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A Level - Chemistry > 12: Acids and Bases > Flashcards

12: Acids and Bases Flashcards

1
Q

What is a bronsted-lowry acid?

A

Proton donors

Releases H+ in water

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2
Q

What is the standard method for writing an acid?

A

HA

A - other element

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3
Q

What is the equation when an acid dissociates in water?

A

HA(aq) + H2O(l) → H3O+(aq) + A-(aq)

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4
Q

What is a bronsted-lowry base?

A

Proton acceptors

Bind to H+ if they are present in solution

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5
Q

What is the formula for bases reacting with water?

A

B(aq) + H2O(aq) → BH+(aq) + OH-(aq)

B = a base

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6
Q

What is a strong acid?

A

One which dissociates almost completely in water

Meaning almost all H+ ions are released

Equilibrium lies to the right

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7
Q

What is a strong base?

A

A base which dissociates almost completely in water

OH- produced

Equilibrium lies to the right

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8
Q

What are some examples of strong acids?

A

HCl - Hydrochloric acid

H2SO4 - Sulphuric acid

HNO3 - Nitric acid

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9
Q

What are some examples of strong bases?

A

NaOH - Sodium hydroxide

KOH - Potassium hydroxide

Ba(OH)2 - Barium hydroxide

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10
Q

What is a weak acid?

A

Dissociate only very slighlty in water

Small numbers of H+ produced

Equilibrium lies to the left

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11
Q

What is a weak base?

A

Only slightly protonate in water

Not as much OH- produced

Equilibrium lies to the left

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12
Q

What are some examples of weak acids?

A

Ethanoic acid

Acetic acid

Lactic acid

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13
Q

What are some examples of weak bases?

A

NH3 - Ammonia

K2CO3 - Potassium carbonate

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14
Q

What do acids donate protons to?

A

A conjugate base

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15
Q

What is the standard formula for the reaction between an acid and a base?

A

HA(aq) + B(aq) ⇔ BH+(aq) + A-(aq)

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16
Q

What are conjugate pairs, in reference to acids and bases?

A

Species that are linked by the transfer of a proton

Always on the opposite side of the reaction equation

A- and HA are a conjugate pair

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17
Q

What is the basic neutralisation reaction?

A

HCl + NaOH -> H2O + NaCl

Acid + Base -> Water + Salt

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18
Q

What is a neutral solution?

A

One where [OH-] = [H+]

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19
Q

When is a solution acidic?

A

[H+] > [OH-]

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20
Q

When is a solution basic/alkaline?

A

[OH-] > [H+]

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21
Q

What is the enthalpy change of nuetralisation?

A

Enthalpy change when 1 mole of water is produced via the reaction between an acid and base

Standard conditions

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22
Q

Is enthalpy of neutralisation endothermic or exothermic?

A

Exothermic

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23
Q

What occurs in the enthalpy of neutralisation for weak acids?

A

Very little inital dissociation

As reversible reaction, H+/OH- react and cause equilibrium to shift to the right

Therefore enthalpy change includes the reaction between H+/OH- and enthalpy of dissociation

Value can vary when weak acids and bases reacted together

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24
Q

What occurs in the enthalp of neutralisation of strong acids/bases?

A

As fully dissociate into water

No dissociation enthalpy included

Just reaction between H+/OH-

Means value is similar when strong acids and bases used

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25
Q

What is pH?

A

pH = - log10[H+]

Normally goes from 0 to 14

7 is regarded as neutral

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26
Q

What is 0 and 14 on the pH scale?

A

0 - very acidic

14 - very basic/alkaline

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27
Q

What does the p in pH mean?

A

-log10

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28
Q

What does monoprotic mean?

A

One mole of acid produces one mole of H+ ions

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29
Q

What is the [H+] if there is a monoprotic strong acid and why?

A

[HA] = [H+]

Fully dissociates and produces one H+ per mole of acid

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30
Q

How do you calculate [H+] from pH?

