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a-level chemistry oxford aqa > 3.1.10 Acids and Bases > Flashcards

3.1.10 Acids and Bases Flashcards

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1
Q

Define Brønsted–Lowry Acids

A

Proton donors

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2
Q

When Brønsted–Lowry acids are mixed with water, they ______ ____

A

Release H+

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3
Q

State the equation for when Brønsted–Lowry acids are mixed with water

A
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4
Q

Define Brønsted–Lowry Bases

A

Proton acceptors

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5
Q

When Brønsted–Lowry bases are mixed with water, they grab ___ from ____

A

grab H+ from H2O

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6
Q

State the equation for when Brønsted–Lowry bases are mixed with water

A
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7
Q

Define strong acids

A
  • Dissociate (or ionise) completely in water
  • Nearly all H+ ions released
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8
Q

Where does the equilibrium of strong acids and bases reacting with water lie?

A

To the right

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9
Q

Define Weak Acids

A
  • Dissociate only very slightly in water
  • Small no. of H+ ions formed
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10
Q

Where does the equilibrium of weak acids and bases reacting with water lie?

A

To the left

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11
Q

Acids only get rid of their protons if there’s a ____ to ____ them

A

Acids only get rid of their protons if there’s a base to accept them

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12
Q

Write an equation for when HA (acid) reacts with B (base)

A
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13
Q

What happens to the position of equilibrium if you add more HA or B?

A

Shifts to right

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14
Q

What happens to the position of equilibrium if you add more BH+ or A-?

A

Shifts to left

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15
Q

When acid is added to water, water acts as ____ and _____ the ____

A

When acid is added to water, water acts as base and accepts the proton

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16
Q

Equilibrium’s far to the ___ for weak acids

A

left

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17
Q

Equilibrium’s far to the ___ for strong acids

A

right

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18
Q

What does water dissociate into?

A

Dissociates into hydroxonium ions and hydroxide ions

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19
Q

Which side does the equilibrium lie on?

A

To the left

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20
Q

State the ionic product of water (Kw) & the units

A
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21
Q

Describe how Kw is derived from the equilibrium constant

A
  • Can work out normal equilibrium constant from equation: H2O ⇌ H+ + OH-
    • ​Kc = [H+] [OH-] / [H2O]
  • So much more water compared to H+ & OH- that water is considered to have constant value
  • ∴ if you multiply expression fo Kc (which is a constant) by [H₂O] (another constant), you get a constant
  • New constant = Kw
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22
Q

State what Kw is at 298K

A

1.00 x 10-14 mol² dm-6

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23
Q

State what the Kw expression is at pure water

A

Kw = [H+]2

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24
Q

Explain why Kw = [H+]2 in pure water

A

In pure water, there’s always one H+ ion for each OH- ion

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25
Q

What is the pH scale?

A

Measure of hydrogen ion concentration

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26
Q

The smaller the pH, the greater…

A

conc. of H+ ions

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27
Q

State the equation you can use to work out pH

(used for strong acids directly)

A
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28
Q

State the equation for calculating hydrogen ion concentration

A
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29
Q

What is meant by strong monoprotic acids?

e.g. HCL and HNO3 (nitric acid)

A
  • 1 mole of acid produces 1 mole of hydrogen ions when it dissociates
  • [H+] = [Acid]
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30
Q

What is meant by strong diprotic acids?

e.g. sulfuric acid

A
  • Produces 2 mole of H+ for 1 mole of acid when it dissociates
  • [H+] = 2[Acid]
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31
Q

What should you use to calculate pH of a strong base?

A
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32
Q
A
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33
Q

Why do you have to use Ka (acid dissociation constant) to work out the [H+] for weak acids?

A
  • Weak acids (e.g. any that end in ‘oic’) dissociate only slightly in aq solution
  • ∴ [H+] isn’t equal to acid concentration
  • ∴ have to use equilibrium constant Ka
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34
Q

Why can you assume?

A

As only a tiny amount of HA dissociates

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35
Q

State the expression for Ka

(used when calculating pH for weak acids)

A
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36
Q

State Ka expression you use for weak acids in aq solution with nothing else added. Include the units.

