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Chemistry OCR AS Level > PH, Acid, Base and Buffer > Flashcards

PH, Acid, Base and Buffer Flashcards

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1
Q

What is a Bronsted-Lowry Acid?

A

A Bronsted-Lowry Acid is any substance that can donate a proton.

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2
Q

What is a Bronsted-Lowry Base?

A

A Bronsted-Lowry Base is any substance that can accept a proton.

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3
Q

Why is HCl and H₂SO₄ a Bronsted Lowry acids?

A

This is because when dissolved in water, they dissociate to donate a H+ ion/proton.

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4
Q

Why is NH₃ a Bronsted Lowry base?

A

This is because when dissolved in water, they accept a H+ ion/proton.

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5
Q

Write out the dissociation equations for the:

  • dissociation of HCl in water
  • dissociation(s) of H₂SO₄ in water
  • reaction of NH₃ with water
A

HCl (aq) —> H+(aq) + Cl-(aq)

H₂SO₄ (aq) —> H+(aq) + HSO₄- (aq)
HSO₄- (aq) —> H+(aq) + SO₄²⁻ (aq)

NH₃(aq) + H+(aq) —> NH₄+(aq)

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6
Q

Write out the equations for the:

  • dissociation of HCl in water
  • dissociation(s) of H₂SO₄ in water
  • reaction of NH₃ with water
A

HCl (aq) —> H+(aq) + Cl-(aq)

H₂SO₄ (aq) —> H+(aq) + HSO₄- (aq)
HSO₄- (aq) —> H+(aq) + SO₄²⁻ (aq)

NH₃(aq) + H+(aq) —> NH₄+(aq)

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7
Q

Acids can be ________, __________ or _________ depending on the number of protons that each acid molecule can release.

A

Monobasic
Dibasic
Tribasic

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8
Q

What is a monobasic acid? Give an example and write out the equation.

A

A monobasic acid is acid molecule that can only release one proton/H+ ion. An example of this would be HCl:
HCl (aq) —> H+(aq) + Cl-(aq)

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9
Q

What is a dibasic acid? Give an example and write out the equation.

A

A dibasic acid is acid molecule that can only release two protons/H+ ions. An example of this would be H₂SO₄:
H₂SO₄ (aq) —> H+(aq) + HSO₄- (aq)
HSO₄- (aq) —> H+(aq) + SO₄²⁻ (aq)

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10
Q

What is a tribasic acid? Give an example and write out an equation.

A

A tribasic acid is an acid molecule that can only release three protons/H+ ions. An example of this would be H₃PO₄:
H₃PO₄ (aq) —> H+(aq) + H₂PO₄- (aq)
H₂PO₄- (aq) —> H+(aq) + HPO₄²⁻ (aq)
HPO₄²⁻ (aq) —> H+(aq) + PO4³⁻ (aq)

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11
Q

What is a conjugate acid-base pair?

A

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

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12
Q

Below is the acid-base equilibria involving two acid base pairs. This equilbrium shows the dissociation of nitrous acid, HNO₂, in water:

HNO₂(aq) + H₂O(aq) ⇌ H₃O+(aq) + NO₂-(aq)

Determine this acid base conjugate pairs.

A

1) HNO₂ is acid 1 as it releases a proton to form its conjugate base NO₂- (which is base 1 as it would accept a proton to reform HNO₂)
2) H₂O is base 2 as it accepts a proton to form its conjugate acid H₃O+ (which is acid 2 as it would release a proton to reform H₂O).

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13
Q

Aqueous acids take part in reactions with acids carbonates, oxides, hydroxides, bases and alkalis called? What is always produced?

A

1) Neutralisation

2) Salt and water

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14
Q

When acid dissociates in water, what does H+ represent?

A

The hydronium ion, H₃O+.

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15
Q

Write the ionic equation for the following reaction:

2HCl(aq) + Na₂CO₃(aq) —> 2NaCl(aq) + CO₂(g) + H₂O(l)

A

2H+(aq) + CO₃²⁻ (aq) —> CO₂(g) + H₂O(l)

You should be able to write ionic equations for reactions between acids and other bases and alkalis. You also need the ionic equations when metals react with acids.

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16
Q

What kind of reactions are neutralisation reactions not?

A

Neutralisation reactions are not redox reactions.

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17
Q

Give an example of a acid reaction that is redox? What does this reaction always produce?

A

acid + metal —> salt + hydrogen

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18
Q

What is the general equation for the dissociation of an acid?

A

HA(aq) —> H+(aq) + A-(aq)

This is the same as HA(aq) + H₂O(l) –> A-(aq) + H₃O+(aq)

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19
Q

What is the strength of an acid?