A

[H+] = 10-pH

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31
Q

What is a polyprotic acid?

A

An acid that releases more than one proton per molecule upon dissociation

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32
Q

What does diprotic mean?

A

2 moles of H+ ions are released for every mole of acid which dissociates

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33
Q

What is Ka?

A

Acid dissociation constant

Applies to a particular acid at a specific temp regardless of concentration

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34
Q

What is the formula of Ka?

A

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

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35
Q

How is Ka used for determining the pH of a weak acid?

A

Assume [HA]inital ≈ [HA]equilibrium

Assume acid dissociates more than water so all protons are from the acid, meaning [H+] ≈ [A-]

Therefore Ka = [H+]2/[HA]inital

Then use to find [H+] and pH

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36
Q

Why does the assumption that [HA]inital ≈ [HA]equilibrium not work for strong acids?

A

Strong acids dissociate more in solution so the difference is significant

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37
Q

What can water act as?

A

Acid - donating a proton

or Base - accepting a proton

Always OH- and H3O+ in water

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38
Q

What equation is constantly occuring in water?

A

H2​O + H2​O ⇔ H3O+ + OH-

Simplified to: H2O ⇔ H+ + OH-

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39
Q

What is Kw?

A

The ionic product of water

Kw = [H+][OH-]

Always the same at a specific temperature

40
Q

How is Kw derived?

A

Kc = [H+][OH-] / [H2O]

Water only dissociates a little bit, equilibrium lies on the left. [H2O] is considered to have a constant value (much larger than others)

Kc * [H2O] = Kw

Kw = [H+][OH-]

41
Q

What is Kw in a solution of pure water?

A

[H+] = [OH-]

Kw = [H+]2

42
Q

What is Kw at 25ºC?

A

Kw = 10-14 mol2 dm-6

43
Q

If a base only donates one OH- what can be said about the concentration?

A

[OH-] = [B]

B = a base

44
Q

How is the pH of a strong base calculated?

A

Kw = [H+][OH-]

[H+] = Kw / [OH-]

pH = - log10[H+]

45
Q

What is pKw?

A

pKw = -log10Kw

46
Q

What is a pH meter?

A

Probe and digital display allowing for the measurement of pH

47
Q

How are pH meters calibrated correctly?

A

Place probe into pH meter into deionised water and adjust the reading to read 7.0

Do the same with a standard solution of pH 4 and another of pH 10

Rinse the probe with deionised water between each reading

48
Q

What can the pH of chemicals tell you?

A

Compare to the concentration to determine if strong or weak

49
Q

How can masses and pH be used to calcualte Ka?

A

Calculate moles from mass and RFM of compound

Concentration of acid = moles*1000 / volume(cm3)

[HA]inital = [HA]equilibrium

[H+] = 10-pH

Ka = [H+]2/[HA]

50
Q

What occurs to the pH of an acid when diluted?

A

[H+] decreases

pH increases

51
Q

What occurs when a strong acid is diluted by a factor of 10?

A

Diluted by factor of 10, increases pH by 1

52
Q

What occurs if a weak acid is diluted by a factor of 10?

A

Diluted by a factor of 10, increases pH by 0.5

53
Q

How can the [base] be calculated by titration?

A

Measure base using a pipette and put in flask with indicator

Rinse burette with standard solution of acid then fill with standard solution

Rough titration to an idea of rough end point, do this by adding acid to base and swirling and stopping when colour change

Accurate titration done within 2cm3 of end point then drop-by-drop, and work out the amount of acid needed to neutralise the base

Repeat a few times to get an average titre

54
Q

What are the graphs of pH vs volume of base added for strong acids and bases?

A

Starts low, steep equivalence point to a high plateau

55
Q

What is the pH curves for a reaction between a strong acid and a weak base?

A

Starts low and steep curve to just above 7 where it plateaus

56
Q

What is the pH curves for a reaction between a weak acid and a strong base?

