation constant
16
Q
What conditions need to be met to use degree of ionisation or dissociation?
A
- same temperature
- same concentration
17
Q
What is the formula for degree of ionisation/dissociation?
A
α = amount of molecules which ionised at equilibrium/amount of molecules present initially
18
Q
What is the degree of ionisation/dissociation for strong acids/bases?
A
α = 1
19
Q
What is the degree of ionisation/dissociation for weak acids/bases?
A
0 < α < 1
20
Q
What is the relationship of α and the strength of acid/base?
A
The greater the value of α, the stronger the acid/base
21
Q
What is the degree of ionisation depend on and what affects it? (limitation)
A
- concentration of acids/bases
- HA (aq) + H₂O (l) ⇌ H₃O⁺ + A⁻ (aq)
- When water is added, [HA], [H₃O⁺], [A⁻] decreases, most species on right is diluted, POE shift right and degree of ionisation increases
- α increases with decreasing concentration of acid/bases
22
Q
What do hydrogen ions do in aqueous solutions?
A
due to its high charge density, it is always dative bonded to water molecule to form H₃O⁺
23
Q
What is the assumption when using pH and pOH?
A
assuming same concentration of acid/base
24
Q
What is the equation of auto-ionisation of water?
A
H₂O (l) ⇌ H⁺ (aq) + OH⁻ (aq)
25
What is Kw?
* ionic product of water
* Kw = [H⁺][OH⁻] = 10⁻¹⁴ at 25 degree celsius
26
Is the auto-ionisation exothermic or endothermic?
endothermic
27
How does Kw vary with temperature?
* H₂O (l) ⇌ H⁺ (aq) + OH⁻ (aq)
* As temperature increases, POE shift right to absorb heat
* [H⁺] and [OH⁻] increase
* Kw increases
28
What is the graph of Kw against temperature?
Curve with increasing gradient
29
What is the equation of pKw?
pKw = pH + pOH
30
What needs to be constant for equilibrium constant to be used to indicate strength of acids and bases?
same temperature
31
What is Ka?
* acid dissociation constant
* HA (aq) ⇌ H⁺ (aq) + A⁻ (aq)
* Kₐ = [H⁺][A⁻]/[HA]
32
What does greater Ka indicate?
Stronger the acid
33
What does smaller pKa indicate?
stronger the acid
34
What is Kb?
* base dissociation constant, Kb
* B (aq) + H₂O (l) ⇌ BH⁺ (aq) + OH⁻ (aq)
* Kb = [BH⁺][OH⁻]/[B]
35
What does greater Kb indicate?
Stronger base
36
What does smaller pKb indicate?
stronger base
37
What is a conjugate base?
When acid HA donates a proton, the resulting product A⁻ is called the conjugate base of HA
38
What is conjugate acid?
When base B accepts a proton, the resulting product BH⁺ is called conjugate acid of B
39
What is unique for a conjugate acid-base pair?
Ka × Kb = Kw = 10⁻¹⁴ at 25 degree celsius
40
What is the relationship between conjugate acid-base pair?
* stronger the acid, the weaker the conjugate base and vice versa
* stronger the base, the weaker the conjugate acid and vice versa
41
What is pKw equation?
pKa + pKb = pKw = 14 at 25 degree celsius
42
What is the best indicator for strength of acid/base
* Ka/Kb is best indicator as it is constant at constant temperature and does not vary with concentration
* pH and degree of isonisation varies with concentration
43
When is a salt produced?
When acid react with bases
44
What is a salt made up of?
Conjugate acid and conjugate base from the acid and base it was made from
45
What happens when a salt dissolves in water?
* it completely ionises
* the resultant cation or anion may undergo hydrolysis to produce acidic or alkaline solutions
46
What happens when salt of strong acid and strong base is dissolved in water?
* pH = 7
* no salt hydrolysis occurs
47
What happens when weak acid + strong base salt is dissolved in water?
* pH > 7
* relatively strong conjugate base of weak acid hydrolyses in water to give OH⁻
* weak conjugate acid does not undergo hydrolysis
48
What happens when strong acid + weak base salt is dissolved in water?
* pH < 7
* relatively strong conjugate acid of weak base hydrolyses in water to give H₃O⁺
* weak conjugate base does not undergo hydrolysis
49
What happens when weak base + weak acid salt is dissolved in water?
* pH (depends on relative strength of conjugate acid and base)
* Both strong conjugate acid and base undergoes hydrolysis in water
50
What is a buffer solution?
A buffer solution is one that can resist a change in pH when a small amount of acid/base is added to it
51
What is a buffer made up of?
conjugate acid-base pair
52
What does acidic buffer consist of?
weak acid and its salt (which contains conjugate base of weak acid)
53
What does alkaline buffer consist of?
weak base and its salt (which contains conjugate acid of weak base)
54
How is an acidic buffer created?
* Salt, CH₃COONa ionises completely to give a relatively high concentration of CH₃COO⁻
* CH₃COOH being a weak acid is partially dissociated, the high concentration of CH₃COO⁻ from the complete ionisation of CH₃COONa further suppresses the ionisation of CH₃COOH
* there is a high concentration of both CH₃COOH and CH₃COO⁻
55
What happens when a small amount of acid is added to acidic buffer solution?
