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A level Chemistry > Acids, Bases and Buffers > Flashcards

Acids, Bases and Buffers Flashcards

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1
Q

What is an acid?

A

proton donors

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2
Q

What is a base?

A

proton acceptors

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3
Q

The definitions we use for acids and bases are called

A

Bronsted Lowry acids and bases

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4
Q

oxonium ion?

A

H3O+ - H2O forms a dative covalent bond to H+

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5
Q

when acids dissociate?

A

(x)H+ + negative ion always forms

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6
Q

When a monoprotic acid dissociates?

A

1 (mol) of H+ forms

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7
Q

When a diprotic acid dissociates?

A

2 H+ form

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8
Q

When a triprotic acid dissociates?

A

3 H+ form

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9
Q

Idm if acid is di/triprotic, when it dissociates how many moles of the salt forms?

A

1 mole

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10
Q

could say ? instead of protic

A

basic

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11
Q

Acid base pairs?

A
  • involves 2 acid base pairs

* 2 particles that change into each other by losing/ gaining a H+

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12
Q

ABP: what does the acid form?

A

Conjugate base by donating a H+

the CB has a negative charge

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13
Q

ABP: what does the base form?

A

Conjugate acid.

the CA has a positive charge

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14
Q

water is amphoteric meaning?

A

can act as acids and bases

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15
Q

strong acid definition?

A

Fully dissociate into H+ e.g. HCl

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16
Q

weak acid definition?

A

only partially dissociate into H+

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17
Q

conc acid definition?

A

high no. of moles per dm3

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18
Q

dilute acid definition?

A

fewer moles per dm3

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19
Q

for the reaction to be a neutraliation?

A

water must be formed, so metal + acid is not a neutralisation

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20
Q

⭐ionic equations

A

only include ions that change e.g. state

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21
Q

pH equation?

A

-log [H+]

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22
Q

[H+] =

A

10 to the power of -pH

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23
Q

calc pH of strong acid?

A

• in strong acid, all H+ dissociate so [H+] dissociated = [H+] in acid

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24
Q

give pH to?

A

2dp

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25
Q

calc pH of di/triprotic acid?

A

x the [H+] by 2/3 b4 putting in pH equation

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26
Q

What is Kw used for?

A
  • the ionic product of water, 10^-14mol2dm-6 at 298K

* used to find the pH of alkaline solutions

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27
Q

Kw equation?

A

Kw/ [OH-] = [H+]

28
Q

when do u use Kw?

A

when the [OH-] is given, get [H+] and then calc pH

29
Q

why does an increase in T change the pH of water?

A

• endo forwards reaction favoured, eq shifts to the right
• [H+] inc
so pH ⬇

30
Q

remember that lower pH =

A

more acidic

31
Q

to calc pH of weak acids?

A

DIFFERENT METHOD - can’t just use [acid] as [H+]

32
Q

neutral solution =

A

equal conc of H+ and OH- - does not have to be at pH 7

33
Q

when 2x OH in the alkali?

A

times the [OH-] by 2

34
Q

dissociation of a weak acid is?

A

reversible

35
Q

Ka =

A
  • the acid dissociation constant
  • larger the Ka value, the stronger the acid - eq lies towards products
  • Units = moldm-3
36
Q

Ka equation?

A

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

simplified to Ka = [H+]2/ [HA]

37
Q

Ka (in relation to pKa)?

A

Ka =10-^pKa

38
Q

10 is not just 10 it’s…

A

shift log

39
Q

pKa =

A

pKa = -logKa

think of Ka like [H+]

40
Q

the stronger the acid, the lower the?

A

pKa value

41
Q

3 steps to calculating the pH of a weak acid?

A
  1. get Ka if pKa given
  2. get [H+]
  3. get pH
42
Q

weak acid: depending on the no. of marks,

A

show the rearranging of the Ka equation

43
Q

how can u tell if an acid is weak?

A

must be given Ka or pKa

44
Q

relationship between pH and [H+]

A

as pH ⬇, [H+] ⬆

45
Q

relationship between pKa and Ka?

A

as pKa ⬇, [Ka] ⬆

same relationship as pH and [H+]

46
Q

What is a bufffer?

A
  • solution that can resist a change in pH when small amounts of acid or base are added
  • can be acidic or basic
47
Q

What do buffer contain?

A

• a weak acid and a large amount of a salt of a strong base

e.g. ethanoic acid and sodium ethanoate - G1 metals used cos soluble

48
Q

What’s the point of the salt of a strong base?

A

forms the same negative ion as the acid when the acid dissociates. creates a reserve

49
Q

What happens to a buffer when H+ are added?

A
  • equilibrium shifts to the left
  • uses up H+
  • we’d run out of the negative ion
  • but bc of the salt, there’s a reserve of negative ions and pH doesn’t change
50
Q

if OH- are added to the buffer solution?

A
  • would react w H+, reducing H+ so eq would shift to the right inc amount of H+
  • not gonna run out of acid bc it’s weak so only a small amount has ionised - lots left
  • this is why weak acid is used
  • pH doesn’t change

salt not involved here

51
Q

why can we used the moles instead of conc to calc Ka of buffer solution?

A

bc all solutions are in the same total V so conc ratio = moles ratio

52
Q

for buffers don’t ⭐

A

simplify [H+][A-} to [H+]2

53
Q

buffers work out?

A

moles of both then do Ka

54
Q

If acid added to buffer?

A

amount of acid inc and salt decreases by that amount of moles

55
Q

If you want an buffer of a specific pH the choose a an acid of a similar ?

A

pKa value

56
Q

3 factors that influence pH?

A

Ka, [acid], [base]

57
Q

What is the most important factor that determines the pH of a buffer?

A

Ka/ pKa

58
Q

what can alter the pH of a buffer solution?

A

ratio of acid to salt

59
Q

how is the pH of a buffer affected by dilution?

A

not diluted, Salt and acid are in the same container so the ratio of them doesn’t change - stays constant and so pH doesn’t change

60
Q

Weak acid weak base indicator?

A

unclear where the equivalence point is so diff to select an indicator

61
Q

indicator pHs?

A

over 7: P

under 7: methyl orange

62
Q

equivalence point?

A

[H+] = [OH-]

63
Q

What can be done from an acid base/ graph?

A

read pH and calc [H+]

64
Q

strong acid and strong base equivalence point at?

A

ph 7

65
Q

For an indicator to work in a titration?

A

the range of the indicator’s colour change must be within the range of the pH change at the equivalence point

A level Chemistry (39 decks)

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