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CHEM 110 > Block 5 - Acids and Bases > Flashcards

Block 5 - Acids and Bases Flashcards

1
Q

Brønsted acid

A

A substance that has a proton (H+) which can be taken by a base
Any molecule that contains H joined to a more electronegative atom

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2
Q

Brønsted base

A

A substance that can take a proton from an acid

Has a lone pair of electrons

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3
Q

Amphoteric species

A

Species that act as both acids and bases

e.g. H2O, H2PO4

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4
Q

Diprotic, tripotic acids

A

Some acids can give up more than one proton, where removal of each proton is a distinct step

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5
Q

Conjugate base

A

The species left behind after the Brønsted acid has transferred a proton

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6
Q

Conjugate acid

A

The species produced when the Brønsted base accepts a proton

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7
Q

Equilibrium constant (Kw)

A

Kw = [H3O+][OH-] = 10^-14

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8
Q

What does Kw depend on

A

Temperature dependant

Since auto-ionisation of water is endothermic, Kw increases with temperature

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9
Q

What is ‘p’

A

A short-hand for -log10

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10
Q

pH < 7, pH = 7, pH > 7

A

pH < 7 = acidic
pH = 7 = neutral
pH > 7 = basic

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11
Q

pH < 0

A

pH can be less than 0, but rare and only happens if solution is highly acidic

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12
Q

Strong acid

A

Where equilibrium lies far to the right

Uses full arrow (–>)

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13
Q

Weak acid

A

Equilibrium arrow used

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14
Q

Strong and weak acids - concentration

A

Definition of strong and weak acids is independent of concentration

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15
Q

Proton transfer reactions - speed

A

Fast, so acid-base systems reach equilibrium rapidly

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16
Q

Strong and weak acids - equilibrium

A

Strong acid: equilibrium lies far to the right

Weak acid: equilibrium lies more to the left

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17
Q

Ka

A

Known as acidity constant, acid dissociation constant, acid ionisation constant

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18
Q

Strength of acid - Ka

A

Large value of Ka –> more reaction lies to right –> stronger acid

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19
Q

Strength of acid - pKa

A

Smaller pKa –> stronger acid

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20
Q

Strong acid and strong base - pKa

A

Strong acid: pKa < 0

Strong base: pKa > 14

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21
Q

As an acid gets weaker…

A

Its conjugate base gets stronger

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22
Q

Bonds are strongest when…

A

They are short (joined atoms are close together) and electron density between the two joined atoms is large

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23
Q

What affects acidity (strength of acid)

A

More positive or negative charge
Charge stabilisation:
- Inductive effects
- Delocalisation

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24
Q

Acidity - positive and negative charges

A

It’s easier for a positively charged molecule to lose a positively charged proton
Ease of proton transfer decreases as charge on molecule becomes more negative

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25
Q

Acidity - charge stabilisation by inductive effects

A

Presence of nearby electronegative atoms / EWGs move charge away from acidic H –> -H bonds weaker –> stronger acid

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26
Q

Acidity - charge stabilisation by delocalisation

A

Allows charge to be spread out over a large number of atoms

Can occur through resonance

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27
Q

To make a stronger acid…

A

Make the conjugate base as stable as possible

28
Q

Relationship between pKa and pH

A

pH < pKa : more acid form (HA)
pH = pKa : HA and A- in equal conc
pH > pKa : more base form (A-)

29
Q

Di/tri-protic acids - pKa

A 
Have 2 (or 3 respectively) pKa values - one for loss of first proton, and one for loss of second
pKa increases (i.e. pKa1 < pKa2)
30
Q

Amino acids - general structure

A

NH2CHRCO2H

where R defines the amino acid

31
Q

Amino acids - groups

A

Amine group: -NH2 (conjugate acid -NH3+)

Carboxyl group: -CO2H (conjugate base -CO2-)

32
Q

Isoelectric point

A

The point at which the zwitterion for amino acids has max conc

33
Q

Amino acids - electrophoresis

A

Allows separation of mixtures of amino acids by their charge

34
Q

How many species exist at one point in time

A

For acids and bases, the normal situation is that a max of two species exist significantly in any conditions

35
Q

When will both species of a conjugate acid/base pair be present

A

Within 1 pH unit (+/- 1) of the pKa of the acid form

36
Q

Amino acids - pKa

A

If pH < pKa, exists as acidic form (NH3 or COOH)
If pH > pKa, exists as basic form (NH2 or COO-)
If pH = pKa, exists as acidic and basic form (mixture)

37
Q

Strong acid calculations - assumption(s)

A

Only the strong acid contributes to [H3O+]

This is not true if pH > 6

38
Q

Strong base calculations - assumption(s)

A

Only the strong base contributes to [OH-]

This is not true if pH < 8

39
Q

Predicting pH of weak acids

A

Weak acid –> doesn’t dissociate to large extent –> dominant species is acid form –> pH < pKa

40
Q

Weak acid calculations - assumption(s)

