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a-level chemistry > Topic 12 Acid-Base Equilibria > Flashcards

Topic 12 Acid-Base Equilibria Flashcards

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1
Q

Define an acid

A

a substance that can donate a proton i.e. a proton donor

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2
Q

Define an alkali

A

a substance that can accept a proton i.e. a proton acceptor

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3
Q

What is a requirement of an alkali/proton acceptor?

A

needs to have a lone pair of electrons to accept a proton and form a dative covalent bond

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4
Q

What is a conjugate acid-base pair?

A

consists of either a base and its conjugate acid or an acid and its conjugate base

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5
Q

What is a conjugate acid?

A

when a base accepts a proton

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6
Q

What is a conjugate base?

A

when an acid donates a proton

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7
Q

Define an amphoteric substance

A

a substance that can act as a base and an acid e.g. water

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8
Q

What ion causes a solution to be acidic?

A

H+ ion, hydrogen ion or H3O+ ion, oxonium ion (protons react with H2O to form it)

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9
Q

What ion causes a solution to be alkaline?

A

OH- ion, hydroxide ion

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10
Q

Write an equation for the ionisation of water

A 
2 H2O(l) ---> H3O+(aq) + -OH(aq) OR
H2O(l) ---> H+ (aq)+ -OH(aq)
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11
Q

Define a monoprotic acid

A

an acid that releases one H+ ion per molecule

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12
Q

Define a diprotic acid

A

an acid that releases two H+ ions per molecule

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13
Q

How do you work out pKa?

A

pKa= -lgKa

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14
Q

How do you work out pH using H+ ions?

A

pH= -lg[H+] or pH=lg 1/H+

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15
Q

What is Kw/ how do you work it out?

A

Kw is the ionic product of water Kw=[H+(aq)][OH-(aq)]

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16
Q

What is the equation to work out pH using acid and salt concentrations?

A

pH=pKa-lg[acid]/[salt]

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17
Q

What is a buffer solution?

A

a solution that minimises the change in pH when a small amount of either acid or base is added

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18
Q

What do acid-base reactions involve the transfer of?

A

protons

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19
Q

What are the 2 equations for the ionisation of water?

A

1) H2O(l) –> H+(aq) + OH-(aq)

2) 2H2O(l) –> H3O-(aq) + OH-

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20
Q

What is Kw value for water?

A

Kw= [H+][OH-]

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21
Q

What is the value of Kw at 298K?

A

1.00 x 10-14 (to the power of -14)

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22
Q

What physical factor effects the value of Kw and how?

A

temperature, an increased temperature moves the equilibrium to the right so Kw increases and the pH of pure water decreases

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23
Q

How do you calculate pKw?

A 
pKw= -logKw
Kw= 10-pKw (to the power of -pKw)
24
Q

If 2 solutions have a pH difference of 1, what is the difference in [H+]?

A

a factor of 10

25
Q

What is different when finding [H+] from the concentration of diprotic and triprotic acids?

A

need to multiply the concentration of the acid by the number of protons to find [H+]

26
Q

Define a strong acid

A

one which fully dissociates in water

27
Q

Define the term strong base

A

one which fully dissociates in water

28
Q

What is the difference between strong and concentrated?

A

concentrated means many mols per dm3. strong refers to the amount of dissociation

29
Q

What is a weak acid and weak base?

A

one which partially dissociates

30
Q

Give some examples of strong acids

A
  • HCL
  • H2SO4
  • H3PO4
31
Q

Give some examples of strong bases

A
  • NaOH
  • CaCO3
  • NaCO3
32
Q

Give some examples of weak acids

A
  • CH3COOH

- any organic acids

33
Q

Give some examples of weak bases

A

-NH3

34
Q

What is Ka

A

acid dissociation constant:

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

35
Q

Define equivalence point

A

the point at which the exact volume of base is added to the acid to neutralise it, or acid added to base

36
Q

What generally happens to the pH of the solution around the equivalence point?

A

there is a large and rapid change in pH, except in a weak base and weak acid titration

37
Q

What is the end point?

A

the volume of acid or alkali added when the indicator just changes colour, with the right indicator equivalence point= end point

38
Q

State the properties of a good indicator

A
  • sharp colour change- no more than a drop of acid or alkali needed to change colour
  • end point must be the same as the equivalence point
  • distinct colour change
39
Q

What indicator would be used for a strong-acid strong-base titration?

A

phenolphthalein or methyl orange- phenolphthalein is the clearest colour change

40
Q

What indicator would be used for a strong-acid and weak-base titration?

A

methyl orange

41
Q

What indicator would be used for a strong base and weak-acid titration?

A

phenolphthalein

42
Q

What indicator would be used for a weak base and weak acid titration?

A

neither would be suitable as there is no steep change

43
Q

What is the half-neutralisation point?

A

when volume= half the volume that has been added at the equivalence point

44
Q

Define a buffer solution

A

a solution that resists changes to pH when small amounts of acid or alkali are added

45
Q

What do acidic buffer solutions contain?

A
  • weak acid

- soluble salt that an acid fully dissociates

46
Q

Write an equation for an acidic buffer with added alkali

A

HA + OH- –> H2O + A-

47
Q

What do basic (base) buffer solution contain?

A
  • weak base

- soluble salt of that weak base

48
Q

How can you calculate the pH of buffer solutions?

A

use the Ka of the weak acid, sub in [A-] and [HA], calculate [H+] –> pH

49
Q

How can you calculate the new pH of a buffer solution when acid or base is added?

A

calculate the number of moles of H+ and A- and HA before acid or base is added (initial moles). use equations to work out the new moles of A- and HA, work out [H+] –> pH

50
Q

Why is there a difference in enthalpy changes of neutralisation values for strong and weak acids?

A
  • enthalpy changes of neutralisation are always exothermic, value is similar for weak and strong acids because same reaction is occurring: H+ + OH- –> H2O
  • weak acids have a less exothermic value because energy is absorbed to ionise the acid and break the bond to the hydrogen in the un-dissociated acid
51
Q

What does a lower pKa value tell you?

A

that the acid is strong

52
Q

Give the equation for the ionisation of HCl

A

HCl(aq) –> H+(aq) + Cl- (aq)

53
Q

What does a large Ka value tell you?

A

that the acid is strong (small pKa value also tells you the acid is strong)

54
Q

What is a neutral solution?

A

where the concentration of hydrogen ions and hydroxide ions are equal

55
Q

How do you determine pKw?

A

pKw= -lgKw

56
Q

What does diluting a strong acid by a factor of 10 do?

A

increase the pH by 1

57
Q

What does diluting a weak acid by a factor of 10 do?

A

increase the pH by 0.5

a-level chemistry (20 decks)

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