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A-level chemistry unit 4 > unit 4 section 4 Acids, Bases and PH > Flashcards

unit 4 section 4 Acids, Bases and PH Flashcards

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1
Q

what is a Brønsted-lowry acid

A

they are proton donors. they release hydrogen ions when mixed with water.

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2
Q

what is a Brønsted-lowry base

A

they accept protons. when in a solution they take hydrogen ions from water molecules.

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3
Q

what does dissociation mean

A

it means that they break up into their positive and negative ions.

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4
Q

what dictates the amount of dissociation in a reaction between an acid and an alkali

A

how weak or strong an acid or base is.
- strong acids almost completely dissociate in water
- strong bases ionise almost completely in water
- weak acids dissociate very slightly in water and a small amount of H+ ions are released ( equilibrium lies to the left )
- weak bases only slightly dissociate in water too ( equilibrium lies to the left )

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5
Q

what can water dissociate into and show the formula for this reaction and the ionic formula for this reaction

A

only a very small amount of water can dissociate, they dissociate into hydroxonium ions and hydroxide ion.
H2O + H2O = H3O+ + OH-
the ionic equation:
H2O = H+ + OH-

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6
Q

since only a small amount of water dissociates, where does the equilibrium lie and explain why.
(3)

A

the equilibrium lies way over to the left. this is because there is a huge amount of water compared to the H+ & OH- ions that the amount of water is considered to be constant.

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7
Q

what is the equation for the ionic product of water and how is it dervided
(2)

A

ionic product of water
Kw = [H+] [OH-]

it is dervided by times the kc for water by the concentration of water Kc of water = [H+] [OH-] / [H2O]

kw = [H+] [OH-] / [H2O] X [H2O] = [H+] [OH-]

Kw = [H+] [OH-]

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8
Q

what are the conditions that give you the same value of Kw of a aqueous solution, and what is that value
(2)

A

temp - 298k or 25 degrees
the same value that is given is
1.00x10^-14 mol^2 dm^-6

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9
Q

what is the formula for the kw of pure water and explain why there is a difference
(2)

A

Kw = [H+]^2
this is because pure water there is always 1 H+ ion for each OH- ion.

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10
Q

what is the PH scale
(1)

A

it is the measure of the hydrogen ion concentration of a solution

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11
Q

what is the equation for PH
(1)

A

PH = -log10 [H+]

-the 10 next to the log is the base so it goes at the bottom on the calculator
- [H+] concentration of H+ ions in solution in mol dm^-3

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12
Q

how do you calculate the concentration of hydrogen ions (H+) when you are given the PH.
(1)

A

you use the equation

[H+] = 10^-PH

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13
Q

what is a monoprotic acid and give 2 examples
(3)

A

a monoprotic acid is an acid that for every molecule of the acid, it will release 1 proton when it dissociates. this means that the H+ concentration is the same as the concentration of the acid.
hydrochloric acid (HCL) & nitric acid (HNO3)

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14
Q

what is a diprotic acid and give an example of it.
(2)

A

a diprotic acid is an acid that for every molecule of acid, it will release 2 protons when it dissociates. this means the concentration of the [H+] ions is double the concentration of the acid
- sulfuric acid (H2SO4)

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15
Q

why do you have to use the Ka formula instead of the Kw formula when you are dealing with a weaker base or acid
(2)

A

this is because weak acids and bases only slightly dissociate in water, this means that the concentration of the H+ ions is not the same as the concentration of the reaction.

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16
Q

what is the formula for Ka (the acid dissociation constant )
(1)

A

Ka = [H+] [ A-] / [HA]
this can be simplified to (because we can assume that all the H+ comes from the acid)
Ka = [H+]^2 / [HA]

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17
Q

what is pKa and why is it used, also give the formula for pKa
(3)

A

pKa is calculated from Ka in the same way that PH is calculated from [H+]. they use pKa because the value of Ka varies massively between different acids, this can make the numbers hard to deal with.

pKa = -log10 (ka)

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18
Q

what is
a buffer and what are the two types
(1)

A

a buffer is a solution that resists changes in ph when a small amount of acid or alkali is added to the solution
- acidic buffers & base buffers

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19
Q

what is an acidic buffer
(3)

A

it is a buffer that has a PH lower than 7, they contain a mixture of a weak acid and one of its salt. they can resist change when an acid or a base is added to it.