* nearly all the added H⁺ are neutralised by the large amount of CH₃COO⁻ (from salt)
* CH₃COO⁻ + H⁺ → CH₃COOH
* [H⁺] does not increase appreciably and pH is kept approximately constant
56
What happens when a small amount of base is added to acidic buffer solution?
* nearly all the added OH⁻ are neutralised by the large amount of un-ionised CH₃COOH (weak acid)
* CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O
* [OH⁻] does not increase appreciably and pH is kept approximately constant
57
How is an alkaline buffer created?
| from NH₃ and NH₄Cl
* NH₄Cl (salt) ionises fully to give a relatively high concentration of NH₄⁺
* NH₃ is partially ionised, high concentration of NH₄⁺ from complete ionisation of NH₄Cl further suppresses the ionisation of ammonia
* relatively high concentration of un-ionised NH₃
58
What happens when a small amount of acid is added to alkaline buffer solution?
* nearly all the added H⁺ are neutralised by the large amount of un-ionised NH₃ (weak base)
* NH₃ + H⁺ → NH₄⁺
* [H⁺] does not increase appreciably and pH is kept approximately constant
59
What happens when a small amount of base is added to alkaline buffer solution?
* nearly all the added OH⁻ are neutralised by the large amount of NH₄⁺ (salt)
* NH₄⁺ + OH⁻ → NH₃ + H₂O
* [OH⁻] does not increase appreciably and pH is kept approximately constant
60
What is the Henderson-Hasselbalch equation for acidic buffer?
* pH = pKa + lg ([A⁻]/[HA])
* At equilibrium,
* [HA] ≈ [HA] initial (because weak acid remains largely undissociated)
* [A⁻] ≈ [A⁻] salt (because only small amount of acid dissociates while the salt completely ionises)
61
What is the Henderson-Hasselbalch equation for alkaline buffer?
* pOH = pKb + lg([BH⁺]/[B])
* [BH⁺] ≈ [BH⁺] salt (because only small amount of base dissocaites while the salt completely ionises)
* [B] ≈ [B] initial (because weak base remains largely dissociated)
62
What is the effect of dilution on pH of buffer solution?
* no effect as both [salt] and [acid] are reduced to the same extent
* however buffer may not be so effective at controlling pH
63
What is buffering capacity?
It refers to the amount of acid or base that may be added to a buffer solution before its pH changes appreciably (big enough to be impactful)
64
When is a buffer solution most effective?
* the amounts of both the weak acid or base and its salt are large relative to that of the acid or base to be added to the buffer solution
* equal amounts of conjugate acid-base pair, [HA]=[A⁻]
65
When does acidic buffer have maximum buffering capacity and what happens as a result?
* When [HA] = [A⁻]
* pH = pKa
66
When does alkaline buffer have a maximum buffering capacity and what happens as a result?
* [B] = [BH⁺]
* pOH = pKb
67
What is the effective buffer range for acidic/alkaline buffer?
pH/pOH = pKa/pKb ± 1
68
What are some uses of buffer solutions?
* blood is buffered at a pH of 7.4 so that enzymes in the blood can function
* buffering agents present in antacid tablets to lower acidity of stomach
* dyeing fabrics where pH must not deviate much from optimum value
69
What is equivalence point?
Point during titration at which stoichiometric amounts of reactants have reacted
70
What is end-point?
Point during titration where the indicator changes colour
71
What is an acid-base indicator?
* substance which shows different colours in solutions of different pH
* either weak acid or bases for which the acid and its conjugate base have different colours
72
How does an acid-base indicator work?
| HIn (aq) ⇌ H⁺ (aq) + In⁻ (aq)
HIn: colour A
In⁻: colour B
* In acidic medium, POE shift left and indicator exists mainly in the form of HIn, colour A dominates
* In alkaline medium, POE shifts right and inidicator exists mainly in the form of In⁻ thus colour B dominates
73
When will colour A be observed and when will colour B be observed?
pH = pKᵢₙ + lg([In⁻]/[HIn])
| HIn (aq) ⇌ H⁺ (aq) + In⁻ (aq)
* when [HIn]/[In⁻] ≥ 10, lg([In⁻]/[HIn]) pH ≤ pKᵢₙ - 1, colour A is observed
* when [HIn] = [In⁻], pH = pKᵢₙ, a combination of A and B are observed
* when [In⁻]/[HIn] ≥ 10, lg([In⁻]/[HIn]) ≥ 1, pH ≥ pKᵢₙ + 1, colour B is observed
74
What is the pH range of an indicator?
pKᵢₙ ± 1
75
Why is the pH working range and colour of methyl orange?
* pH 3-5
* red, orange, yellow
76
What is the pH working range and colour of screened methyl orange?
* pH 3-5
* violet, grey, green
77
What is the colour and pH working range of thymol blue?
* pH 8-10
* red/yellow
* yellow (when acid is added to strong base) or blue (when strong base is added to acid)
* blue
77
What is the pH working range and colour of thymolphtalein?
* pH 8-10
* colourless, pale blue, blue
78
What is the pH working range and colour of phenolphthalein?
* pH 8-10
* colourless, pale pink, pink
79
How to choose suitable indicator?
working pH range of indicator must lie within sharge charge in pH near equivalence point