A
  1. The only source of H3O+ is from the dissociation, so [conjugate base] = [H3O+]
  2. The dissociation is small compared to the original amount, so
    [HA] = [HA]0 - [conjugate base] ≈ [HA]0
41
Q

Weak acid calculations - checking assumptions

A
  1. pH more than 1 below 7
  2. pH more than 1 below pKa
  3. Check pH is on acid side of pKa: pH < pKa
42
Q

Predicting pH of weak bases

A

Weak base –> base form dominates –> pH > pKa

43
Q

Weak base calculations - assumption(s)

A
  1. The only source of OH- is from the reaction, so
    [conjugate acid] = [OH-]
  2. The amount of reaction is small compared to the original amount, so
    [base] = [base]0 - [conjugate acid] ≈ [base]0
44
Q

Weak base calculations - checking assumptions

A
  1. pH is more than 1 above 7
  2. pH is more than 1 above pKa
  3. pH is on base side of pKa: pH > pKa
45
Q

Amphoteric species calculations - determining which pKa are involved and predicting

A

Relevant ones where species of interest is involved in equation
Expect pH between pKas of relevant equations

46
Q

Amphoteric species calculations - assumption(s)

A

[H3O+] and [OH-] are much less than the conc of the amphoteric species, so the only reaction the amphoteric species undergoes is with itself

47
Q

Amphoteric species calculations - checking assumption

A

[H3O+] &laquo_space;original conc

48
Q

Buffer solutions: Henderson-Hasselbach equation - assumption(s)

A

Can only use for buffers!!
Extent of dissociation of weak acid and its conjugate base is small, so
[HA] = [HA]initial and
[A-] = [A-]initial
When HA and A- are of similar conc, the log term is small and pH is approx equal to pKa and will only change by +/- unit

49
Q

Buffer solution

A

Solution where pH changes much less than that of pure water when acid or base is added, i.e. resists changes in pH
Typically contain either equal amounts of a weak acid and its conjugate base (where buffer pH < 7) or equal amounts of a weak base and its conjugate acid (where buffer pH > 7)

50
Q

How does a buffer work

A

It contains both species that can accept protons (to react with added acids) and that can donate protons (to react with added base)

51
Q

When does a buffer solution work best

A

When [HA] is close to [A-], i.e. most suitable at pH around pKa of the acid
More effective with higher conc of the two species as there is more to react with added acid or base

52
Q

When a buffer is fully used up…

A

It has exceeded its capacity, and pH will change significantly

53
Q

pH of unbuffered solution - addition of acid or base

A

pH changes significantly

54
Q

pH of unbuffered solution - dilution

A

pH changes significantly

55
Q

pH of buffer system - dilution

A

Doesn’t change (much)

56
Q

pH of buffer system - addition of acid or base

A

pH of solution changes by a small amount

57
Q

Titration

A

A method of volumetric analysis for determining conc of an unknown solution by letting it react with another whose conc is known

58
Q

Titration - equivalence point

A

Volume at which the reaction is just completed

59
Q

Titration of strong base with strong acid

A

Before any acid is added, pH is high
As acid is added, pH of solution falls, gradually at first, then faster as equivalence point is approached
After EP, rate of pH change slows
Well beyond the EP, pH is low
Since acid and base are strong, both 100% dissociated and pH at EP is 7

60
Q

Titration of weak acid/base with strong base/acid - exact pH of EP depends on…

A

pKa of acid

61
Q

Titration of weak acid with strong base

A

Beginning of titration, weak acid only
pH rises fairly steeply when first drops of base are added - only little OH- required to react with free H3O+
Buffer region - pH changes slowly
Half-equivalence point - reaction is half complete, so there are equal amounts of acid and base present
Equivalence point - reaction is 100% complete, only conjugate base present –> pH > 7
After EP when all acid has been deprotonated, solution is essentially a strong base and curve flattens out; pH determined by strong base

62
Q

Titration - half-equivalence point

A

pH = pKa
Can only work out pKa for weak acid/base involved in titration, not strong acid/base, as no equilibrium between conjugate acid and base for strong acids/bases so at 1/2 equivalence point, pH only determined by amount of unreacted acid present, not due to an equilibrium

63
Q

Titration of weak base with strong acid

A

Beginning of titration - weak base only
Buffer region - pH changes slowly
Half-equivalence point - reaction is half complete, so there are equal amounts of acid and base present
Equivalence point - reaction is 100% complete; only conjugate acid present; pH < 7
After EP when all base has been protonated, solution is strong acid and curve flattens out; pH determined by strong acid

64
Q

End point

A

When we decide to stop, generally because observable change has occurred
Ideally, end point is at equivalence point

65
Q

Indicators

A

Compounds that are diff colours in their acid and base forms, i.e. they are acids or bases too
Colour change occurs around at pH around pK(In)
pK(In) should be around 1 pH unit of pH of equivalence point

66
Q

Titration curves for polyprotic acids

A

Multiple equivalence points and buffer regions

CHEM 110 (6 decks)

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