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OCR A LEVEL CHEMISTRY > 5.1.3 Acids, Bases and Buffers > Flashcards

5.1.3 Acids, Bases and Buffers Flashcards

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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?

A

H+ (hydrogen ion)

H3O+ (oxonium ion) as protons react with H2O to form it

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6
Q

What ions causes a solution to be alkaline?

A

OH-

hydroxide ion

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7
Q

Write an equation for the ionisation of water

A

2 H2O (l) H3O+ (aq) + OH- (aq)

H2O (l) H+ (aq) + OH- (aq)

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8
Q

Give an example of a monobasic acid

A

HCL

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9
Q

Give an example of a dibasic acid

A

H2SO4

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10
Q

Give an example of a tribasic acid

A

H3PO4

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11
Q

Identify the acid pairs for the reaction below

CH3COOH + H2O CH3COO- + H3O+

A 
CH3COOH = ACID 1
CH3COO- = BASE 1
H30+ = ACID 2
H2O = BASE 2
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12
Q

Define strong acid

A

Acids dissociates 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 the 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 acids

A

Methanoic acid, any organic acid

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17
Q

What is the 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

Write the acid dissociation constant expression

A

For acid HA,
HA H+ + A-

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

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20
Q

What does a larger Ka value mean?

A

Larger the Ka. the greater the extent of disccociation

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21
Q

Write the equation used to convert Ka into pKa

A

pka = -log (Ka)

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22
Q

Write the equation used to convert pKa into Ka

A

ka = 10 ^-pKa

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23
Q

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

A

Smaller the pKa, stronger the acid

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24
Q

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

A

pH = -log [H+]

25
Q

Write equation used to convert pH into conc of H+

A

[H+] = 10^-ph

26
Q

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

A

pH scale allows a wide ranger of H+ concentrations to be expressed as a simple positive value

27
Q

What is the relationship between pH and [H+]

A

High pH value means a small [H+]

28
Q

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

A

a factor of 10

29
Q

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

A

[H+] = [HA]

30
Q

Write the equation used to calculate [H+] of weak acids

A

[H+] = sqrt ka x [HA]

31
Q

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

A

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

32
Q

Write the expression for ionic product of water, kw

A

kw = [H+][OH-]

33
Q

What is the units for kw

A

mol2dm-6

34
Q

What is the value of kw at 298k?

A

1..0 x 10^-14

35
Q

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

A

Temperature only

if temp increases, the equilibrium moves to the right so kw increases and the pH of pure water decreases

36
Q

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

A

-14

37
Q

Define the term strong base

A

Base that dissociates 100% in water

38
Q

Give examples of some strong bases

A

NaOH
KOH
Ca(OH)2

39
Q

Give example of a weak base

A

Ammonia

40
Q

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

A

[H+] = kw / [OH-]

41
Q

Define a buffer solution

A

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

42
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

43
Q

In which direction does 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+

44
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 concentrations of H+ reacts with OH- to restore the H+ ions HA dissociates shifting the equilibrium

45
Q

Write the equation used to calculate [H+] of buffer solution

A

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

46
Q

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

A

H+ + HCO3 CO2 + H2O

Adding OH-
reacts with H+ to form H2O
then shifts equilibrium left to restore H+ lost

Adding H+
equilibrium shifts to the right to remove excess H+

47
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 neutralisation has occurred, as is a pH meter

48
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

49
Q

What is the end point?

A

the point at which pH changes rapidly

50
Q

What are the good properties of a good indicator for a reaction?

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

51
Q

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

A

phenolphthalein or methyl orange

Phenolphthalein is usually used as clearer colour change

52
Q

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

A

Methyl orange

53
Q

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

A

Phenolphthalein

54
Q

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

A

neither methyl orange of phenolphthalein is suitable as neither gives a sharp change at the end point

55
Q

What colour is methyl orange in acid or alkali?

A

Red in acid

Yellow in alkali

56
Q

What colour is phenolphthalein in acid or alkali?

A

Colourless in acid

red in alkali

57
Q

What colour is bromothymol blue in acid or alkali?

A

yellow in acid

blue in alkali

58
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

OCR A LEVEL CHEMISTRY (48 decks)

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