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Z A Level Chemistry > Acids and Bases > Flashcards

Acids and Bases Flashcards

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1
Q

What is the Arrhernius model of acids and bases?

A
  1. Acids dissociate and release H+ ions in aqueous solution
  2. Alkalis dissociate and release OH- ions in aqueous solution
  3. H+ ion are neutralised by OH- ions to form water:
    H+(aq) + OH-(aq) —> H2O (l)
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2
Q

What is an acid?

A

A species that releases H+ ions in aqueous solutions

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3
Q

What is an alkali?

A

A type of base that dissolves in water forming hydroxide ions, OH-(aq) ions

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4
Q

What is a base?

A

A compound that neutralises an acid to form a salt

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5
Q

What is a Bronsted-Lowry acid?

A

A proton donor

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6
Q

What is a Bronsted-Lowry base?

A

A proton acceptor

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7
Q

What is a conjugate acid-base pair?

A
  • Contains two species that can be interconverted by transfer of a proton
  • HCL(aq) (equilibrium sign) H+ (aq) + Cl- (aq)
  • E.G HCL and Cl-
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8
Q

What happens in the forward direction?

A

HCL releases a proton to form its conjugate base Cl-

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9
Q

What happens in the reverse direction?

A

Cl- accepts a proton to form its conjugate acid HCL

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10
Q

How can neutralisation be down?

A

H+ (aq) + OH- (aq) –> H2O (l)

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11
Q

In Hydrochloric acid how have the H+ ions been supplied by dissociation?

A

HCl (aq) (equilibrium arrow) H+ (aq) + Cl-(aq)

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12
Q

What is the conjugate acid equilibrium for HCL?

A

HCL (aq) + OH- (aq) (equilibrium arrow) H2O (l) + Cl- (aq)

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13
Q

What happens in the forward direction of the conjugate acid equilibrium for HCL?

A
  • HCL is an acid as it donates H+

- OH- is a base as it accepts H+

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14
Q

What happens in the reverse direction of the conjugate acid equilibrium for HCL?

A
  • H2O is an acid is it donates H+

- Cl0 is a base as it accepts H+

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15
Q

What is needed for dissociation?

A
  • In aqueous solution dissociation requires a proton to be transferred from an acid to a base
  • The dissociation does not take place unless water is present
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16
Q

What is the equation for the acid-base equilibrium for hydrochloric acid with the hydronium ion H3O + (aq)?

A

HCL (aq) + H2O (l) (equilibrium arrow) H3O+ (aq) + Cl- (aq)

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17
Q

Describe the acid-base equilibrium for hydrochloric acid with the hydronium ion H3O + (aq)

A
  • Water accepts a proton and is behaving as a base
  • H2) has accepted a proton to form its conjugate acid, the hydronium ion H3O+ (aq)
  • The hydronium ion is very important, as it is the active acid ingredient in any aqueous acid
  • You can use H3O+ in equation but you will see that it is far more common to use the simpler H+ (aq)
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18
Q

What do the terms mono basic, dibasic and tribasic acids refer to?

A
  • The total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction
  • This would typically be replacement of protons by metal ions or ammonium ions to form a salt
  • No of hydrogen atoms in formula gives a clue
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19
Q

What is the role of H+ in the reactions of acids?

A
  • Acids react with metals, carbonates, metal oxides, and alkalis to form salts
  • The active species from the acid is H+ (aq) and ionic equations can be written for each type of reaction to emphasise the role of H+ ions
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20
Q

What is important to remember in ionic equations?

A
  • Split up ionic/aq into it’s ions and keep l,s,g the same and then cancel out spectator ions
  • When writing ionic equations with H+ (aq) the acid does not matter, ti is the same reaction
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21
Q

What are spectator ions?

A
  • Spectator ions are ions that do not change during the reactions
  • In the equations, spectator ions can simply be cancelled out
  • You then know that any acid will react with magnesium to form Mg 2+ (aq) ions and hydrogen gas
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22
Q

What is the redox reaction between an acid and metal?

