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A Level Chemistry OCR A > 5.1.3: Acids, bases and buffers > Flashcards

5.1.3: Acids, bases and buffers Flashcards

1
Q

Define a Bronsted-Lowry acid

A

Proton donor

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2
Q

Define a Bronsted-Lowry base

A

Proton acceptor

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3
Q

Define Lewis acid

A

Electron pair acceptor

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4
Q

Define Lewis base

A

Electron pair donor

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5
Q

What ion causes a solution to become acidic? (2 answers) Name and formula

A

H+ (hydrogen ion) or, more accurately, H3O+ (oxonium ion), as protons react with H2O to form it

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6
Q

What ion causes a solution to become alkaline?

A

-OH (hydroxide ion)

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7
Q

Write an equation for the ionisation of water (2 marks)

A 
2H2O (l) ⇌ H3O+ (aq) + -OH (aq)
OR H2O (l) ⇌ H+ (aq) + -OH (aq)
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8
Q

Give example of a monobasic acid

A

HCl

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9
Q

Give example of a dibasic acid

A

H2SO4

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10
Q

Give example of a tribasic acid

A

H3PO4

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11
Q

Identify the acid base pairs for the reaction below

CH3COOH + H2O ⇌ CH3COO- + H3O+

A

Acid 1: CH3COOH
Base 2: H2O
Base 1: CH3COO-
Acid 2: H3O+

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12
Q

Define strong acid

A

Acids dissociate completely

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13
Q

Give some examples of strong acids

A

● Hydrochloric acid
● Sulfuric acid
● Nitric acid

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14
Q

What is the difference between concentrated and strong?

A

Concentrated means many mol per dm3,

strong refers to amount of dissociation

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15
Q

Define weak acids

A

Acids that only partially dissociate

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16
Q

Give some examples of weak acid

A

Methanoic acid, any organic acid

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17
Q

What is constant that is used to measure the extent of acid dissociation called?

A

Acid dissociation constant

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18
Q

What is the symbol of acid dissociation constant?

A

Ka

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19
Q

What does a larger Ka value mean?

A

Larger the Ka - greater the extent of

dissociation

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20
Q

Write the equation used to convert Ka into pKa

A

pKa = -log10Ka

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21
Q

Write the equation used to convert pKa

into Ka

A

Ka= 10-pKa

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22
Q

What is the relationship between pKa and strength of the acid?

A

Smaller the pKa stronger the acid

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23
Q

Write the equation used to convert concentration of H+ into pH

A

pH = -log[H+]

24
Q

Write the equation used to convert pH into concentration of H+

A

[H+] = 10-pH

25
Q

Why is a pH scale useful compared to concentration of H+?

A

pH scale allows a wide range of H+
concentration to be expressed as simple
positive values

26
Q

What is the relationship between pH and [H+]?

A

High pH value means a small [H+]

27
Q

If two solutions have a pH difference of 1, what is the difference in [H+]?

A

A factor of 10

28
Q

[H+] of a strong acid is equal to what?

A

[H+] = [HA]

29
Q

What is the assumption made when calculating pH of weak acids?

A

It is assumed that the concentration of acid at
equilibrium is equal to the concentration of
acid after dissociation. This is because only
very little of the acid dissociates

30
Q

Write the expression for ionic product of water, Kw

A

Kw = [H+][OH-]

31
Q

What is the units for Kw?

A

mol2dm-6

32
Q

What is the value of Kw at 298 K?

A

1.0 x 10-14

33
Q

What physical factors affect the value of Kw? How do they affect it?

A

Temperature only - if temperature is increased,
the equilibrium moves to the right so Kw
increases and the pH of pure water decreases

34
Q

Indices of of [H+] and [OH-] always adds up to what value?

A

-14

35
Q

Define the term strong base

A

Base that dissociates 100% in water

36
Q

Give examples of some strong bases

A

NaOH
KOH
Ca(OH)2

37
Q

Give example of a weak base

A

Ammonia

38
Q

Write the equation used to calculate [H+] of strong bases

A

[H+] = Kw / [OH-]

39
Q

Define a buffer solution

A

A mixture that minimises pH change on addition of small amounts of an acid or a base

40
Q

What are the 2 ways in which buffers can be made?

A

● Weak acid and its conjugate base

● Weak acid and a strong alkali

41
Q

In which direction does the equilibrium shift when an acid is added to a buffer solution? Why?

A

Equilibrium shifts to the left because [H+] increases and the conjugate base reacts with the H+ to remove most of the H+

42
Q

In which direction does the equilibrium shift when an alkali is added to a buffer solution? Why?

A

Equilibrium shifts to the right, because [OH-] increases and the small concentration of H+ reacts with OH-. To restore the H+ ions HA dissociates shifting the equilibrium

43
Q

Which buffer system maintains blood pH at 7.4? What happens when acid/alkali is
added?

A

H+ + HCO3– ⇌ CO2 + H2O
Add OH- → reacts with H+ to form H2O, then
shifts equilibrium left to restore H+ lost
Add H+ → equilibrium shifts to the right, removing
excess H+

44
Q

What is a titration?

A

The addition of an acid/base of known
concentration to a base/acid to determine the
concentration. An indicator is used to show
that neutralization has occurred, as is a pH
meter.

45
Q

Define the term equivalence point

A

The point at which the exact volume of base has
been added to just neutralise the acid, or
vice-versa

46
Q

What is the end point?

A

The point at which pH changes rapidly

47
Q

What are the properties of a good indicator for a reaction? (3 marks)

A

Sharp colour change (not gradual) - no more than one drop of acid/alkali needed for colour change
End point must be the same as the equivalence point otherwise titration gives wrong answer.
Distinct colour change so it is obvious when the end point has been reached.

48
Q

What indicator would you use for a strong acid-strong base titration?

A

Phenolphthalein or methyl orange, but phenolphthalein is usually used as clearer colour change.

49
Q

What indicator would you use for a strong acid-weak base titration?

A

Methyl orange

50
Q

What indicator would you use for a strong base-weak acid titration?

A

Phenolphthalein

51
Q

What indicator would you use from a weak acid-weak base titration?

A

Neither methyl orange or phenolphthalein is suitable, as neither give a sharp change at the end point.

52
Q

What colour is methyl orange in acid? In alkali?

A

Red in acid; yellow in alkali.

53
Q

What colour is phenolphthalein in acid? In alkali?

A

Colourless in acid; red in alkali

54
Q

What colour is bromothymol blue in acid? In alkali?

A

Yellow in acid and blue in alkali

55
Q

Describe how to use a pH metre

A

● Remove the pH probe from storage solution and
rinse with distilled water
● Dry the probe and place it into the solution with
unknown pH
● Let the probe stay in the solution until it gives a
settled reading

A Level Chemistry OCR A (38 decks)

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