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A-Level Chemistry > pH and Acids > Flashcards

pH and Acids Flashcards

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1
Q

What is the Bronstead-Lowry acid-base theory?

A

An acid is a proton donor

A base is a proton acceptor

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2
Q

What are conjugate acid-base pairs?

A

Once an acid has “donated” a proton it would become able to “accept” a proton back and hence act as a base

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3
Q

Give an example of an acid

A

HCl

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4
Q

Given an example of a base

A

Cl^-

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5
Q

What are the acid base pairs in:

HCl + H2O ⇌ H3O^+ + Cl^-

A

Acid 1 = HCl Base 1 = Cl^-

Acid 2 = H3O^+ Base 2 = H2O

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6
Q

What are the acid base pairs in:

CH3COOH + H2O ⇌ CH3COO^- + H3O^+

A

Acid 1 = CH3COOH Base 1 = CH3COO^-

Acid 2 = H3O^+ Base 2 = H2O

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7
Q

What do strong acids do?

A

Completely dissociate

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8
Q

How do you calculate the pH of a strong acid?

A

-log[H+]

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9
Q

Give an example of a monobasic acid

A

HCl

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10
Q

Give an example of a dibasic acid

A

H2SO4

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11
Q

Give an example of a tribasic acid

A

H3PO4

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12
Q

What do you do if you’re calculating pH when you have a di/tribasic acid

A

Multiple the [H+] by 2 or 3

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13
Q

How do you calculate [H+] for strong acids?

A

10 ^ -pH

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14
Q

How do you calculate pH changes on dilution?

A

Calculate fraction for volume
Divide conc by fraction
The calculate pH using -log[H+]

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15
Q

Calculate pH 25cm3 of 0.05 diluted with 75cm3 of water

A

75/25 = 1/3
0.05/ 3 = 0.0167
pH = -log(0.0167) = 1.78

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16
Q

How do you calculate the pH of a dilute acid?

A

[H+] old X old vol/ new vol

New vol = (original + added vol)

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17
Q

What do weak acids do?

A

Only partially dissociate

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18
Q

What is the equation for a strong acid?

A

HA —-> H+ + A-

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19
Q

What is the equation for a weak acid?

A

HA ⇌ H+ + A-

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20
Q

What is the Ka equation?

A

[HA]

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21
Q

What can the Ka equation be shortened to?

A

[HA]

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22
Q

What assumptions must be made for Ka?

A

1) [H+] = [A-]

2) [HA] start = [HA] eqm

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23
Q

What alters Ka?

A

Temperature

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24
Q

What does the larger the Ka value tell you?

A

Equilibrium lies to the right

= pH lower as more [H+]

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25
Q

Why is it easier to give Ka as its negative logarithm?

A

Because Ka very small so difficult to compare

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26
Q

How do you calculate pKa?

A

-logKa

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27
Q

How do you calculate Ka from pKa?

A

10 ^ -pKa

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28
Q

What does the higher the value of pKa tell you?

A

The weaker the acid

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29
Q

What does the lower the value of pKa tell you?

A

The stronger the acid

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30
Q

How do you calculate the pH of a weak acid?

A 
[H+] = √Ka X [HA]
pH = -log[H+]
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31
Q

What are the steps for calculating pH of a weak acid?

A

Write out Ka expression
Calculate [H+]
Calculate pH

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32
Q

What are the problems with Ka calculations approximations?

A

1) At pH values >6 water dissociation is significant
= doesn’t work for very weak acids/ dilute solutions
2) If [H+] significant there will be a difference between [HA} start and [HA] eqm
= doesn’t work for stronger acids with Ka> 10-2 / very dilute solutions

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33
Q

What does the [H+] of a weak acid depend on?

A

Value of Ka and [HA]

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34
Q

What happens if a di/tribasic weak acid dissociates twice in 2 following reactions?

A

The first reaction will always have the stronger acid (lower pKa value)

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35
Q

What does water do?