A

acid + metal –> salt and hydrogen

  • Dilute acids undergo redox reaction with some metals to produce salts and hydrogen gas
  • E.G for zinc, the ionic equation for a reaction with any dilute acid is 2H+ (aq) + Zn (s) –> Zn2+ (aq) + H2 (g)
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23
Q

What is the neutralisation of acids with carbonates? (reactions between acids and bases)

A

acid + carbonate –> salt + water + carbon dioxide

-Carbonates are bases that neutralise acids to form a salt, water and carbon dioxide

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24
Q

What is the neutralisation of acids with alkalis? (reactions between acids and bases)

A

acid + alkali –> salt + water

-With alkalis, the acid and base are in solution

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25
Q

What is the neutralisation of acids with metal oxides? (reactions between acids and bases)

A

acid + base –> salt + water

-An acid is neutralised by a solid metal side or hydroxide to form a salt and water only

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26
Q

What did Soren Sorensen do?

A
  • Introduced pH as a simple numerical scale for measuring hydrogen ion concentrations
  • Found that hydrogen ion concentrations in solution had a very large range of values with negative powers of 10
  • He used the native logarithm of these hydrogen ion concentrations to produce a more manageable scale
  • Using his scale, hydrogen ion concentration fo 10-1 to 1o-14 are converted to pH values of 2 to 14
  • Simple and easier to sue
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27
Q

For an solution at 25 degrees what does the pH mean?

A
  1. Less than 7 shows decreasing acidity
  2. Greater than 7 shows decreasing acidity
  3. 7 is neutral
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28
Q

How was acidity measured prior?

A
  • Before Sorensen’s pH scale the degree of acidity and basicity in solution was measured as colours using indicators
  • He added numbers to the colours
  • His method relied on measuring hydrogen ion using an electrochemical cell
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29
Q

How is pH measured today?

A
  • pH meters are used for measuring the pH accurately and are based on the same principle
  • pH indicator paper or universal indicator solution is still used as an easier and cheaper, through less accurate alternative
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30
Q

How do you work out pH?

A

pH = -log[H+(aq)]

31
Q

How do you work out [H+(aq)]?

A

[H+(aq)] = 10^-pH

32
Q

How does [H+(aq)] relate to pH?

A
  • A low value of [H+(aq)] matches a high value of pH
  • A high value of [H+(aq)] matches a low value of pH
  • As a logarithmic scale a change in one pH number is equal to a 10 times difference in [H+ (aq)], so there is a large difference in hydrogen ion concentration between solutions with a pH of 2 and a pH of 3
33
Q

Describe some parts of the pH scale

A
  1. A pH of 1 has 10 times the concentration of H+ ions as a solution with a pH of 2
  2. To dilute a solution from a pH 1 to a pH of 4 (just 3 pH units) would require a dilution by 10x10x10 = 1000 times
  3. A solution with a pH of 1 contains 10^13 times more H+ (aq) ions as a solution with a pH of 14
  4. Comparing 1 with 14 is a lot more manageable that comparing 1 x 10-1 with 1 x 10-14
34
Q

What happens in a strong acid?

A

-In aqueous solution a strong mono basic acid, HA completely dissociates
HA (aq) –> H+ (aq) + A- (aq)
(1 mol) (1 mol)

35
Q

In a strong acid what is [H+(aq)] equal to?

A
  • The concentration of the acid [HA(aq)]

- The pH of a strong acid can be calculated directly from the concentration of the acid

36
Q

What is strong and weak acids used to describe?

A
  • Strong and weak acids are terms used to describer the extent of dissociation of an acid
  • A strong acid such as hydrochloric acid HCl completely dissociates in aqueous solution
  • HCl (aq) –> H+(aq) + Cl-(aq)
  • A weak acid such as ethnic acid, CH3COOH, partially dissociates
  • CH3COOH (aq) (equilibrium arrow) H+ (aq) + CH3COO- (aq)
37
Q

What is Ka?

