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A LEVEL CHEM > 1.12 acid base equilibria > Flashcards

1.12 acid base equilibria Flashcards

1
Q

what is a bronsted-lowry acid ?

A
  • a substance that can donate a proton
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2
Q

what is a bronsted-Lowry base

A
  • a substance that can accept a proton
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3
Q

how can we calculate pH?

A

pH = - log [ H + ]

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4
Q

how many d.p should we give pH values to?

A
  • two decimal places
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5
Q

what do strong acids do?

A
  • completely dissociate
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6
Q

how can we find concentration of H+ from pH?

A

1 x 10 ^ - pH

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7
Q

what is the expression for Kw?

A

Kw = [ H+ (aq) ] [ OH- (aq) ]

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8
Q

why can we assume [ H20 (l) ] is constant

A
  • concentration is much bigger than the concentration of the ions
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9
Q

what is the value for Kw at 298K

A

1.00 x 10 ^ -14

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10
Q

why are pure water solutions neutral?

A

[ H+] = [ OH- ]

concentration of H+ is equal to the concentration of OH-

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11
Q

what is the equation of Kw for pure water solutions?

A

Kw = [ H+ (aq) ] ^2

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12
Q

what effect does increasing the temperature on the dissociation of water have on the pH?

A
  • forwards reaction is endothermic
  • increasing temp causes equilibrium to shift to RHS, to oppose the increase in temp
  • this increases concentration of [ H + ]
  • so lower pH
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13
Q

what do strong bases do

A
  • completely dissociate into their ions
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14
Q

how can we calculate the pH of a strong base?

A
  • we are normally given the concentration of hydroxide ion
  • rearrange with Kw equation, to find H
  • use pH equation
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15
Q

what do weak acids do

A
  • only slightly dissociate in water
  • give an equilibrium mixture
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16
Q

what is the expression for weak acids dissociation?

A

Ka = [ H+ ] [ A- ] / [ HA ]

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17
Q

whats the relationship between Ka and acids

A
  • the larger the Ka the stronger the acid
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18
Q

how can we calculate Ka from pKa?

A

Ka = 10^-pKa

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19
Q

what two assumptions are made when calculating pH of a weak acid

A

[ H+ ] = [ A- ] , because they have dissociated in a 1:1 ratio

  • as the amount of dissociation is small, assume that initial concentration of the undissociated acid has remained constant
20
Q

how can we calculate pH from strong acid and strong base neutralisations?

A
  • work out moles of original acid, so moles of H+
  • work out moles of base added, so moles of OH-
  • work out which one is in excess
21
Q

in strong acid and strong base neutralisations, what happens if excess acid is present?

A

new conc of H+ = moles excess / total volume (dm3)

pH = - log [ H+ ]

22
Q

in strong acid and strong base neutralisations what happens if base is in excess

A

[ OH ] = moles excess OH / total volume

[ H+ ] = Kw / [ OH- ]

pH = - log [ H+ ]

23
Q

what happens if the bases / acids are diprotic ?

A
  • double the moles of the acid
  • double the moles of the base
24
Q

in weak acids and strong base neutralisations what happens if excess alkali is present

A
  • use same method as strong acid strong base
25
Q

in weak acid and strong base neutralisations what happens if excess acid is present?

A

[ HA ] = inital moles of HA - moles of OH / tv

[ A- ] = moles of OH added / total volume

rearrange Ka = [H] [A] / [HA]

pH = - log [ H ]

26
Q

how do you work out the pH of a weak acid at half equivalence

A

[ H + ] = Ka

  • we can assume [ HA ] = [ A- ]
27
Q

how can we calculate the pH of a diluted strong acid ?

A

[ H+ ] = [ H+ old ] x old volume / new volume

pH = -log[H]

remember to add the volumes for the new volume

28
Q

what is a buffer solution?

A
  • a solution where the pH does not change significantly if small amounts of acid or alkali are added to it
29
Q

how can i calculate the pH of a diluted base?

A

new OH = OH x old volume / new volume

H+ = Kw / OH

pH = -log[H+]

30
Q

what is an acidic buffer solution made from?

A
  • a weak acid and a salt of that weak acid
31
Q

what is a basic buffer solution made from?

A
  • a weak base and a salt of that weak base
32
Q

explain why the pH stays fairly constant when small amounts of alkali/acid are added

A
  • are there is a large concentration of salt ion in the buffer, the ratio or acid / salt stays almost constant
33
Q

how can we calculate the pH of making a buffer by adding salt solution / solid salt

A
  • find moles of both solutions
  • use given Ka

[H+] = Ka [ HA ] / [ A- ]

we can enter moles straight away as they have same volume

34
Q

how can we calculate the pH of a buffer made by adding sodium hydroxide

A
  • calculate moles of solutions
  • find moles in excess
  • moles excess / total volume = conc of acid
  • rearrange Ka
  • moles of OH added / total volume = conc of salt
35
Q

whats the steps for constructing a pH curve

A
  • transfer 25cm^3 of acid to a conical flask
  • measure initial pH
    -add alkali in small amounts
  • stir mixture to equalise pH
  • measure and record the pH
  • repeat steps 3-5
  • add until alkali in excess
36
Q

what are the 4 main types of pH curves

A
  • strong acid strong base
  • strong acid weak base
  • weak acid strong base
  • weak acid weak base
37
Q

what is the pH at equivalent point of a strong acid and strong base curve

A

pH = 7 at equivalence point

38
Q

what are the key points to sketching a curve

A
  • inital and final pH
  • volume at neutralisation
  • general shape
39
Q

what is the pH at equivalence point for weak acid strong base

A

pH > 7

40
Q

what is the pH at equivalence point for strong acid weak base curves

A

pH < 7

41
Q

what is an indicator

A
  • changes colour from Hln to ln- over a narrow range.
  • different indicators change colours over different ranges
42
Q

what is the equilbrium for indicators?

A

Hln <> ln- + H+

43
Q

what happens at low pH

A

low pH means high concentration of [H+]

so equilbrium favors left hand side of equation ( colour A ) is present

44
Q

what happens at high pH

A

at high pH, low concentration of [H+]

equilbrium favors right hand side, so colour B is present

45
Q

what happens at transition point

A

[Hln] ≈ [ln-]

both colours may be visible as a mixture

46
Q

what is the colour change for methyl orange and over what pH range?

A

red to yellow

around 3.1 - 4.4

47
Q

what is the colour change of phenolpthalein and over what pH range?

A

colourless to pink

around 8.3 - 10.0

A LEVEL CHEM (20 decks)

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