A

Starts higher (nearer to 7) and curves up to high (near 14)

57
Q

What is the pH curves for the reaction between a weak acid and weak base?

A

Starts high (near 7) and curves to just above 7

58
Q

What does a pH curve show?

A

Initial pH depends on strength of the acid and small amounts of base has little impact with strong acids

Vertical equivalence lines at the end point where [H+] ≈ [OH-] as the acid has been neutralised, tiny increase causes big change in pH

pH change is less pronounced when strong acids added to strong bases compared to strong acids added to weak bases (or strong base to weak acid)

Final pH depends on strength of the base - stronger the base, the higher the final pH

59
Q

What does an indicator do?

A

Changes colour at the end point (neutralisation) of a reaction

60
Q

What is required when choosing an indicator?

A

Pick one which changes colour exactly at the end point, shown on the graph by the sharp increase

Changes colour over narrow pH range entirely on the vertical part of the pH curve

61
Q

Describe the features of methyl orange

A

Colour at low pH = red

Colour at high pH = yellow

Approx. pH of colour change = 3.1-4.4

62
Q

Describe the features of phenolphthalein

A

Colour at low pH = colourless

Colour at high pH = pink

Approx. pH of colour change = 8.3 - 10

63
Q

Which indicator should be used for the titration of a strong acid and strong base?

A

Either methyl orange or phenolphthalein

Rapid pH over range for both

64
Q

Which indicator should be used for the titration of a strong acid and a weak base?

A

Methyl orange

pH rapidly changes over methyl orange range but not phenolphthalein range

65
Q

Which indicator should be used for the titration of a weak acid with a strong base?

A

Phenolphthalein

pH changes rapidly over phenolphthalein range

66
Q

Which indicator should be used for the titration of a weak acid with a weak base?

A

Neither methyl orange nor phenolphthalein

Just use a pH meter as no indicators work

67
Q

When can you use a titration curve to find the pKa of a weak acid?

A

Titration curve for a weak acid and strong base

68
Q

What is half-equivalence?

A

Stage of titration when half of hte acid has been neutralised

For using this: half of equivalence of strong base has been added to the weak acid

69
Q

How do you obtain the pKa of a weak acid from a titration curve?

A

Weak acid dissociates HA ⇔ H+ + A-

At half-equivalence point [HA] = [A-]

Therefore Ka = [H+] and pKa = pH

pH at half-equivalence point is the pKa of the weak acid

70
Q

What is a pH chart?

A

Charts which show the colour of indicators at different pH

71
Q

Define a buffer

A

A solution that minimises changes in pH when small amounts of acid or base are added

72
Q

When do buffers work?

A

Only can minimise small changes

Does not work with large amounts of acid/base

73
Q

What is an acidic buffer comprosed of?

A

Weak acid and conjugate base

pH less than 7

74
Q

How is an acidic buffer made?

A
  1. Mix a weak acid with salt of conjugate base, salt dissociates into ions and the weak acid only slightly dissociates
  2. Mix excess of weak acid with a strong base, all base reacts with acid, as xs of weak acid still some left in the solution
75
Q

What occurs to the equilibrium if OH- or H+ ions are added to an acidic buffer?

A

e.g. CH3COOH ⇔ H+ + CH3COO-

Addition of H+ shifts equilibrium to the left, reducing H+ value to near original

Addition of OH- reacts with H+, equilibrium shifts to the right meaning H+ conc returns to near original

76
Q

What are alkaline buffers made of?

A

Weak base and one of its salts

E.g. Ammonia solution and ammonium chloride

77
Q

What does alkaline mean?

A

A basic solution that’s soluble in water

78
Q

How are alkaline buffers formed using ammonia as an example?

A

Salt of weak base is fully dissociated in the solution NH4Cl → NH4+ + Cl-

Equilibrium set up between the ammonium ions and ammonia

NH4+ ⇔ H+ + NH3

79
Q

How do alkaline buffers work?

