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A level Chem OCR B > O - Equilibria (acid–base) *01 *02 > Flashcards

O - Equilibria (acid–base) *01 *02 Flashcards

1
Q

what are acids?

A

proton donors

they release H+ ions when they are mixed with water

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2
Q

Why would you never get H+ ions by themselves in water?

A

because they are always combined with water

HA (aq) + H2O (l) —> H3O+ (aq) + A- (aq)

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3
Q

What is a base?

A

proton acceptors

when in solution they grab H+ ions from water molecules

B (aq) + H2O (l) —> BH+ (aq) + OH- (aq)

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4
Q

What is a strong acid?

A

completely dissociate in water

HCl (g) + water —> H+ (aq) + Cl- (aq)

(reversible but equilibrium lies very far to the right)

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5
Q

What is a strong base?

A

completely dissociate in water

NaOH (s) + water —> Na+ (aq) + OH- (aq)

(reversible but equilibrium lies very far to the right)

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6
Q

What is a weak acid?

A

only dissociate slightly in water

CH3COOH (aq) + water < —> CH3COO- (aq) + H+ (aq)

an equilibrium is set up which lies well over to the left

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7
Q

What is a weak base?

A

only dissociates slightly in water

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8
Q

What happens when acids and bases react?

A

protons are transferred

base accepts protons, acid gets rid of them

HA (aq) + B (aq) < —-> BH+ (aq) + A- (aq)

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9
Q

base + acid reaction [HA (aq) + B (aq) < —> BH+ (aq) + A- (aq)] - what would shift equilibrium right?

A

adding more HA or B

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10
Q

base + acid reaction [HA (aq) + B (aq) < —-> BH+ (aq) + A- (aq)] - what would shift equilibrium left?

A

adding more BH+ or A-

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11
Q

What happens when an acid is added to water?

A

water acts as a base

HA (aq) + H2O (l) < —> H3O+ (aq) + A- (aq)

for weak acids equilibrium left

for stong acids equilibrium right

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12
Q

Identify the acid and base in this reaction

HCl (aq) + NH3 (aq) —> Cl- (aq) + NH4+ (aq)

A

HCl = acid - it donates a proton to NH3 so it becomes NH4+

NH3 = base - it accepts a proton from HCl to become NH4+

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13
Q

What do acids and bases form?

A

Conjugate pairs

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14
Q

What are conjugate pairs?

A

eg.

HA + H2O < —> H3O+ + A-
acid 1 base 2 acid 2 base 1

HA and A- are conjugate pairs

  • in the forward reaction HA donates a proton to form A-
  • in the reverse reaction A- acts as a base and accepts a proton from the H3O+ to form HA

H2O and H3O+ is also a conjugate pair

HA < —> H+ + A- and H+ + H2O < –> H3O+

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15
Q

Identify the conjugate pairs

HCl (aq) + H2O (l) < —-> H3O+ (aq) + Cl- (aq)

A

HCl (aq) + H2O (l) < —-> H3O+ (aq) + Cl- (aq)
acid 1 base 2 acid 2 base 1

HCl and Cl-

H2O and H3O+

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16
Q

a neutral solution is one in which…..

A

[H+] = [OH-]

17
Q

what does the pH scale measure?

A

the hydrogen ion concentration

18
Q

How can you work out pH

A

pH = -log10 [H+]

19
Q

what are monoprotic acids?

A

strong acids (ionise fully)

each mole of acid produces one mole of hydrogen ions

[H+] = [acid]

eg.
0.1 mol dm^-3 HCl [H+] = 0.1 mol dm^-3
so pH = -log10 [H+]

20
Q

How do you work out [H+] from pH?

A

[H+] = 10^-pH

21
Q

Ionic product of water =

A

Kw = [H+] [OH-]

Kw is always 1.00 x 10^-14
at 298K

22
Q

Where does equilibrium lies - when water dissociates

H2O < —-> H+ + OH-

A

far to the left

23
Q

in pure water relationship between [H+] and [OH-]

A

[H+] = [OH-]

so Kw = [H+]^2

24
Q

How do you find pH of a strong base

A

Kw = [H+] [OH-]

if conc. of base = 0.02mol dm^-3, [OH-] = 0.02mol dm^-3

[H+] = Kw / [OH-]

pH = -log10[H+]

25
Q

what is the acid dissociation constant (Ka) ?