A

Ionises only slightly, acting as both an acid and base - setting up equilibrium

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36
Q

How much does water dissociate?

A

Very small amount

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37
Q

How do we know what is dissociates only a very small amount?

A

Conducts electricity

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38
Q

What is the equilibrium equation for water?

A

H2O ⇌ H+ + OH-

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39
Q

Where does equilibrium lie in the equation for water?

A

Left

40
Q

What is the constant for “ionic product of water”?

A

Kw

41
Q

What is the equation for Kw?

A

Kw = [H+] [OH-]

42
Q

What is the K equation for water?

A

K = [H+][OH-]
—————
[H2O]

43
Q

What is Kw dependent on?

A

Temperature

44
Q

How do you calculate the pH of 0.4 moldm-3 of NaOH?

Using Kw

A 
Kw = [H+] [OH-]
[H+] = 1X10-14 / 0.4 = 2.5X10-14
pH = -log[H+]
-log(2.5X10-14)
= 13.60
45
Q

Why is it useful to know the value of Kw?

A

Can rearrange the equation give [H+] or [OH-]

46
Q

What are the concentrations of H+ and OH- in a solution of pH 3.25 at 25 degrees?

A 
[H+] = 10 ^-pH = 10^ -3.25 = 5.62X10-4
[OH-] = 1X10-14 / 5.62X10-13 = 1.78X10-11
47
Q

Why is it easy to calculate pH value that are whole numbers?

A 
As [H+] and [OH-] indices add up to 14 
eg. Acid solution = 3
[H+] = 10^ -3
14-3 = 11
[OH-] = 10^ -11
48
Q

What does the value of Kw control?

A

Concentration of each ion

49
Q

What is the solution if [H+] > [OH-]?

A

Acidic

50
Q

What is the solution if [H+] < [OH-]?

A

Alkaline

51
Q

What is the solution if [H+] = [OH-]?

A

Neutral

52
Q

What is a neutral solution?

A

When there is an equal number of moles of H+ and OH- ions

NOT when it has a pH of 7

53
Q

What can K w also tell us?

A

The pH of pure water

54
Q

What assumption can we make when H2O splits into equal concentrations of OH- and H+?

A 
Kw = [H+] [OH-] = 1X10-14 
[H+]^2 = 1X10-14 - This is squared as [H+]=[OH-]
[H+] = √1X10-14 
pH = -log(1X10-7)
= 7
55
Q

What happens to the value of Kw when temp is increased?

A

Kw increases

56
Q

What does the pOH scale measure?

A

OH- conc

57
Q

What is the value for the ionic product of water at 25 degrees?

A

1X10^-14 mol^2dm^-6

58
Q

What are buffer solutions?

A

Systems that minimise pH changes on the addition of small amounts of an acid or base

59
Q

When is the buffer most effective?

A

When [HA]=[A-]

60
Q

When is the buffer no longer effective?

A

When one of the components are completely used up

61
Q

Why shouldn’t a buffer be considered a constant?

A

As its pH will vary slightly when reacting with any added species

62
Q

What happens to a buffer if there is an addition of an acid?

A

[H+] increases
Conjugate base (A-) reacts
Equilibrium shifts left
[H+] removed

63
Q

What happens to a buffer if there is an addition of an alkali?

A

[OH-] increases
H+ reacts
Equilibrium shifts right
HA dissociates restoring [H+]

64
Q

What pH must blood maintain?

A

7.4 +/-0.05

65
Q

What is the most important buffer in the body (blood)?

A

Carbonic acid - hydrogencarbonate

66
Q

What happens to a person if their blood becomes too acidic?

A

Fatigue
Shortness of breath
Shock/death

67
Q

What happens to a person if their blood becomes too alkaline?

A

Spasms
Light-headed
Nausea

68
Q

What is called when the blood becomes too acidic?

A

Acidosis

69
Q

What is called when the blood becomes too alkaline?

A

Alkalosis

70
Q

What happens when the body produces more acidic products?