A
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37
Q
A
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38
Q

State the equation for calculating pKa

A

pKa = -logKa

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39
Q

State the equation for calculating Ka

A

Ka = 10-pKa

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40
Q
A
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41
Q

When a ___ acids reacts with a ___ base, for every mole of OH- added, ___ mole of HA is used and ___ mole of A- is formed

A

When a weak acids reacts with a strong base, for every mole of OH- added, 1 mole of HA is used and 1 mole of A- is formed

42
Q

6 moles of HA with 1.3 moles of Ba(OH)2. State the moles before and after the reaction.

A
43
Q

Weak Acid + Strong Base

Calculate the pH of the solution formed when 30 cm3 of 0.200 mol dm-3 ethanoic acid (pKa = 4.76) is added to 100 cm3 of 0.100 mol dm-3 NaOH.

A
44
Q

Weak Acid + Strong Base

Calculate the pH of the solution formed when 50 cm3 of 0.500 mol dm-3 ethanoic acid (pKa = 4.76) is added to 75 cm3 of 0.200 mol dm-3 NaOH

A
45
Q

When ___ of the HA molecules have reacted with OH-, ____ = _____ ∴ ___ = _____ or ____ = ____

A

When half of the HA molecules have reacted with OH-, [HA] = [A-] ∴ Ka = [H+] or pKa = pH

46
Q

Calculate the pH of the solution formed when 100 cm3 of 0.2 mol dm-3 ethanoic acid (pKa = 4.76) is added to 40 cm3 of 0.250 mol dm-3 KOH.

A
47
Q

Draw the pH curve for a strong acid/strong base. Include where the graph starts and where it levels off.

A
48
Q

Draw the pH curve for a strong acid/weak base. Include where the graph starts and where it levels off.

A
49
Q

Draw the pH curve for a weak acid/strong base. Include where the graph starts and where it levels off.

A
50
Q

Draw the pH curve for a weak acid/weak base. Include where the graph starts and where it levels off.

A
51
Q

What is the name given to the section that is vertical on the pH curve?

A

Equivalence point or end point

52
Q

At equivalence point or end point, a tiny amount of base/acid causes a …

A

sudden, big change in pH

53
Q

What happens to the pH curves when you titrate a base with an acid instead?

A

Shapes of the curves stay the same but they flip over

54
Q

How do you deicide which indicator to use?

A

Need to pick one that changes colour over a narrow pH range that lies entirely on the vertical part of the pH curve

55
Q

Name 2 indicators

A
  • Methyl orange
  • Phenolphthalein
56
Q

State the colour of methyl orange at low pH (in acid)

A

Red

57
Q

State the colour of methyl orange at high pH (in base)

A

Yellow

58
Q

State the approx. pH of colour change for methyl orange

A

3.1-4.4

59
Q

State the colour of phenolphthalein at low pH (in acid)

A

Colourless

60
Q

State the colour of phenolphthalein at high pH (in base)

A

Pink

61
Q

State the approx. pH of colour change for phenolphthalein

A

8.3-10

62
Q

State an indicator you can use for a strong acid/strong base titration

A

Methyl orange or phenolphthalein

(Rapid pH change over range for both indicators)

63
Q

State an indicator you can use for a strong acid/weak base titration

A

Methyl orange

64
Q

State an indicator you can use for a weak acid/strong base titration

A

Phenolphthalein

65
Q

State an indicator you can use for a weak acid/weak base titration

A

Can’t use indicator

66
Q

Why can’t you use a indicator for weak acid/weak base titrations?

A

Don’t get sharp change in weak acid/weak base titration

67
Q

What should you use instead of an indicator for weak acid/weak base titrations?

A

pH meter

68
Q

Name 3 things you can do to make your titration results as accurate as possible

A
  1. Measure neutralisation volume as precisely as possible
    • (i.e. to nearest 0.05 cm3)
  2. Repeat titration at least 3 times and take mean titre value
    • Makes result reliable
  3. Don’t use anomalous results
    • Results should be within 0.1 cm3 of each other
69
Q

If you use a pH meter, describe how you can work out how much acid or base is needed for neutralisation.