A

E.g. NH4+ ⇔ H+ + NH3

Small amount of acid added causes equilibrium to shift to the less, reducing H+ near to orignial value

Small amount of base added reacts with H+, equilibrium shifts to the right due to less H+ so returns to original value. Can occur due to plenty of NH4+ due to the salt

80
Q

What causes the shape of a titration curve for a weak acid and strong base to be as it is?

A

Initial quick increase in pH as base is strong

Curve levels off as buffer solution of conjugate base (salt) in weak acid is formed which resists big pH change

Eventually all weak acid used up and equivalence point is reached

81
Q

What is the pH of the blood?

82
Q

How is pH controlled in the blood?

A

Carbonic acid-hydrogencarbonate buffer system

83
Q

What is the equation of the carbonic acid-hydrogencarbonate buffer system and how does it respond to slight pH change?

A

H2CO3 ⇔ H+ + HCO3-

Increase in H+ causes equilibrium to shift to the left, reducing it to the regular value

Decrease in H+ causes equilibrium to shift to the right, increasing [H+] to regular value

84
Q

How are levels of carbonic acid regulated in the body?

A

H2CO3 ⇔ H2O + CO2

Controlled by respiration

Breathing out CO2 reduces H2CO3

Levels of HCO3- controlled by the kidneys, excess excreted in the urine

85
Q

What are the assumptions when calculating the pH of the buffer?

A

Salt of conjugate base is fully dissociated, [A-]initial of salt ≈ [A-]equilibrium

HA is only slightly dissociated so assume [HA]initial ≈ [HA]equilibrium

86
Q

Why does [H+] not equal [A-] in a buffer?

A

Conjugate base doesn’t only come from dissociation of the weak acid

87
Q

How do you calculate the pH of an acidic buffer?

A

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

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

Use data value of Ka and assumptions that [HA] and [A-] are equal to inital concentrations

88
Q

What is the Henderson-Hasselbalch equation?

A

pH = pKa + log10([A-]/[HA])

Relies on [A-]initial of salt ≈ [A-]equilibrium, and [HA]initial ≈ [HA]equilibrium

Allows for formation of a buffer with a specific pH

89
Q

How do you calculate the pH of a solution when an acid is diluted?

A

Calculate [H+] in original solution

Calculate [H+] in diluted solution = [H+] * (old/ new volume)

Calculate pH using new [H+]

90
Q

What occurs to Kw and pH of water as temperature increases?

A

Temp increases, equilibrium moves to the right

[H+] and [OH-] increase

Kw increases and pH decreases

Water still neutral as [H+] = [OH-] but lower pH

91
Q

How do you calculate the pH of a strong base?

A

[OH-] in dilute = [OH-] in original * (old/new volume)

[H+] = Kw/[OH-]

Kw = 10-14 at room temp

92
Q

How do you calculate the pH of strong acids and bases?

A

Calculate moles of H+

Calculate moles of OH-

Work out which in excess and by how many moles

Calculate conc. of excess

Calculate pH

93
Q

How do you calculate the pH of a weak acid?

A

Ka = [H+]2/[HA]

Calculate [H+] and pH

Remember [HA]initial = [HA]equilibrium

94
Q

How do you calculate the pH of the reaction between a weak acid and a strong base?

A

Calculate moles of HA

Calculate moles of OH-

Calculate which is in excess

If XS HA, calculate moles and conc of HA and A-, use Ka to find [H+]

If XS OH-, use Kw to find [H+]

If equal moles, pH = pKa of weak acid

95
Q

What are some common ionic equations for titration calculations?

A

H+ + OH- → H2O

2H+ + CO32- → H2O + CO2

H+ + HCO3- → H2O + CO2

H+ + NH3 → NH4+

96
Q

How do you calculate the pH of a basic buffer?

A

HA + OH- → A- + H2O

Calculate amount of A- formed and HA, then conc

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

A Level - Chemistry (26 decks)

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