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A-Level Chemistry > Physical Unit 1.12: Acids & Bases > Flashcards

Physical Unit 1.12: Acids & Bases Flashcards

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1
Q

what is the Bronsted-Lowry definition for an acid, base & acid-base reaction/equilibria?

A

acid: proton donor
base: proton acceptor
acid-base reaction/equilibria: involves the transfer of protons

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2
Q

what is the pH scale?

A

logarithmic scale
a measure of hydrogen ion concentration

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3
Q

formula for pH (strong acids)

A

pH = -log[H+]
always to 2dp

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4
Q

[H+] =

A

10^-pH

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5
Q

define monobasic & dibasic acid

A

monobasic acid: acids where each molecule dissociates to form 1H+ ion e.g. HNO3, HCl
= monoprotic

dibasic = diprotic: each molecule dissociates to form 2 H+ ions e.g. H2SO4

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6
Q

define strong acid

A

(in solution) all molecules dissociate to form H+ ions

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7
Q

dilution of a strong acid

A

[H+] = (volume of acid/total volume) x conc. acid

it’s the proportion of the volume x conc.

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8
Q

what is Kw, the ionic product of water?

A

water dissociates slightly:
H2O <–> H+ + OH- endothermic
conc. water is 55.6 moldm-3
Kw is derived from Kc of this dissociation

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9
Q

what is the formula for Kw? & derivation

A

Kc = [H+][OH-] / [H2O]

Kw = [H+][OH-]

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10
Q

define neutral

A

[H+] = [OH-]

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11
Q

how does temperature affect pH of water & neutrality of water?

A

pure water is neutral

increasing temp. shifts equilibrium in endothermic direction, in forwards direction
= which increases [H+] & [OH-]
= pH changes
but pure water stays neutral as [H+] = [OH-]

as temp. increases, Kw increases

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12
Q

what is Kw @ room temp.?

A

1x10^-14

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13
Q

@ stp in pure water, Kw =
pH =

A

Kw = [H+]^2 = 1x10^-14
[H+] = root 1x10^-14 = 10^-7
pH = 7.00

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14
Q

define monoprotic & diprotic base

A

monoprotic base: 1 mole of base accepts 1 mole of H+ ions
/
each molecule accepts 1 H+ ion

diprotic base: 1 mole of base accepts 2 moles of H+ ions
each molecule accepts 2 H+ ions

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15
Q

how do you calculate pH of a strong base?

A

use Kw = 1x10^-14 @stp
use [H+] = Kw/[OH-]

to calculate conc. of OH-, multiply conc. of substance by # moles of OH- in one molecule

to calculate conc. of base, divide conc. of substance by # moles of OH- in one molecule

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16
Q

how do you calculate the pH of a solution formed from the reaction b/w a strong acid & strong base?

A
  1. calculate moles H+
  2. calculate moles OH-
  3. calculate leftover moles at end, either of H+ or OH-
  4. calculate [H+] or [OH-] of leftover at end by mol/total vol.
  5. calculate pH
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17
Q

define weak acid

A

only a small fraction of the molecules dissociate to form H+ ions
e.g. carboxylic acids

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18
Q

weak acid equilibrium

A

HA <–> H+ + A-
HA = weak acid
A- = salt of weak acid

19
Q

what is the salt of a weak acid?

A

slightly basic bc proton accepted by A- to form HA

20
Q

what is the formula for Ka, the acid dissociation constant for a weak acid?
what are the assumptions made?

A

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

assumptions:
1. [HA] = original [HA], the conc. doesn’t change much due to equilibrium
2. [H+] only comes from the dissociation of HA & not from water/Kw

21
Q

what is the formula for pKa?

A

pKa = -log(Ka)
Ka = 10^-pKa

22
Q

important A- info

A

in a solution of pure weak acid dissolved in water, A- comes from the dissociation of weak acid (HA)
so [A-]=[H+]

if any ionic salt is dissolved in water, [A-] = [salt] & A- from the dissociation of weak acid is negligible

23
Q

how do you calculate the pH of solution formed from the reaction b/w weak acid & strong base?