A
  1. Draw pH curve
  2. Find equivalence point (mid-point of the line of rapid pH change) and draw a vertical line downwards until it meets the x-axis
  3. Value at x-axis = volume of acid or base needed
70
Q
A
71
Q

When a diprotic acid reacts with a base, the reaction occurs in __ ____

A

2 stages

72
Q

Why is it that when you react a diprotic acid with a base, the reaction occurs in 2 stages?

A

∵ 2 protons are removed from acid separately

73
Q

Diprotic acid (e.g. ethanedioic acid) + strong base results in a pH curve with __ equivalence points

A

2

74
Q
A
75
Q

Write the equation for when hydrogen ions react with hydroxide ions

A

H+ + OH- → H2O

76
Q

Write the equation for when hydrogen ions react with carbonate ions

A

2 H+ + CO32- → H2O + CO2

77
Q

Write the equation for when hydrogen ions react with bicarbonate ion

A

H+ + HCO3- → H2O + CO2

78
Q

Write the equation for when hydrogen ions react with ammonia

A

H+ + NH3 → NH4+

79
Q

What is a buffer?

A

Solution that resists changes in pH when small amounts of acid or base are added, or when it’s diluted

80
Q

How are acidic buffers made?

A

Made by mixing weak acid with one of its salts

e.g. ethanoic acid and sodium ethanoate

81
Q

Explain what happens when add a small amount of acid to this acidic buffer solution

A
  • H+ conc. ↑
  • Most of extra H+ ions combine with CH3COO- ions to form CH3COOH
  • Shifts equilibrium to left = reducing H+ conc. close to its original value
  • ∴ pH doesn’t change
82
Q

Explain what happens when add a small amount of base to this acidic buffer solution

A
  • OH- conc. ↑
  • Most of extra OH- ions react with H+ ions to form water = removing H+ ions from solution
  • Causes more CH3COOH to dissociate to form H+ ions = shifting equilibrium to the right
  • H+ conc. increases until it’s close to original value ∴ pH doesn’t change
83
Q

How are basic buffers made?

A

Made by mixing weak base with one of its salt

e.g. solution of ammonia and ammonium chloride acts as a basic buffer

84
Q

In a solution of ammonia and ammonium chloride acts a ____ ___, the salt …

A

basic buffer, the salt fully dissociates in solution

85
Q

In a solution of ammonia and ammonium chloride, some of NH3 will…

A

react with water molecules

86
Q

Explain what happens when you add a small amount of acid to this basic buffer solution

A
  • H+ conc. ↑
  • Some of H+ ions react with OH- ions to make H2O
  • ∴ equilibrium position moves to right to replace OH- ions that have been used up
  • Some of H+ ions react with NH3 = NH4+
  • These reactions remove most of extra H+ ions ∴ pH won’t change much
87
Q

Explain what happens when you add a small amount of base to this basic buffer solution

A
  • OH- conc. ↑
  • Most of extra OH- ions will react with NH4+ ions to form NH3 and H2O
  • Equilibrium will shift to left, removing OH- ions from solution
  • Stops pH from changing much
88
Q

Making a buffer by adding a salt solution

Calculate the pH of a buffer made from 45cm3 of 0.1 mol dm-3 ethanoic acid and 50cm3 of 0.15 mol dm-3 sodium ethanoate (Ka = 1.7 x 10-5)

A
89
Q

Making buffer by mixing weak acids and strong bases

55cm3 of 0.5 mol dm-3 CH3CO2H is reacted with 25cm3 of 0.35 mol dm-3 NaOH. Calculate the pH of the resulting buffer solution. Ka = 1.7 x 10-5 mol dm-3

CH3CO2H + NaOH → CH3CO2Na + H2O

A
90
Q

Calculating change in pH of buffer on addition of acid/alkali

2cm3 of 0.10 mol dm-3 NaOH is added to 100cm3 of a buffer solution containing 0.15 mol dm-3 ethanoic acid and 0.10 mol dm-3 sodium ethanoate (Ka ethanoic acid = 1.74 x 10-5 mol dm-3). Calculate the change in pH of the buffer solution.