A
  • The acid dissociation constant is one of several special equilibrium, constant used for acid-base equilibria
  • All the constants are just version of the equilibrium constant Kc in terms of concentration of mild-3
38
Q

How do you calculate Ka?

A

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

only for weak acids!

39
Q

How does the Ka value change?

A
  • All with all equilibrium consents Ka change with temperature and recorded Ka values are usually standardised at 25 degrees
  • The larger the numerical value of Ka, the further the equilibrium is to the right
  • If you look at the Ka expression, the concentration of dissociated ions are on top and so the large the Ka value, the greater the dissociation and the greater the acid strength
40
Q

What is the problem with Ka values?

A
  • It is difficult to compare number with negative idiocies
  • Solved by converting the Ka value into a negative logarithm called pKa
  • pKa values are much more manageable than Ka and it is much easier to compare relative acidic strength using pKa values than Ka values
41
Q

How do you convert Ka into pKa?

A

pKa = -logKa

42
Q

How do you convert pKa into Ka?

A

Ka = 10^-pka

43
Q

What happens to Ka and pKa the stronger the acid?

A

The stronger the acid, the larger the Ka value and the smaller the pKa value

44
Q

What happens to Ka and pKa the weaker the acid?

A

The weaker the acid, the smaller the Ka value and the larger the pKa value

45
Q

When is pKa used?

A

pKa values are used extensively for campaign the strengths of weak acids particularly in biological system

46
Q

How does pH change on dilution?

A

A pH of 1 has 10 times the concentration of H+ (aq) ions as a solution with a pH of 2

47
Q

What do you have to watch out for in dibasic and tribasic strong acids?

A 
[H+(aq)] = 2 x [H2A]
[H+(aq)] = 3 x [H3A]
48
Q

What is a note for Ka values?

A

Ka values are always given for monoprotic acids where a weak acid is diprotic, it will have two values: Ka (1) for the removal of the first proton and Ka (2) for the removal of the second proton
A triprotic acid has three values

49
Q

What happens for a mono basic acid HA in aqueous solution in strong/weak acid?

A
  • A strong acid HA completely dissociates

- A weak acid HA partially dissociates

50
Q

What is the equilibrium for a weak acid?

A

HA (aq) (equilibrium arrow) H+ (aq) + A- (aq)

51
Q

What does [H+(aq)] depend on in a weak acid?

A
  • The concentration of the acid

- The acid dissociation constant

52
Q

What happens when HA molecules dissociate?

A

H+ (aq) and A-(aq) ions are formed in equal quantities

53
Q

How can Ka be calculated using equilibrium concentrations?

A

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

54
Q

What is the first approximation used for the Ka calculation?

A

[HA (aq)]initial = [H+(aq)]equilibrium

  • The undissociated concentration of HA is greater than the concentration of equilibrium concentration and as the dissociation of weak acid is small you can assume [HA(aq)]start&raquo_space;> [H+(aq)] and you can neglect any decrease in the concentration fo HA from dissociation
  • The initial [HA] is the same as the [HA} at equilibrium for weak acids (this does not hold for a strong weak acid where there is appreciable dissociation
55
Q

What is the second approximation used for the Ka calculation?

A

[H+(aq)]eqm=[A-(aq)]eqm

  • HA dissociates to produce equilibrium concentrations of H+ (aq) and A-(aq)that are equal. There will also be a very small concentration of H+(aq) from the dissociation of water but this will be extremely small and can be neglected compared with H+ concentration from the acid
  • Ignoring the dissociation of water
56
Q

What is the simplified expression for Ka?

A

Ka = [H+(aq)]^2 / [HA(aq)] (starting conc)

57
Q

What is the simplified expression for Ka rearranged for [H+(aq)]?

A

[H+(aq)]=sqrt(Ka x [HA(aq)])

pH=-log[H+(aq)]

58
Q

How can Ka be determined?

