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Edexcel A Level Chemistry > ✅12 - Acid Base Equilibria > Flashcards

✅12 - Acid Base Equilibria Flashcards

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1
Q

What is a Bronsted-Lowry acid?

A

A proton donor

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2
Q

What is a Bronsted-Lowry base?

A

A proton acceptor

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3
Q

What is a conjugate acid?

A

A species formed by the reception of a proton (H+) by a base in other words, it is a base with a hydrogen ion added to it.

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4
Q

What is a conjugate base?

A

What is left after an acid has donated a proton in a chemical reaction.

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5
Q

What is a diprotic/dibasic acid?

A

One which can donate two protons, such as H2SO4

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6
Q

What is a diprotic/diacidic base?

A

One which can accept two protons, such as CO3 2-

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7
Q

What are amphoteric substances?

A

One which can act either as an acid or a base, such as water

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8
Q

What is a strong acid?

A

One which is almost completely dissociated in aqueous solution

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9
Q

How are strong acids represented?

A

With a single arrow in the dissociation equation, not reversible

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10
Q

What is a weak acid?

A

One that is only partially dissociated in aqueous solution

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11
Q

How are weak acids represented?

A

With a reversible arrow in the dissociation equation

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12
Q

How are hydrogen ion concentration and and concentration linked for strong acids?

A

Directly, so HCl of 0.100 moldm-3 will have an H+ concentration of 0.100 moldm-3

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13
Q

How would you calculate pH from H+ concentration?

A

pH = -log [H+]

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14
Q

How would you calculate [H+] from pH?

A

[H+] = 10^-pH

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15
Q

What is Ka?

A

The acid dissociation constant

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16
Q

What is the equation to work out Ka of a weak acid?

A

[H+] x [A-]
_______
[HA]

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17
Q

What can the expression for Ka be simplified to?

A

[H+]^2
_____
[HA]

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18
Q

How can you work out [H+] for a weak acid from Ka?

A

By rearranging the equation to find a value for [H+]^2, then square rooting the value.

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19
Q

How would you work out pKa?

A

-log Ka

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20
Q

The larger the value of Ka…

A

…the stronger the acid

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21
Q

The larger the value of pKa…

A

…the weaker the acid

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22
Q

How would you calculate the pH of a dibasic acid?

A

Ka HSO4 = 0.0100
In 0.500 moldm-3 solution of H2SO4, contribution to [H+] from H2SO4 will be 0.500moldm-3. If we assume that contribution to [H+] from HSO4- is (x)moldm-3:

Ka(HSO4-) = 0.0100 = (0.500- x) x/0.500
Solving this equation gives x = 0.0098
So total [H+] = 0.05098 with a pH of 0.293

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23
Q

What is Kw?

A

The ionic product of water

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24
Q

What is the equation to calculate Kw?

A

[H+] x [OH-]
_________
[H2O]

25
Q

What can the equation for Kw be simplified to?

A

[H+] x [OH-], as pure water is neutral

26
Q

How would you calculate pKw?

A

-log Kw

27
Q

Why do even the most alkaline solutions have some hydrogen ions?

A

Because water ionises to form H+ and OH-

28
Q

How would you calculate the pH of a strong base?

A

Find the concentration of the base
Divide the Kw by the base concentration
This will give you [H+], so you can then calculate pH

29
Q

How can the relative strengths of different acids/bases be determined?

A

By measuring the pH of equimolar aqueous solutions of the acids/bases, at the same temperature

30
Q

What is the pH of a salt of a strong acid and strong base?

A

7

31
Q

What is the pH of a salt of a weak acid and strong base?

A

> 7, alkaline

32
Q

What is the pH of a salt of a strong acid and weak base?

A

<7, acidic

33
Q

How does dilution affect the pH of solutions of strong acids?

A

pH increases by one for each 10-fold dilution

34
Q

How does dilution affect the pH of solutions of weak acids?

A

pH increases by 0.5 for each 10-fold dilution

35
Q

How can Ka of a weak acid be determined experimentally?

A

By dissolving a known mass of acid in a small volume of water and making it up to a known volume, then measuring its pH
Calculate concentration of solution
Work backwards to calculate Ka

36
Q

What is the end point of a titration?

A

When the acid and base are mixed in exactly the right proportions to react in equivalent amounts

37
Q

What is the equivalence point of a titration?

A

When the acid and base have reacted together in the exact proportions dictated by the stoichiometric equation

38
Q

What does the titration curve for strong acid strong base look like?

A

Starts at high pH, sudden drop as equivalence point reached, ends at low pH

39
Q

What does the titration curve for weak acid strong base look like?

A

Starts at high pH, equivalence point around pH 8/9, shorter middle section, ends at pH 3/4

40
Q

What does the titration curve for strong acid weak base look like?

A

Starts at pH 11, slight decrease in pH before equivalence point, around pH 5/6, ends around pH 1

41
Q

What does the titration curve for weak acid weak base look like?

A

Starts around pH11, no steep section, short, equivalence point at pH 7, ends around pH 4/5

42
Q

What is the point of inflexion?

A

The point on a weak acid/base titration curve where the equivalence point lies, no steep section

43
Q

For a strong acid/weak base titration, why does the pH fall sharply at first but then level off?

A

Because a buffer solution is formed

44
Q

What is the equation for K(in)?

A

K(in) = [H+] x [In-]
________
[HIn]

45
Q

What is the rule of thumb for when indicators change colour?

A

The red colour of methyl orange will first predominate when [HIn] is ten times [In-], and the yellow colour will predominate when [In-] is ten times [HIn].

46
Q

How can the approximate pH for when each colour predominates be calculated?

A

When [HIn] = 10[In-]:

[H+] x [In-]
________ = 2.00 x 10^-4 mol dm-3 (K(In))
10[In-]
[H+] + 2.00 x 10^-3

47
Q

A good indicator shows a colour change when…

A

…one drop of acid is added from the burette to accurately determine the end point of titration

48
Q

What is a buffer solution?

A

A solution whose pH remains the same when small amounts of acid are base are added

49
Q

What are the two most common ways of making a buffer solution?

A

Mixing a weak acid with its conjugate base

Mixing a weak base with its conjugate acid

50
Q

What is the simplest example of a buffer solution?

A

Ethanoic acid and sodium ethanoate

51
Q

How can the pH of a buffer solution be calculated?

A

Find Ka
Then find [H+] by rearranging the equation for Ka with the acid or base and the conjugate ion
Use [H+] to calculate pH

52
Q

How does a buffer solution work when acid is added?

A

When small amounts of acid are added, majority of H+ react with the CH3COO- ions to form CH3COOH molecules.

53
Q

How does a buffer solution work when base is added?

A

Majority of OH- added react with CH3COOH to form CH3COO- and H2O

54
Q

What is the Henderson-Hasselbalch equation?

A

[H+] = Ka x [acid]
____
[salt]

55
Q

What does the Henderson-Hasselbalch equation show?

A

The pH of a buffer solution

56
Q

How would you make a buffer solution with a pH less than 7?

A

A mixture of a weak acid and its conjugate base

57
Q

How would you make a buffer solution with a pH more than 7?

A

A mixture of a weak base and its conjugate acid

58
Q

How can Ka be determined from a pH titration curve?

A

At the half equivalence point, pH = pKa

59
Q

What are the two processes involved in neutralisation?

A

Dissociation (endothermic) and hydration (exothermic)

Edexcel A Level Chemistry (20 decks)

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