A
91
Q

Draw pH curve when…

Flask: 25 cm3 0.10 mol dm-3 HNO3

Burette: 50 cm3 0.20 mol dm-3 NaOH

A
92
Q

Dilution of a Strong Acid

Calculate the pH of the solution formed when 250 cm3 of 0.300 mol dm-3 H2SO4 is made up to 1000 cm3 solution with water

A
  • [H+] in original H2SO4 solution
    • 2 x 0.300 = 0.600
  • [H+] in diluted solution
    • 0.600 x 250/1000 = 0.150
  • pH = -log(0.150)

= 0.82

93
Q

Reaction between strong acid & strong base

Calculate the pH of the solution formed when 50 cm3 of 0.100 mol dm-3 H2SO4 is added to 25 cm3 of 0.150 mol dm-3 NaOH

A
94
Q

Reaction between strong acid & strong base

Calculate the pH of the solution formed when 25 cm3 of 0.250 mol dm-3 H2SO4 is added to 100 cm3 of 0.2 mol dm-3 NaOH

A
95
Q

5 cm3 of 0.10 mol dm-3 hydrochloric acid is added to 1 dm3 of a buffer solution containing 2.35 x 10-2 mol of methanoic acid and 1.84x10-2 mol of sodium methanoate (Ka methanoic acid = 1.78 x 10-4 mol dm-3). Calculate the pH of the buffer solution after this addition. (4)

A
  • Mol H+ added: 5 x 10-3 x 0.1 = 5 × 10–4
  • Mol HCOOH: 2.35 x 10-2 + 5 x 10-4 = 2.40 × 10–2
  • Mol HCOO–: 1.84 x 10-2 – 5 x 10-4 = 1.79 × 10–2
  • [H+] = Ka × [HA] / [A-]
    • 1.78 × 10-4 × 2.40 × 10-2 / (1.79 × 10-2 ) = 2.39 × 10-4
  • ​pH = 3.62
96
Q

Describe how to investigate of how the pH of a solution of ethanoic acid changes as sodium hydroxide solution is added

A
  1. Calibrate the pH meter
    1. Place pH probe in standard buffer solutions (e.g. pH 4, 7, 9) & record pH readings
    2. Plot graph of pH reading (y-axis) against the pH of the buffer solution
  2. Rinse pipette with ethanoic acid & fill it with this
    • Transfer 20cm3 of ethanoic acid to beaker
  3. Rinse burette with NaOH & then fill it with this
  4. Rinse pH probe with distilled water and clamp it so bulb is immersed in ethanoic acid
    • Use to rod to stir solution & record pH
  5. Add NaOH to ethanoic acid in small regular intervals
    • e.g. 2cm3 and then change to smaller intervals around 22cm3 e.g. 0.2cm3
    • Repeat until alkali in excess
    • Add in smaller increments near endpoint
  6. Stir solution and take a reading after each addition
97
Q

Explain briefly why a pH meter should be calibrated before use (1)

A

Over time/after storage meter doesn’t give accurate readings

98
Q

State why water at 50°C is neutral (1)

A

[H+] = [OH−]

99
Q

Describe breifly how you would ensure that a reading from a pH meter is accurate (2)

A
  • Calibrate meter with solution(s) of known pH/buffer(s)
  • Adjust meter/plot calibration curve
100
Q

Two solutions, one with a pH of 4.00 and the other with a pH of 9.00, were left open to the air. The pH of the pH 9.00 solution changed more than that of the other solution. Suggest what substance might be present in the air to cause the pH to change. Explain how and why the pH of the pH 9.00 solution changes. (3)

A
  • CO2
  • pH decreases
  • acidic (gas) reacts with alkali / OH−
    • ​or CO2 + 2OH− → CO32− + H2O
    • CO2 + OH− → HCO3−
101
Q

Use information from the curve in the figure above to explain why the end point of this reaction would be difficult to judge accurately using an indicator. (2)

A
  • The change in pH is gradual at the end point
  • An indicator would change colour over a range of volumes of sodium hydroxide / indicator would not change colour rapidly with a few drops of NaOH

a-level chemistry oxford aqa (34 decks)

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