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chemistry a-level copy > Acids, Bases and pH (Chapter 20) > Flashcards

Acids, Bases and pH (Chapter 20) Flashcards

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1
Q

Describe the Arrhenius model about acids and base

A

1) acids - dissociate and release H+ ions in aqueous solutions
2) alkalis - dissociate and release OH- ions in aqueous solutions
3) H+ ions are neutralised by OH- ions to form water

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2
Q

What is an alkali?

A

A soluble base

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3
Q

Define a Bronsted-Lowry acid

A

A proton donor

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4
Q

Define a Bronsted-Lowry base

A

A proton acceptor

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5
Q

What does a conjugate acid-base pair contain?

A

2 species that can be interconverted by transfer of a proton

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6
Q

Describe an example of a conjugate acid-base pair

A
  • HCl (conjugate acid) and Cl- (conjugate base)
  • Forward reaction: HCl releases a proton to form Cl-
  • Reverse reaction: Cl- accepts a proton to form HCl
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7
Q

What is the equilibrium between HCl and Cl-?

A

HCl + OH- <=> Cl- + H2O

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8
Q

What does dissociation require in aqueous solution?

A

Water bc a proton needs to be transferred from an acid to a base

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9
Q

Describe the dissociation of HCl

A
  • HCl + H2O <=> H3O+ + Cl-
  • H2O accepts a proton to form the conjugate base, the hydronium ion (H3O+/H+) which is the active acid ingredient in any aqueous acid
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10
Q

What do the words monobasic, dibasic and tribasic refer to?

A

The total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction

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11
Q

What is the spectator ion?

A

The ion in an acid which isn’t H+ (normally not include in the ionic equation)

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12
Q

What are the 4 important equations for acids?

A

1) acid + metal => salt + hydrogen
2) acid + base/alkali => salt + water
3) acid + carbonate => salt + water + CO2
4) H+ + OH- => H2O

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13
Q

What are bases normally

A

Metal oxides or metal hydroxides

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14
Q

What is the pH equation?

A

pH = -log10[H+]

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15
Q

What is the equation for [H+]?

A

10^-pH

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16
Q

What is the effect of pH being a logarithmic scale?

A

A change of one pH number is equal to 10x the difference in [H+]

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17
Q

Describe the [H+] in strong monoprotic acids

A

[H+]=[HA] bc strong acids completely dissociate in water

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18
Q

Describe the [H+] in strong diprotic acids

A

[H+] = 2[HA]

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19
Q

What is the difference between the dissociation equation for weak and strong acids?

A

Weak acids are in equilibrium, strong acids are not:

HA => H+ + A- vs HA <=> H+ + A-

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20
Q

What is the Ka equation?

A

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

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21
Q

What are the units of Ka?

A

Mol/dm3

22
Q

What is Ka just a version of?

A

Kc

23
Q

What does Ka change with?

A

Temperature (but normally standard at 25 degrees

24
Q

What is the magnitude of Ka show?

A

The larger the value of Ka, the further the equilibrium to the right ∴ the greater the dissociation and the acid strength

25
Q

What is the equation for pKa?

A

-logKa

26
Q

What is the equation of Ka?

A

10^-pKa

27
Q

Why is pKa used to compare weak acids instead of Ka?

A

It is difficult to compare numbers with negative indices ∴ Ka is converted into the negative logarithm pKa

28
Q

What are Ka and pKa used for?

A

To measure the strength ([H+]) of weak acids

29
Q

What happens to Ka and pKa with stronger acids?

A

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

30
Q

What happens to Ka and pKa with weaker acids?

A

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

31
Q

Describe the dissociation of sulfurous acid

A

Sulfurous acid = H2SO3

  • H2SO3 <=> H+ + HSO3-
  • HSO3- <=> H+ + SO32-
32
Q

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

A

The concentration of the acid - [HA] and Ka

33
Q

What happens when the weak acid HA dissociates?

A

H+ and A- are formed in equal quantities

34
Q

What is the concentration of the weak acid HA at equilibrium?

A

[HA]start - [H+]eqm

35
Q

What is the concentration of H+/A- at equilibrium?

A

[H+]eqm and [A-]eqm

36
Q

What is the complicated equation for the Ka of a weak acid at equilibrium?

A

Ka = ([H+]eqm x [A-]eqm)/[HA]start-[H+]eqm

37
Q

What are the two approximations used to simplify the the Ka equation of a weak acid?

A

1) [H+]eqm = [A-]eqm - bc, even though there will be a very small amount of H+ ([H+]) from the dissociation of water, this can be neglected as it is so small compared with the [H+] from the acid
2) [HA]eqm = [HA]start - as the dissociation of weak acids is small, we can assume that the [HA]start is significantly larger than [H+]eqm and can ∴ neglect any decrease in the [HA] from dissociation

38
Q

What is the simplified equation for the Ka of a weak acid?

A

Ka = [H+]^2/[HA]

39
Q

How can the Ka of a weak acid be determined?

A

1) prepare a standard solution of the weak acid of known concentration
2) measure the pH using a pH meter

40
Q

When does approximation 1 ([H+]eqm = [A-]eqm) for the Ka of a weak acid break down?

A

For very weak acids (pH>6) or very dilute solutions - bc then [H+] from the dissociation fo water will be significant compared with the dissociation from weak acids

41
Q

When does approximation 2 ([HA]eqm = [HA]start) for the Ka of a weak acid break down?

A

For stronger weak acids (Ka>10^-2) or very dilute solutions - bc here, [H+] comes significant and there is a different between [HA]eqm and [HA]start-[H+]eqm

42
Q

Describe the ionisation of water

A
  • Water ionises very slightly, acting as both an acid and a base, setting up the acid-base equilibrium:
  • H2O + H2O <=> H3O+ + OH- (H2O <=> H+ + OH-)
  • ∴ treating water as a weak acid,
    Ka = [H+][OH-]/[H2O] OR Ka x [H2O] = [H+][OH-}
  • The dissociation of water is very small ∴ 1dm3 of water is mainly undissociated H2O - [H2O] = 55.6mol/dm3 (constant)
43
Q

Describe Kw

A
  • Kw = ionic product of water (ions in water multiplied together)
  • Kw = [H+][OH-]
  • Kw varies with temperature - the value of Kw at 298K is 1x10^-14, this value sets up the neutral point on the pH scale
  • Kw controls the [H+] and [OH-] in aqueous solutions
44
Q

How does the Kw equation give pH 7?

A

1) [H+]=[OH-]
2) ∴ Kw = [H+][OH-]=[H+]^2
3) ∴ [H+] = rootKw = 1 x 10^-7
4) ∴ pH = -log(1 x 10^-7)=7

45
Q

When is a solution acidic?

A

When [H+]>[OH-]

46
Q

When is a solution neutral?

A

When [H+]=[OH-]

47
Q

When is a solution alkaline?

A

When [H+]

48
Q

What is try about acidic and alkaline solutions?

A

An acidic solution still contains OH- ions and an alkaline solution still contains H+ ions

49
Q

What is a strong base?

A

An alkali that completely dissociates in solution

e.g. NaOH => OH- + Na+ (monobasic)

50
Q

What are the two ways to calculate the pH of a base?

A

1) [H+] = Kw/[OH-]

2) pH = 14-pOH where pOH=-log[OH-]

51
Q

Show how a weak base (NH3) partially dissociates

A

NH3 + H2O <=> NH4+ + OH-

52
Q

What happens to the concentration of something when an equal volume of water is added to it?

A

The concentration halves

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