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A-Level Chemistry > Physical Unit 1.12: Acids & Bases (A2) > Flashcards

Physical Unit 1.12: Acids & Bases (A2) 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

define pH

A

-log[H+]

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5
Q

[H+] =

A

10^-pH

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6
Q

define monobasic & dibasic acid & give e.g.s

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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7
Q

define strong acid

A

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

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8
Q

[H+] in 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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9
Q

dilution of weak acid

A

[HA] = volume of acid/total volume x conc. acid

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10
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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11
Q

what is the formula for Kw? & derivation

A

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

Kw = [H+][OH-]

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12
Q

define neutral

A

[H+] = [OH-]

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13
Q

how does temperature affect pH & neutrality of water & the value for Kw?

A

pure water is neutral

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

as temp. increases, Kw increases

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14
Q

what is Kw @ room temp.?

A

1x10^-14

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15
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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16
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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17
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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18
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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19
Q

define weak acid & give e.g.

A

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

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20
Q

weak acid equilibrium

A

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

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21
Q

describe the salt of a weak acid

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A

A- is slightly basic bc is accepts H+ to form HA

22
Q

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

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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

23
Q

what is the formula for pKa?

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A

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

24
Q

important A- info

Study These Flashcards
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

25
how do you calculate the pH of solution formed from the reaction b/w weak acid & strong base?
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
26
what is half neutralisation of weak acid?
when half of HA molecules have reacted with OH-
27
what is true at half neutralisation of weak acid?
[HA]=[A-] so Ka = [H+] pKa = pH
28
define indicator
weak acids where HA & A- are different colours (the pH at which the indicator changes colour varies b/w indicators)
29
for indicators: at low pH at high pH
at low pH, HA is the main species present at high pH, A- is the main species present
30
what does universal indicator contain?
it is a mixture of different indicators so shows several colours at each pH changes colour gradually, not a sharp colour change
31
table of methyl orange & phenolphthalein, colour of HA, colour of A-, pH range of colour change
methyl orange: red yellow 3.2-4.4 phenolphthalein: colourless pink 8.2-10.0
32
for an indicator to change colour where moles acid = moles base,
it must change colour within the range of rapid pH change at the end point of the titration = vertical/steep part on curve
33
define equivalence point
point when moles acid = moles alkali - but pH not always 7 rapid pH change around equivalence point = small additions of OH- cause large increases in pH curve steep/vertical here
34
define end point
when indicator changes colour - should coincide with equivalence point if correct indicator is used
35
what do pH curves look like for: strong acid-strong base strong acid-weak base weak acid-strong base weak acid-weak base
see booklet weak acid-weak base curve has no vertical section
36
define buffer solution
solution that resists changes in pH when small amounts of acid or alkali are added so an approximately constant pH is maintained
37
define acidic buffer & define basic buffer
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-]
38
how can an acidic buffer also be made? basic?
excess HA & strong alkali --> HA & A- mixture excess weak base & strong acid --> weak base + its salt
39
[H+] = [H+] α
Ka[HA] / [A-] [H+] α HA/A-
40
what is the effect of adding small amounts of acid or alkali on the ratio of HA:A-
the ratio stays roughly constant so pH hardly changes
41
describe what happens when a little H+ is added to acidic buffer
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
42
describe what happens when a little H+ is added to basic buffer
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
43
describe what happens when a little OH- is added to acidic buffer
write equilibrium equation related to Q HA <--> H+ + A- the added OH- is removed by reaction with HA (or with H+) so some HA breaks down/dissociates to replace H+ so the position of equilibrium shifts right
44
describe what happens when a little OH- is added to basic buffer
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
45
how do you calculate the pH of acidic buffers made by reacting weak acid with strong base?
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
46
how do you calculate pH of acidic buffer solution made from weak acid & its salt?
mol of A- [A-] Ka [H+] = Ka.[HA given in Q] / [A-]
47
explain why the expression for Kw does not include the concentration of water
the concentration of water is almost constant water only slightly dissociates so its concentration is much greater than [H+] or [OH-]
48
how does the value of Kw change as temperature increases?
Kw increases the forward reaction is endothermic so as temperature increases, the position of equilibrium shift right to decrease temperature
49
why is [H+] squared in Ka & Kw expressions?
for Kw, in pure water [H+] = [OH-] for Ka, [H+] = [A-] because the ratio is 1:1
50
describe the method for calculating pH of acid added to acidic buffer solution
write out HA <--> H+ + A- 1. mol of H+, A- & HA at end = IE table HA: I=y, E=y+x H+: I=x, E=0 A-: I=z, E=z-x 2. Ka 3. pH
51
what are the equations for: acidic buffer/weak acid + strong base acid added to acidic buffer base added to acidic buffer reaction of strong acid + strong base?
acidic buffer/weak acid + strong base: HA + OH- --> A- + H2O weak acid + its salt: HA <--> H+ + A- acid + acidic buffer: HA <--> H+ + A- base + acidic buffer: HA + OH- --> A- + H2O strong acid + strong base: H+ + OH- --> H2O
52
why does the pH stay approximately constant?
the ratio of [A-]:[HA] stays roughly constant

A-Level Chemistry (22 decks)

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