A

The strength of an acid is the extent of dissociation into H+ and A- ions.`

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20
Q

What is the concentration of an acid?

A

The concentration of an acid is how many moles of the acid are present in a given volume.

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21
Q

What is a strong acid?

A

An acid that fully dissociates in water/aqueous solution.

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22
Q

State the strong acids that we need to know.

A 
HCl (Hydrochloric acid)
H₂SO₄ (Sulfuric acid)
HNO₃ (Nitric acid)
HBr (Hyrobromic acid)
HI (Hydroiodic acid)
HClO₄ (Chloric (VIII) acid)
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23
Q

What is a weak acid?

A

A weak acid only partially dissociates in aqueous solutions.

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24
Q

What is the general equation for weak acids?

A

HA(aq) ⇌ H+(aq) + A-(aq)

  • The equilbrium position lies well over to the left.
  • There are only small concentration of H+(aq) and A-(aq) compared with the concentration of HA(aq)
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25
Q

What is the acid dissociation constant, Ka?

A

The actual extent of acid dissociation.

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26
Q

What is the equation to calculate acid constant, Ka?

A

Ka = [H+(aq)] [A-(aq)]/[HA(aq)]

This is not used for calculating the pH of a strong acid, usually a weak acid or buffer (but for a weak acid this equation can be edited).

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27
Q

What is the units for Ka?

A

moldm⁻³

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28
Q

What does a large value of Ka indicate?

A

A large value of Ka indicates a large extent of dissociation - the acid is strong.

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29
Q

What does a small value of Ka indicate?

A

A small value of Ka indicates a small extent of dissociation - the acid is weak.

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30
Q

The range of values for Ka is vast so pKa values are used as they are more manageable. What two equations link pKa and Ka?

A 
pKa = -log(Ka)
10⁻ᵖᴷᵃ = Ka
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31
Q

What does a low value of pKa tell us about the strength of an acid?

A

Low value of pKa means high value of Ka, and thus stronger acid.
High value of pKa means low value of Ka and thus weaker acid.

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32
Q

Equations that allow you to convert between pH and H+ concentration?

A 
pH = -log[H+(aq)]
10⁻ᵖᴴ = [H+(aq)]
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33
Q

What decimal places should you always give PH to?

A

2 decimal places

34
Q

A low pH value means?

A

Large concentration of H+ ions

35
Q

A high pH value means?

A

Small concentration of H+ ions

36
Q

A pH change of one changes [H+] by

A

10 times (increasing by one unit of pH, causes a decrease in concentration by 10 times).

37
Q

How do you work out the pH of a strong acid? (monobasic, dibasic and tribasic)

A

Strong acids fully dissociate in water. Therefore for a monobasic acid, [H+(aq)] = [HA(aq)].
Once [H+(aq)] calculated, use equation pH = -log[H+(aq)]

For a dibasic acid: 2[H+(aq)] = [HA]
For a tribasic acid: 3[H+(aq)] = [HA]

38
Q

How to calculate the PH of a weak acid?

A

Use the equation: Ka = [H+(aq)]²/[HA]

Rearrange to get [H+]

Use equation pH = -log[H+(aq)]

39
Q

When using the equation to calculate the concentration of a weak acid, what must we assume?

A

When HA molecules dissociate, [H+(aq)] = [A-(aq)]. Therefore the ‘[H+(aq)] [A-(aq)]’ in the acid dissociation constant becomes [H+(aq)]². This is assuming that any water molecules present will have dissociated to such a negligible amount that it will not affect the concentration of H+ ions.
Furthermore, as HA dissociates, [HA(aq)] decreases, however we assume that the concentration of [HA] (even at equilibrium) is the same as the initial concentration of the acid.

40
Q

What does water exist as normally?

A

Water exists in equilibrium:
2H₂O(l) –> H₃O+((aq) + OH-(aq)

This can be simplified to :
H₂O(l) ⇌ H+(aq) + OH-(aq)

The equilibrium lies well to the left.

41
Q

When can water act as an acid? Equation

A

H₂O(l) –> H+(aq) + OH-(aq)

42
Q

When can water act as an base? Equation?

A

H₂O(l) + H+(aq) –> H₃O+(aq)

43
Q

What is Kw?

A

Kw is the ionic product of water.

As water exists as an equilibrium, we can write an expression for Kc for this equilbrium:

Kc = [H+(aq)][OH-(aq)]/[H₂O(l)]

This can be rearranged to
Kc x [H₂O(l)] = [H+(aq)][OH-(aq)]

Kc x [H₂O(l)] is equal to Kw, the ionic prouct of water, therefore:
Kw = [H+(aq)][OH-(aq)]

44
Q

What is the units for Kw?