A

[H+] increases
Conjugate base (HCO3-) will react with H+
Equilibrium shifts left to remove H+ from blood

71
Q

What happens if acid levels continue to rise in the body?

A

pH would fall leading to acidosis
Symptoms would show
As [HCO3-] could be used up
So buffer system will not function

72
Q

What will the body do to prevent the build up of CO2?

A

Body convert H2CO3 to CO2 to be expelled from lungs

73
Q

What are the two ways to form a buffer?

A

Adding solutions of weak acid + its conjugate base

Partial neutralisation of weak acid

74
Q

How can adding solution of weak acid + its conjugate form a buffer?

A

Dissociation of weak acid = very low = assume conc of acid remains same
As salts are ionic = completely dissociate in solution to provide conc for conjugate base

75
Q

How can partial neutralisation of a weak acid form a buffer?

A

Adding an alkali to an excess of weak acid

= neutralise some of acid to form conjugate base + leave some acid over

76
Q

What the equation you would use for buffers?

A

[H+] = [HA]
Ka ————
[A-]

77
Q

How do you calculate the pH of a buffer?

A

Write the Ka expression
Rearrange expression for [H+]
Convert [H+] to pH

78
Q

What must you do if you need to calculate the pH of a buffer but are not given the concs?

A

Use conc = moles X1000 / vol
Use ice table to calculate final conc
Place in [H+] = Ka[HA]/[A-]
Calculate pH = -log

79
Q

What does the shape of titration curve depend on?

A

Substance being titrated = base or acid

Type of acid or base = strong or weak

80
Q

Why is the equivalence point of a strong acid-strong base titration pH 7?

A

H+ completely neutralised by OH- so only water + salt present = pH 7

81
Q

Why is the equivalence point of a strong acid-weak base titration pH <7?

A

Weak base (NH3)will have strong conjugate base (NH4+), which will react with water to produce H3O+

82
Q

Why is the equivalence point of a weak acid-strong base titration pH >7?

A

Weak acid, Ha, will have a strong conjugate base, A-, which reacts with water to produce OH-

83
Q

What is an acid-base indicator?

A

Weak acids that have different coloured conjugate bases

84
Q

What happens in acidic conditions to the indicator?

A

Indicator equilibrium shifted towards right

85
Q

What happens in basic conditions to the indicator?

A

Equilibrium shifts towards conjugate base

86
Q

When is the endpoint reached?

A

When equal HIn and In- are present

87
Q

What should happen to the colour when the end point is reached?

A

Colour lie between 2 extremes

88
Q

Why will each indicator have a different pH value for the end point?

A

Because they each have different Ka values

89
Q

Which indicator can be used for a strong acid-strong base titration?

A

Methyl orange/ phenolthanein

90
Q

Which indicator can be used for a strong acid-weak base titration?

A

Methyl orange

91
Q

Which indicator can be used for a weak acid-strong base titration?

A

Phenolthanein

92
Q

What would be used in a strong acid-base titration?

A

pH meter to monitor the change in pH as an acid is added to a base

93
Q

Describe the method for the titration of strong acid- base

A

Pipette 10cm3 of 0.5M NaOH into conical flask
Place pH probe into flask + record pH
Fill burette with 0.5M HCl
Add 1cm3 of acid to flask, then swirl + record pH
Repeat step 4 until pH changes rapidly
Now add acid dropwise till pH changes less rapidly
Continue adding 1cm3 at a time until acid in excess
Plot data on graph of acid vol to pH, including line of best fit

94
Q

LEARN GRAPHS FOR TITRATIONS

A

DO IT

95
Q

Describe the graph of a strong acid-base

A

Excess of pH = pH decreases as acid added
Vertical section = acid-base conc similar, pH alters rapidly
Excess of acid = pH decreases slightly as acid added

96
Q

Where is the equivalence point on the titration graphs?

A

Halfway up the vertical section of the graph

A-Level Chemistry (26 decks)

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