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Chemistry - Physical > Acids, bases, buffers & pH > Flashcards

Acids, bases, buffers & pH Flashcards

1
Q

Acid definition

A

A proton donor in aqueous solution

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2
Q

Strong acid definition

A

Fully dissociated in aqueous solution

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3
Q

Weak acid definition

A

Partially dissociates in aqueous solution

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4
Q

Weak acid dissociation equation

A

Use reversible reaction

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5
Q

Base definition

A

A proton acceptor

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6
Q

Base examples

A

Metal oxides
Metal hydroxides
Metal carbonates
Ammonia

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7
Q

Alkali definition

A

A soluble base
that dissolves in water
to produce hydroxide ions

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8
Q

Salt definition

A

Formed when hydrogen ions from an acid
are replaced by metal/ammonium ions

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9
Q

Typical acid reactions

A

Acid + base -> salt + water
Acid + carbonate -> salt + water + CO2
Acid + ammonia -> ammonium salt
Acid + metal -> salt + hydrogen

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10
Q

Hydrated definition

A

A crystalline compound
that contains water molecules

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11
Q

Anhydrous definition

A

A substance that contains
no water molecules

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12
Q

Ionic equation for acid + base

A

H+(aq) + OH-(aq) -> H2O(l)

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13
Q

Water of crystallisation definition

A

Water molecules that form an essential part of the crystalline structure

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14
Q

Bronsted-Lowry Acid

A

A proton donor

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15
Q

Bronsted-Lowry Base

A

A proton acceptor

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16
Q

Monobasic acid

A

Acid that can release 1 proton per molecule

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17
Q

Dibasic acid

A

Acid that can release 2 protons per molecule

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18
Q

Tribasic acid

A

Acid that can release 3 protons per molecule

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19
Q

Ionic equation for solutions of carbonates

A

H+(aq) + CO3-2(aq) -> CO2(g) + H2O(l)

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20
Q

Ionic equation for acid reaction with metal

A

eg. 2H+(aq) + Mg(s) -> Mg2+(aq) + H2(g)

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21
Q

Role of water in acid behaviour

A

Acid + base (water) -> Hydronium ion (H3O+)
eg. HCl + H2O -> H3O+ + Cl-

22
Q

Acid-base pair definition

A

A pair of species that transform into each other by gain or loss of a proton

23
Q

pH

A

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

24
Q

Calculating pH after dilution

A
  1. Find dilution factor
    new vol / old vol
  2. Find new [acid]
    old conc / dilution factor
  3. Find new pH
25
Q

Acid dissociation constant (Ka)

A

Ka = [H+][A-] / [HA]
Only weak acids have Ka value

26
Q

What does Ka value tell us

A

Relative strength of acid
Relative extent of dissociation
Relative position of equilibrium

27
Q

pKa calculation

A

pKa = -log(Ka)

28
Q

pKa to determine acid strength

A

As pKa value decreases
acid strength increases

29
Q

pH of weak acid approximations

A
  1. [HA] start = [HA] equilibrium
  2. [H+] = [A-] (ignores dissociation of water)
30
Q

Ionic product of water

A

Kw = [H+][OH-]

31
Q

Pure water

A

[H+] = [OH-]
NOT pH 7

32
Q

Affect of temperature change on pH of water

A

As temperature increases
pH decreases
[H+] increases
equilibrium shifts right
∴ forward reaction endothermic

33
Q

Calculating pH after a reaction

A

Need [H+] of resulting solution
1. Calculate starting moles
2. Find [H+] left
(eg [H+]start - [OH-]start = [H+] left)
3. Calculate pH

34
Q

Equivalence point on pH curve

A

The point where the volume of one solution reacts exactly with the volume of the other solution

35
Q

pH curves graph

A

y-axis = pH
x-axis = volume of base /cm3

36
Q

Drawing pH curves

A

Consider:
1. Position of vertical section
[acid] x vol(acid) = [base] x vol(base)
2. Shape
acid->base (bottom -> top) / base->acid (top -> bottom)
3. Strength of acid/base
Weak = closer to 7
Strong = further from 7
4. Equivalence point
strong->strong = EP pH7
weak->weak = EP pH7
strong->weak = EP pH<7
weak->strong = EP pH>7

37
Q

Drawing pH curve with scale

A
  1. Start/end points
  2. Vertical section
  3. Shape
  4. Equivalence point
  5. Proportions:
    slopes = 2pH units
    vertical section = minimum 3pH units
38
Q

Indicators

A

Acid -> alkaline
Phenolphthalein: colourless -> pink
Methyl orange: red -> orange -> yellow
orange = end point [HA]=[A-]

39
Q

Explaining colour changes of indicators (adding base to acid)

A

H+ + OH- = H2O
HA will dissociate
Equilibrium shifts to the right
[A-] increases

40
Q

Explaining colour changes of indicators (adding acid to base)

A

H+ reacts with A-
Equilibrium shifts to the left
[HA] increases

41
Q

How to choose indicator based on pH curve

A

pH of end point close as possible to pH of equivalence point
Colour change within pH range of vertical section

42
Q

Buffer definition

A

A mixture that
minimises pH change
on addition of
small amounts of
acid or base

43
Q

Buffers on addition of acid

A

[H+] increases
A- reacts with H+
Equilibrium shifts left
Most of added H+ is removed

44
Q

Buffers on addition of base

A

[OH-] increases
OH- + H+ -> H2O
HA dissociates
Equilibrium shifts to the right

45
Q

Determining most suitable buffer

A

Most effective when [HA] = [A-]
∴pKa = pH

46
Q

Calculating pH when mixing acid with its salt

A
  1. Find start moles of acid & salt
  2. n(salt) = n(conjugate base)
  3. Calculate [H+] with moles
    [H+] = Ka x (HA / A-)
  4. Calculate pH
47
Q

Buffers made by partial neutralisation

A

Aqueous alkali added to excess weak acid
Acid partially neutralised to form conjugate base

48
Q

Calculating pH of buffer from partial neutralisation

A
  1. Write equation
  2. Calculate moles conjugate base formed
  3. Calculate moles acid left
    (start moles - moles conjugate base)
  4. Calculate pH
49
Q

Buffers in blood

A

Blood has pH of 7.40

50
Q

Concentration ratio for buffers

A

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

Chemistry - Physical (14 decks)

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