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Chemistry > Theories of acids and bases > Flashcards

Theories of acids and bases Flashcards

1
Q

What is an Arrhenius acid?

A

A compound that dissolves in water to yield hydrogen ions

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2
Q

What is an Arrhenius base?

A

A compound that dissolves in water to yield hydroxide ions

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3
Q

What is an example of Arrhenius acid?

A

HCl (g) → H⁺ (aq) + Cl⁻ (aq)

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4
Q

What is an example of Arrhenius base?

A

NaOH (s) → Na⁺ (aq) + OH⁻ (aq)

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5
Q

What is a Bronsted-lowry acid?

A

Proton donor

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6
Q

What is a Bronsted-Lowry base?

A

Proton acceptor

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7
Q

What is a Lewis acid?

A

electron-pair acceptor

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8
Q

What is a Lewis base?

A

electron-pair donor

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9
Q

What is a strong acid or base?

A

One that ionises completely in aqueous solutions

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10
Q

What are examples of strong acids?

A

Mineral acids such as H₂SO₄, HCl, HNO₃

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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
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12
Q

What are weak acid or bases?

A

One that ionises partially in aqueous solution

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13
Q

What are some examples of weak acids?

A
  • most organic acids such as HCOOH
  • some inorganic acids like HCN or HNO₂
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14
Q

What are some examples of weak bases?

A
  • NH₃
  • Organic amines like CH₃NH₂
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15
Q

How to measure the relative strength of acids or bases?

A
  • degree of ionisation
  • pH
  • dissociation constant
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16
Q

What conditions need to be met to use degree of ionisation or dissociation?

A
  • same temperature
  • same concentration
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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

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18
Q

What is the degree of ionisation/dissociation for strong acids/bases?

A

α = 1

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19
Q

What is the degree of ionisation/dissociation for weak acids/bases?

A

0 < α < 1

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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

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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
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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⁺

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23
Q

What is the assumption when using pH and pOH?

A

assuming same concentration of acid/base

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24
Q

What is the equation of auto-ionisation of water?

A

H₂O (l) ⇌ H⁺ (aq) + OH⁻ (aq)

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25
Q

What is Kw?

A
  • ionic product of water
  • Kw = [H⁺][OH⁻] = 10⁻¹⁴ at 25 degree celsius
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26
Q

Is the auto-ionisation exothermic or endothermic?

A

endothermic

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27
Q

How does Kw vary with temperature?

A
  • H₂O (l) ⇌ H⁺ (aq) + OH⁻ (aq)
  • As temperature increases, POE shift right to absorb heat
  • [H⁺] and [OH⁻] increase
  • Kw increases
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28
Q

What is the graph of Kw against temperature?

A

Curve with increasing gradient

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29
Q

What is the equation of pKw?

A

pKw = pH + pOH

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30
Q

What needs to be constant for equilibrium constant to be used to indicate strength of acids and bases?

A

same temperature

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31
Q

What is Ka?

A
  • acid dissociation constant
  • HA (aq) ⇌ H⁺ (aq) + A⁻ (aq)
  • Kₐ = [H⁺][A⁻]/[HA]
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32
Q

What does greater Ka indicate?

A

Stronger the acid

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33
Q

What does smaller pKa indicate?

A

stronger the acid

34
Q

What is Kb?

A
  • base dissociation constant, Kb
  • B (aq) + H₂O (l) ⇌ BH⁺ (aq) + OH⁻ (aq)
  • Kb = [BH⁺][OH⁻]/[B]
35
Q

What does greater Kb indicate?

A

Stronger base

36
Q

What does smaller pKb indicate?

A

stronger base

37
Q

What is a conjugate base?

A

When acid HA donates a proton, the resulting product A⁻ is called the conjugate base of HA

38
Q

What is conjugate acid?

A

When base B accepts a proton, the resulting product BH⁺ is called conjugate acid of B

39
Q

What is unique for a conjugate acid-base pair?

A

Ka × Kb = Kw = 10⁻¹⁴ at 25 degree celsius

40
Q

What is the relationship between conjugate acid-base pair?

A
  • stronger the acid, the weaker the conjugate base and vice versa
  • stronger the base, the weaker the conjugate acid and vice versa
41
Q

What is pKw equation?

A

pKa + pKb = pKw = 14 at 25 degree celsius

42
Q

What is the best indicator for strength of acid/base

A
  • 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
Q

When is a salt produced?

A

When acid react with bases

44
Q

What is a salt made up of?

