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AQA A Level Chemistry > 1.12 Acids and Bases > Flashcards

1.12 Acids and Bases Flashcards

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1
Q

Define a Bronstead-Lowry acid

A

A proton donor

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2
Q

Define a Bronstead-Lowry base

A

Proton acceptor

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3
Q

What ion causes a solution to be acidic ?

A

•H+ (hydrogen ion)
•H3O+ (Oxonium ion).

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4
Q

What ion causes a solution to be alkaline ?

A

Hydroxide ion (OH-)

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5
Q

Write an equation for the ionisation of water

A

•2H2O (l) ⇌ H3O+ (aq) + OH- (aq)
•H2O (l) ⇌ H+ (aq) + OH- (aq).

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6
Q

Derive Kw using the formula for the ionisation of water

A

•Kc = [H+][OH-] / [H2O]
•Kc [H2O] = [H+][OH-]
•[H2O] is so large that it can be assumed that it is constant
•Therefore Kw = [H+][OH-].

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7
Q

What is the value of Kw at 298K

A

1.0 x 10 ⁻¹⁴

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8
Q

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

A

•Only temperature effects the value of Kw
•If temperature increases, the equilibrium moves to the right
•Kw increases therefore and the pH of pure water increases.

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9
Q

Why is pure water still neutral, even if pH does not equal 7 ?

A

[H+] = [OH-]

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10
Q

Give an expression for pH in terms of H+

A

pH = -log([H+])

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11
Q

What is the relationship between pH and concentration of H+?

A

Lower pH = higher concentration of H+.

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12
Q

By what scale does the concentration of H+ increase if you jump from pH 2 to 1 ?

A

x10.

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13
Q

How do you find the [H+] from pH ?

A

[H+] = 10 ⁻ᵖᴴ

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14
Q

How do you find [OH-] from pH ? (298K)

A

•Find [H+]
•Use Kw / [H+] = [OH-].

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15
Q

What is different when finding the concentration of H+ in diprotic and triprotic acids ?

A

You need to multiply the concentration of the acid by the number of protons to find [H+].

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16
Q

How do you calculate the pH of a strong alkaline solution ?

A

•Use Kw to calculate [H+] from [OH-]
•Use pH = -log([H+])

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17
Q

Define the term strong acid

A

One which fully dissociates into water : XH -> X- + H+.

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18
Q

Define the term strong base

A

One which fully dissociates into water: XOH -> X+ + OH-

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19
Q

What is the difference between a concentrated acid and strong acid ?

A

•Concentrated means many moles per dm3
•Strong refers to amount of dissociation.

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20
Q

What are weak acids and bases ?

A

Acids / bases which only partially dissociate into their ions in a solution.

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21
Q

Give some examples of strong acids

A

HCl, H2SO4, HNO3, H3PO4

22
Q

Give some examples of strong bases

A

NaOH, CaCO3, Na2CO3

23
Q

Give an example of a weak acid

A

CH3COOH, any organic acid.

24
Q

Give an example of a weak base

A

NH3

25
Q

What is Ka ?

A

•Equilibria value for weak acid dissociation
•Eg. HA ⇌ H+ + A-
•Ka = [H+][A-] / [HA].

26
Q

How would you calculate the pH of a weak acid ?

A

•Use the formula for Ka to find value for [H+]
•Use -log([H+]).

27
Q

What is a titration ?

A

•The addition of an acid/base of a known solution to an acid/base of an unknown solution to determine its concentration
•An indicator or pH meter is used to see when neutralisation occurs.

28
Q

Describe the pH dissociation curves for all combinations of weak acid/bases and strong acid/bases

A

•Strong acid-strong base starts very high, has a large sudden drop and finishes very low
•Strong acid-weak base starts slightly above 7, has a drop then finishes low
•Weak acid-weak base starts slightly above 7 and finishes slightly below 7, not much of a clear drop.

29
Q

Define the term equivalence point

A

The point at which the exact volume of the base has been added to to exactly neutralise the acid, or vice Versa

30
Q

What generally happens to the pH of the solution around the equivalence point ?

A

There is a large and rapid change in the pH, except in the weak-weak neutralisation

31
Q

How would you calculate the concentration of a reactant in a reaction if you know it’s volume and the other reactants volume/conc ?

A

•Calculate mols of one reactant
•Use balanced equation to work out the mols of the other
•Use conc=mol/vol to get answer

32
Q

What is the end point in a titration ?

A

•The volume of acid or alkali when the just changed colour: showing neutrality
•If the correct indicator is used, equivalence point should equal end point.

33
Q

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

A

•A sharp colour change to show when end point has been reached
•End point should be equivalence point.

34
Q

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

A

•Phenolphthalein
•Colourless in acid and pink in base

35
Q

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

A

•Methyl orange
•Red in acid, yellow in bases.

36
Q

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

A

•Phenolphthalein
•Colourless in acid, pink in base.

37
Q

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

A

•Neither methyl orange or phenolphthalein are suitable
•This is because there will be no sharp change in colour at the end point.

38
Q

What colour is methyl orange in acid/alkali ? At what pH does it change ?

A

•Red in acid, yellow in alkali
•Changes in around pH 4-5.

39
Q

What colour is phenolphthalein in acid/alkali ? At what pH does it change ?

A

•Colourless in acid, pink in weak alkali, red in strong alkali
•Change around pH 9-10.

40
Q

What is the half neutralisation point ?

A

When volume = half volume that has been added at the equivalence point

41
Q

Define a buffer solution

A

A solution that can resist changes in pH when a small amount of acid/alkali is added

42
Q

What do acid buffer solutions contain in general terms ?

A

A weak acid and a soluble salt of that acid that fully dissolves

43
Q

Write an equation for an acidic buffer with added acid

A

A- + H+ -> HA (salt which opposes addition of H+)

44
Q

Write an equation for an acidic buffer with added alkali

A

HA + OH -> H2O + A-

45
Q

How else can you achieve an acid buffer solution other than just mixing it’s constituents ?

A

By neutralising half of a weak acid with an alkali - forming a mixture of the unneutralised acid and soluble salt.

46
Q

What do basic buffer solutions contain in simple terms ?

A

A weak base and a soluble salt of the weak base

47
Q

How can you calculate the pH of buffer solutions ?

A

Use the Ka of the weak acid, sub in [A-] and [HA], calculate [H+] -> pH.

48
Q

How can you calculate the new pH of a buffer solution when an acid or base is added ?

A

•Calculate the number of moles of H+, A- and HA before the new acid or base is added
• Use equations to work out new moles of A- and HA -> find [H+] -> find pH

49
Q

Which buffer system maintains blood at pH 7.4 ? What happens when an acid or base is added ?

A

•H+ + HCO3- ⇌ CO2 + H2O
•Adding OH- -> reacts with H+ to form H2O, then shifts equilibrium to left to restore H+
•Adding H+ -> equilibrium shifts to the right, removing excess H+.

50
Q

What products are buffers found in ?

A

Skin and beauty products.

AQA A Level Chemistry (26 decks)

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