A
  • Experimentally the Ka of a weak acid can be determined by
    1. Preparing a standard solution of the weak acid of known concentration
    2. Measuring the pH of the standard solution using a pH meter
59
Q

Describe the ionisation of water

A

-Water ionises very slightly acting as both an acid and as a base setting up this acid-base equilibrium: (look at notes)

60
Q

What is the Ka expression for water treating it as a weak acid?

A

Ka = [H+(aq)][OH-(aq)] / [H2O (l)]

Rearranging: Ka x [H2O(l)] = [H+(aq)] x [OH-(aq)]

61
Q

Why is the dissociation of water very small?

A
  • The dissociation of water is very small
    1. 1dm^3 (1000g) of water is mainly undissociated
    2. So [H2O(l)] = 1000/18.0 = 55.6moldm-3 a constant
62
Q

What is Kw?

A

-Kw is called the ionic product of water and the ions in water (H+ and OH-) multiplied together
Kw = [H+(aq)] x [OH-(aq)]

63
Q

What is the value of Kw and does it change?

A
  • As with all equilibrium constants Kw varies with temperature
  • The value of Kw at 298K (25 degrees C) is 1.00 x 10^-14mol2dm-6
64
Q

What is the importance of Kw?

A
  • The significance of Kw having this value at 25 degrees C os huge and the value sets up the neutral point in the pH scale
  • Kw controls the concentration of H+(aq) and OH-(aq) ions in aqueous solutions
65
Q

What happens to water on dissociation?

A

Water is neutral and it produces an the same number of H+(aq) and OH-(aq) ions
So [H+(aq)] = [OH-(aq)]
(look at notes)

66
Q

What will there always be in an aqueous solution?

A
  • In any aqueous solution there will always be both H+ (aq) and OH- (aq) ions present such that [H+(aq)] x [OH-(aq)]=Kw
  • So a solution that is acidic still contains OH-(aq) ions it is just that there are more H+ (aq) ions (and vice versa for an alkaline solution)
67
Q

When is a solution acidic?

A

[H+(aq)] > [OH-(aq)]

68
Q

When is a solution neutral?

A

[H+(aq)] = [OH-(aq)]

69
Q

When is a solution alkaline?

A

[H+(aq)] < [OH-(aq)]

70
Q

How do you work out the pH of solutions of strong bases?

A
  1. An alkali is a soluble base that relates OH- ions in aqueous solution
  2. A strong base is an alkali that completely dissociates in solution e.g. NaOH is a strong alkali that completely dissociates
    NaOH (aq) –> Na+ (aq) + OH- (aq)
    1mol 1 mol
  3. NaOH is mono basic as each more of NaOH relates one mole of OH- (aq) ions
71
Q

What can the pH of a strong base be calculated by?

A
  1. The concentration of the base

2. The ionic product of water Kw

72
Q

Describe weak bases

A
  • As with strong bases a weak base is also an alkali. Ammonia gas is an example of a weak base and it dissolves in water releasing OH- ions, from water molecules
  • An equilibrium is set up with the equilibrium positioned well to the the left
  • NH3 (aq) + H2O (l) (equilibrium arrow) NH4+ (aq) + OH-(aq)
  • So one mole of NH3 releases far less than one mole of OH- (aq) ions
  • The pH of weak bases can be calculated using a similar method to that used for weak acids (but calculation of the pH of weak bases goes beyond scope of this chemistry course)
73
Q

Why does phenolphthalein which is colourless in acidic solutions turn pink in alkaline solutions?

A

It forms a pink anion by loss of H+ ions

74
Q

How could a student prove that an acid was weak by taking a single pH measurement and then calculate Ka?

A
  1. Measured pH > 1
  2. [H+] = 10^-pH
  3. Ka = [H+][CH3CH2COO-] / [CH3CH2COOH]
  4. Ka = [H+]^2 / [CH3CH2COOH]

Z A Level Chemistry (13 decks)

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