A

mol²dm⁻⁶

45
Q

What is the value for Kw?

A

Kw = 1x10⁻¹⁴

This is because at 25ᵒC, pH of Water is 7, and therefore [H+(aq)] = 1x10⁻⁷. As pure water dissociates, [H+(aq)] = [OH-(aq)], therefore, [OH-(aq)] is 1x10⁻⁷ also.

For [H+(aq)] = [OH-(aq)], water must be pure and neutral.

46
Q

When is the only time that the value for Kw changes?

A

Kw changes only if temperature changes.

At 25ᵒC, Kw is always is always 1x10⁻¹⁴ for water and all other neutral solutions that contains H+ and OH- ions.

47
Q

In aqueous and neutral solutions….

In acidic solutions…

A

[H+] = [OH-]
[H+] > [OH-]
[H+] < [OH-]

48
Q

How to calculate PH for strong bases?

A

Strong bases fully dissociate in water. Therefore for a monobasic base, [OH-(aq)] = [HA(aq)].
Once [OH-(aq)] calculated, use equation: 1x10⁻¹⁴(Kw)/ [OH-(aq)] = [H+(aq)]
Once [H+(aq] determined use the equation: -log[H+(aq)] to calcualte PH.

For a dibasic base: 2[OH-(aq)] = [HA]
For a tribasic base: 3[OH-(aq)] = [HA]

49
Q

What is a buffer solution?

A

A buffer solution is a mixture that minimises pH changes on addition of small amounts of acid or base.

Minimises means that its PH may alter only slightly.

50
Q

What does a buffer solution consist of?

A
  • A weak acid (HA)

- Its conjugate base (A-)

51
Q

How do we ensure that the solution contains large concentrations of the conjugate base?

A

We do this by adding the salt of the weak acid to the mixture. The salt will completely dissociate, as it is ionic, to provide a large concentration of the conjugate base. E.g. adding sodium ethanoate to a mixture of ethanoic acid.

52
Q

How do we ensure that the solution contains large concentrations of the weak acid.

A

When the weak acid is in aqueous solution, it only partially dissociates, so the position of equilibrium lies to the far left. This means there is already a large concentration of the weak acid present.

53
Q

What other methods of producing a buffer solution are there?

A

A buffer solution can be made from the reaction between excess weak acid and a strong alkali. The neutralisation between the weak acid and the strong base produces the salt of the weak acid ‘in situ’ and, as the weak acid was in excess, there will still be some weak acid in the mixture. The resultant mixture contains both the salt of the weak acid and the weak acid itself so can therefore act as a buffer solution.

54
Q

How does a buffer solution minimise pH changes.

A

In a buffer solution, the equilbrium below is setup, where HA = weak acid, A- = its conjugate base:

HA(aq) ⇌ H+(aq) + A-(aq)

The weak acid, HA, removes added alkali
The conjugate base, A-, removes added acid.

55
Q

When acid is added to a buffer solution, what happens?

A
  • [H+(aq)] increases
  • Backward reaction favoured + equilbrium shifts to the left.
  • In doing so, the conjugate base, A-, reacts with the added H+ ions to form more of the undissociated weak acid, removing the H+ ions and minimising pH change.
56
Q

When alkali is added to buffer solution what happens?

A
  • [OH-(aq)] increases
  • H+ ions in the buffer solution react with the OH- ions to form water.
  • Forward reaction favoured + equilbrium shifts to the right.
  • In doing so, more of the weak acid, HA, dissociates to restore most of the H+ ions that had reacted and thus minimised pH change.
57
Q

What does the pH of the buffer solution depend on?

A
  • The acid dissociation constant, Ka of the buffer system
  • The concentration ratio of the weak acid and its conjugate base

Is the pH of a buffer solution not determined by the concentration of H+ ions present?

58
Q

How to calculate the PH of a buffer solution.

A

We know that a buffer solution consisting f a weak acid, an its conjugate base, A-: (buffers are basically weak acids with a lot more A-)

Ka = [H+(aq)][A-(aq)]/[HA(aq)]

Therefore: [H+(aq)] = Ka x [HA(aq)]/[A-(aq)]

Concentration of HA and Ka will probably be given so just put that in, concentration of A- will be equal to the concentration of the salt of the weak acid, as it fully dissociates, so [CH₃COONa] = [CH₃COO-]. You can put that in all to work out [H+].

Next use -log[H+] to calcualte the pH of the buffer.

59
Q

What pH does healthy human blood need to have?