A

Conjugate acid and conjugate base from the acid and base it was made from

45
Q

What happens when a salt dissolves in water?

A
  • it completely ionises
  • the resultant cation or anion may undergo hydrolysis to produce acidic or alkaline solutions
46
Q

What happens when salt of strong acid and strong base is dissolved in water?

A
  • pH = 7
  • no salt hydrolysis occurs
47
Q

What happens when weak acid + strong base salt is dissolved in water?

A
  • pH > 7
  • relatively strong conjugate base of weak acid hydrolyses in water to give OH⁻
  • weak conjugate acid does not undergo hydrolysis
48
Q

What happens when strong acid + weak base salt is dissolved in water?

A
  • pH < 7
  • relatively strong conjugate acid of weak base hydrolyses in water to give H₃O⁺
  • weak conjugate base does not undergo hydrolysis
49
Q

What happens when weak base + weak acid salt is dissolved in water?

A
  • pH (depends on relative strength of conjugate acid and base)
  • Both strong conjugate acid and base undergoes hydrolysis in water
50
Q

What is a buffer solution?

A

A buffer solution is one that can resist a change in pH when a small amount of acid/base is added to it

51
Q

What is a buffer made up of?

A

conjugate acid-base pair

52
Q

What does acidic buffer consist of?

A

weak acid and its salt (which contains conjugate base of weak acid)

53
Q

What does alkaline buffer consist of?

A

weak base and its salt (which contains conjugate acid of weak base)

54
Q

How is an acidic buffer created?

A
  • 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
Q

What happens when a small amount of acid is added to acidic buffer solution?

A
  • 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
Q

What happens when a small amount of base is added to acidic buffer solution?

A
  • 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
Q

How is an alkaline buffer created?

from NH₃ and NH₄Cl

A
  • 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
Q

What happens when a small amount of acid is added to alkaline buffer solution?

A
  • 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
Q

What happens when a small amount of base is added to alkaline buffer solution?

A
  • 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
Q

What is the Henderson-Hasselbalch equation for acidic buffer?

A
  • 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
Q

What is the Henderson-Hasselbalch equation for alkaline buffer?

A
  • 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
Q

What is the effect of dilution on pH of buffer solution?

A
  • no effect as both [salt] and [acid] are reduced to the same extent
  • however buffer may not be so effective at controlling pH
63
Q

What is buffering capacity?

A

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
Q

When is a buffer solution most effective?

A
  • 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
Q

When does acidic buffer have maximum buffering capacity and what happens as a result?

A
  • When [HA] = [A⁻]
  • pH = pKa
66
Q

When does alkaline buffer have a maximum buffering capacity and what happens as a result?

A
  • [B] = [BH⁺]
  • pOH = pKb
67
Q

What is the effective buffer range for acidic/alkaline buffer?

A

pH/pOH = pKa/pKb ± 1

68
Q

What are some uses of buffer solutions?

A
  • 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
Q

What is equivalence point?

A

Point during titration at which stoichiometric amounts of reactants have reacted

70
Q

What is end-point?

A

Point during titration where the indicator changes colour

71
Q

What is an acid-base indicator?

A
  • 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
Q

How does an acid-base indicator work?

HIn (aq) ⇌ H⁺ (aq) + In⁻ (aq)
HIn: colour A
In⁻: colour B

A
  • 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
Q

When will colour A be observed and when will colour B be observed?
pH = pKᵢₙ + lg([In⁻]/[HIn])

HIn (aq) ⇌ H⁺ (aq) + In⁻ (aq)

A
  • 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
Q

What is the pH range of an indicator?

A

pKᵢₙ ± 1

75
Q

Why is the pH working range and colour of methyl orange?

A
  • pH 3-5
  • red, orange, yellow
76
Q

What is the pH working range and colour of screened methyl orange?

A
  • pH 3-5
  • violet, grey, green
77
Q

What is the colour and pH working range of thymol blue?

A
  • pH 8-10
  • red/yellow
  • yellow (when acid is added to strong base) or blue (when strong base is added to acid)
  • blue
77
Q

What is the pH working range and colour of thymolphtalein?

A
  • pH 8-10
  • colourless, pale blue, blue
78
Q

What is the pH working range and colour of phenolphthalein?

A
  • pH 8-10
  • colourless, pale pink, pink
79
Q

How to choose suitable indicator?

A

working pH range of indicator must lie within sharge charge in pH near equivalence point

Chemistry (21 decks)

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