A

HA + OH- –> A- + H2O
1. calculate moles HA
2. calculate moles OH-
3. calculate excess moles (at end) or HA or OH-

if excess HA:
4. calculate moles HA left & moles A- formed
5. calculate [HA] leftover & [A-] formed
6. use Ka to find [H+]
7. find pH

if excess OH-:
4. calculate [OH-]
5. use Kw to find [H+]
6. find pH

if excess base, it is irrelevant whether acid was weak or strong bc it has all reacted

24
Q

what is half neutralisation of weak acid?

A

when half of HA molecules have reacted with OH-

25
Q

what is true at half neutralisation of weak acid?

A

[HA]=[A-]
so Ka = [H+]
pKa = pH

26
Q

define indicator

A

weak acids where HA & A- are different colours

(the pH at which the indicator changes colour varies b/w indicators)

27
Q

for indicators:
at low pH
at high pH

A

at low pH, HA is the main species present
at high pH, A- is the main species present

28
Q

what does universal indicator contain?

A

it is a mixture of different indicators so shows several colours at each pH

29
Q

table of methyl orange & phenolphthalein, colour of HA, colour of A-, pH range of colour change

A

methyl orange:
red
yellow
3.2-4.4

phenolphthalein:
colourless
pink
8.2-10.0

30
Q

for an indicator to change colour where moles acid = moles base,

A

it must change colour within the range of rapid pH change at the end point of the titration

31
Q

define equivalence point

A

point when moles acid = moles alkali - but pH not always 7
rapid pH change around equivalence point

32
Q

define end point

A

when indicator changes colour - should coincide with equivalence point if correct indicator is used

33
Q

what do pH curves look like for: strong acid-strong base
strong acid-weak base
weak acid-strong base
weak acid-weak base

A

see booklet

34
Q

define buffer solution

A

solution that resists changes in pH when small amounts of acid or alkali are added
so an approximately constant pH is maintained

35
Q

define acidic buffer
& define basic buffer

A

acidic buffer solutions contain a weak acid & the salt of that weak acid
pH < 7
[acid] & [salt] are much higher than [H+]

basic buffer solutions contain a weak base & the salt of that weak base
pH > 7
[base] & [salt] are much higher than [OH-]

36
Q

how can an acidic buffer also be made?
basic?

A

excess HA & strong alkali –> HA & A- mixture

excess weak base & strong acid –> weak base + its salt

37
Q

[H+] =

A

Ka[HA] / [A-]
[H+] α HA/A-

38
Q

what is the effect of adding small amounts of acid or alkali on the ratio of HA:A-

A

the ratio stays roughly constant so pH hardly changes

39
Q

describe what happens when a little H+ is added to acidic buffer

A

write equilibrium equation
HA <–> H+ + A- related to Q
the added H+ is removed by reaction with A- to form HA
this shifts the position of equilibrium to the left

40
Q

describe what happens when a little H+ is added to basic buffer

A

write equilibrium equation
related to Q e.g. NH3 + H2O <–> NH4+ + OH-
the added H+ is removed by reaction with OH-
so some NH3 reacts to replace OH
so the position of equilibrium shifts right

41
Q

describe what happens when a little OH- is added to acidic buffer

A

write equilibrium equation related to Q
HA <–> H+ + A-
the added OH- is removed by reaction with H+
so some HA breaks down/dissociates to replace H+
so the position of equilibrium shifts right

42
Q

describe what happens when a little OH- is added to basic buffer

A

write equilibrium equation related to Q
e.g. e.g. NH3 + H2O <–> NH4+ + OH-
the added OH- is removed by reaction with NH4+ to form NH3
so the position of equilibrium shifts left

43
Q

how do you calculate the pH of acidic buffers?

A
  1. write out:
    Ka = [H+][A-] / [HA]
    [H+] = Ka[HA] / [A-]
  2. write out equation
  3. calculate moles HA left & moles A- formed
  4. calculate [HA] leftover & [A-] formed
  5. use Ka to find [H+]
  6. find pH

A-Level Chemistry (14 decks)

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