A

A pH between 7.35 - 7.45

Anything lower than .2 or higher than .6 = dead

60
Q

What happens if pH fall below 7.35?

A

A condition called acidosis occurs.

61
Q

What happens if pH increases above 7.45?

A

A condition called alkalosis occurs.

62
Q

The pH in the blood is controlled by a mixture of buffers in the blood plasma, which is the most important buffer system?

A

The carbonic acid and hydrogencarbonate ion buffer system.

The carbonic acid acts as the weak, HA.
The hydrogencarbonate ions acts as the conjugate base, A-.

H₂CO₃(aq) ⇌ H+(aq) + HCO₃-(aq)

63
Q

Most materials released into the blood is acidic and these are effectively removed by the hydrogencarbonate ions in the blood. How is carbonic acid removed from the the blood when levels of it gets too high?

A

Carbonic acid is converted to aqueous carbon dioxide through the action of an enzyme. In the lungs the dissolved carbon dioxide is converted into carbon dioxide gas, which is then exhaled.

64
Q

How can the amount of CO2 in teh blood be controlled?

A

Control rate of breathing. Heavy breathing removes more CO2. Breathing more slowly removes less CO2.

65
Q

What is the equivalence point?

A

This is the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution.

In a titration the equivalence point is what you are determining, this will give you the volume of the solution of unknown concentration needed to completely neutralise the other solution.

66
Q

What is the end point?

A

This is the point in a titration at which there are equal concentrations of the weak acid and conjugate base form of the indicator. The colour at the end point is midway between the colours of the acid and conjugate base form.

67
Q

How can you measure the PH of the of the reaction mixture (in the conical flask) as the solution from the burette is added over time?

A

PH meters or data loggers can be used.

68
Q

If PH is measured over time, you can plot a graph known as?

A

An acid-base PH titration curve.

69
Q

What type of solution is an acid-base indicator (changes colour in acid or base)?

A

A weak acid.

HIN ⇌ H+ + In-

70
Q

Why can an acid-base indicator change colour?

Give an example.

A

An indicator has one colour in its acid form (HIn) and a different colour in its conjugate base form (In-).

For example, for methyl orange:
HIN ⇌ H+ + In-

HIN would be red and In- would be yellow.

71
Q

Give an expression to shown when the indicator below is at the end point?
HIN ⇌ H+ + In-

A

[HIn] = [In-]

72
Q

For methyl orange, what colour would the end point be?

A

Methyl orange would be orange at the end point ([HIn] = [In-])

73
Q

Most indicators change PH over a range of two PH units. The end point is usually at the _______ of this range.

A

middle

74
Q

How is an indicator chosen for a titration?

A

The indicator chosen is as close as possible to the PH value of the titration’s equivalence point.
Any indicator that changes colour within the PH range of the vertical section of the graph will be suitable. This indicator’s end point will essentially occur at the same time as the equivalence point of the titration as the steep change in PH occurs over one drop of the solution in the burette.

75
Q

What is the general shape of an acid-base PH titration graph if we started with an acid?

A

Sigmodial shape - slight increase in pH followed by a sharp increase in pH (vertical section), followed by another slight increase in pH.

76
Q

What does the vertical section of an acid-base titration curve indicate if we started with an acid?

A
  • This is the point at which the acid is no longer in excess and any base added after this point. has a large impact on the pH.
  • At the equivalence point (centre of the vertical section), is when the volume of base that has left the burette will exactly react with the volume of acid in the conical flask.
  • The steep change usually occurs with just a single drop of the base.
77
Q

Draw the titration curve for a strong acid-strong base, if we started with the strong acid.

A

pg46

  • Vertical section: pH3 - pH11 (change in PH)
  • Equivalence point: pH7
  • Suitable indicators: methyl orange + phenylphthalein (their end points lies within vertical section)
78
Q

Draw the titration curve for a strong acid-weak base, if we started with the strong acid.

A

pg47

  • Vertical section: pH3 - pH7.5
  • Equivalence point: pH value below 7
  • Suitable indicators: Methyl orange (end point lies within vertical section)
79
Q

Draw the titration curve for a weak acid-strong base, if we started with the weak acid.

A

pg47

  • Vertical section: pH6.5 - pH11
  • Equivalence point: pH value above 7
  • Suitable indicators: Phenylpthalein(end point lies within vertical section)
80
Q

Draw the titration curve for a weak acid-weak base, if we started with the weak acid.

A

pg47

  • No real vertical section
  • No real eqivaluence point - is this true?
  • No suitable indicator as it would change colour gradually over a few cm3 of bases added.

Chemistry OCR AS Level (9 decks)

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