A

to find [H+] of weak acids
HA < —> H+ + A-

Ka = [H+][OH-] / [HA]start in mol dm-3

[H+] = [A-] so:

Ka = [H+]^2 / [HA]

26
Q

pH of a weak acid?

A

use Ka

eg. find pH for 0.02mol dm-3 solution of propanoic acid at 298K. Ka = 1.34 x 10^-5 mol dm-3

Ka = [H+]^2 / [CH3CH2COOH]

[H+]^2 = Ka x [CH3CH2COOH] = 1.34 x 10^-5 mol dm-3 x 0.02mol dm-3 = 2.60 x 10^-7 mol dm-3
[H+] = square root of 2.60 x 10^-7 mol dm-3 = 5.10 x 10^-4 mol dm-3

now pH = -log10[H+] = -log10[5.10 x 10^-4] = 3.292

27
Q

pKa =

A

pKa = -log10Ka

Ka = 10^-pKa

28
Q

how do you find the conc. of an acid - method

A

acid - base titration

add a standard solution of base to a measured quantity of acid

pipettes and burettes used so know precisely how much acid and base is used

indicator added, so know when been neutralised

when you know how much base needed to neutralise an acid can then work out conc. of acid
- conc. = moles / vol

29
Q

What are buffers?

A

a solution that resists changes in pH when small amounts of acid or base are added

30
Q

what are acidic buffers made from?

A

a weak acid and one of its salts, pH less than 7

eg. ethanonic acid and sodium ethanoate

31
Q

How do acidic buffers work?

eg. ethanonic acid and sodium ethanoate

A

weak acid slightly dissociates: CH3COOH (aq) < —-> H+ (aq) + CH3COO- (aq)
salt fully dissociates into its ions (dissolves): CH3COONa (s) + H2O —> CH3COO- (aq) + Na+ (aq)

in solution lots of undissociated ethanoic molecules, and lots of ethanoate ions from the salt

when you alter conc. of H+ or OH- ions equilibrium position moves

CH3COOH (aq) < —-> H+ (aq) + CH3COO- (aq)
addition of base ———————> (reducing OH- conc.)

32
Q

What are buffers used for? - blood

A

needs to be kept at pH 7.4

buffered w/ carbonic acid: H2CO3 (aq) < —> H+ (aq) + HCO3- (aq)
H2CO3 (aq) < —> H2O (aq) + CO2 (g)

levels of H2CO3 controlled by respiration

breathing out CO2 = H2CO3 reduced = equilibrium right

levels of HCO3- controlled by kidney, excess is excreted in urine

33
Q

What are buffers needed for? - cells

A

cells need a contasnt pH to allow the biochemical reactions to take place

pH controlled by a buffer based on the equilibrium between dihydrogen phosphate ions and hydrogen phosphate ions

H2PO4- < —-> H+ + HPO42-

34
Q

What are buffers needed for? - food products

A

used in food products to control the pH

changes in pH caused by bacteria and fungi cause food to deteriorate

common buffer is made using sodium citrate, equli. between citrate ions and citric acid
(phorpooric acid/phosphate ions and benzoic acid/benzoate ions also used)

35
Q

What are buffers needed for? - biological washing powders

A

they need to be at the right pH for the enzymes to work

36
Q

What are buffers needed for? - shampoo

A

pH 5.5 buffer

alkalines makes hair looks dull so a slightly acidic buffer will keep your hair looking shiny

37
Q

how to calculate the pH of a buffer solution?

A

need to know Ka of the weak acid and the concs. of the weak acid and its salt

eg. a buffer solution contains 0.4mol dm^-3 methanoic acid, HCOOH, and 0.6mol dm^-3 sodium methanoate, HCOO-Na+. For methanoic acid Ka = 1.6 x 10^-4mol dm^-3. What is the pH

  1. write expression for Ka
    HCOOH (aq) < —-> H+ (aq) + HCOO- (aq):
    Ka = [H+] x [HCOO-] / [HCOOH]
  2. rearrange: [H+] = Ka x ( [HCOOH] / [HCOO-] )
    = 1.6x10^-4 x ( 0.4 / 0.6 ) = 1.07 x 10^-4mol dm^-3
  3. convert [H+] to pH
    pH = -log10[H+] = -log10[1.07x10^-4] = 4.0

A level Chem OCR B (59 decks)

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