20
Q

what is a base buffer
(3)

A

they have a PH that is higher than 7, they contain a mixture of a weak base and 1 of its salts. they can resist change when an acid or a base is added to it.

21
Q

why can buffer resist change even when they are diluted in water
(3)

A

this is because if a small amount of water is added to the buffer, the water only slightly dissociates. so the extra H+ ions and the OH- ions push the equilibrium the same amount in both directions, therefore the the equilibrium is left unchanged.

22
Q

what are some of the applications of buffers
(3)

A
  • they are used in shampoo, this is to maintain the ph whilst you wash your hair
  • blood - this is in order to keep the blood at a PH very near to 7.4
  • ensuring all the tissues in the body are kept at the right PH.
23
Q

what are the two ways of making a buffer
(2)

A
  • mixing a weak acid with its salts
  • add a small amount of acid to a weak alkali so that only a small amount of the acid is neutralised to make a salt.
24
Q

what does the HA in the acid dissociation constant represent

A

it represent the acid in the equation
e.g.
HA + OH = A^- + H20

25
Q

what does it mean if you see pka or ka in a question

A

it means you are dealing with a weak acid and you make need to use the equation for Ka ( the acid dissociation constant)

26
Q

identify two strong bases

A

NaOH
KOH

26
Q

give an example of a weak acid and what weak acids normally are

A

ethanoic acid
- weak acids tend to be carboxylic acids

27
Q

identify a weak base

A

NH3 ammonia

28
Q

what is the generic formula for an acid reacting with a base

A

HA + B = BH^+ + A^-

29
Q

where does the equilibrium lie in a reaction with a strong acid and water

A

it lies far to the right

29
Q

where does the equilibrium lie in a reaction between a weak acid and water

A

the equilibrium lies far to the left

30
Q

what equation do you use when you are working out the PH of a weak acid that doesn’t contain a buffer

A

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

we can use this equation as we can assume that all of the H+ ions comes from the acid

31
Q

what equation do you use when you are working out the PH of a weak acid that contains a buffer

A

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

32
Q

what is a titration used for

A

to work out the concentration of an acid or base

33
Q

what are the axis on a titration graph

A

ph - y axis
volume of base - x axis

34
Q

how does the titration curve of a strong base strong acid look like

A

-the graph starts at around ph 1 as there is an excess of a strong acid
- ends at ph 13 as there is now an excess of a stong base
- the graph has an S shape with a sharp increase in gradient at the middle of the graph

35
Q

how does the titration curve of a strong base and a weak acid look like

A
  • the graph starts at PH 1 as there is an excess of strong acid
  • the graph ends at a PH of around 9 as there is now an excess of weak base
  • the graph has an S shape and there is a steep gradient around the middle of the graph
36
Q

how does the titration curve of a weak acid and a strong base look like

A
  • the graph starts at a PH around 5 as there is an excess of weak acid
  • the graph ends at PH 13 as there is now an excess of strong base
  • the graph has an S shape and it has a steep gradient at the middle of the graph
37
Q

how does a titration curve between a weak acid and a weak base look like

A
  • the graph starts at a PH around 5 as there is an excess of weak acid
  • the graph ends at a PH around 9 as there is now an excess of weak base
  • the graph has an S shape and still has a steep gradient but isn’t as steep as the over curves
38
Q

what is the equivalence point or the end point on a titration curve

A

it is the point where the acid has been fully nuetralised by the base. this is the point on the graph where there is a sharp change in PH ( steep gradient )

39
Q

what is the half nuetralisation point on a titration curve and what can it be used to do

A

it is the half way point between 0 and the equivalence point on the curve. it can be used to calculate the pKa of a weak acid by taking the PH at this point.

40
Q

what is an indicator used for in a titration

A

it is used to determine the end point of a reaction

41
Q

what are the two most common indicators and at what PH do they change colour

A
  • methyl orange : PH 3 - 4.5
  • phenolpthalein : PH 8.2-4.5
42
Q

how do you decide which indicator to use for a titration when you are given the titration curve

A

you at equivalence point/ end point of the reaction. this is where the solution is fully neutralised and there is a steep change in PH. you then look for the indicator that will change colour around that point

43
Q

what is the colour change for methyl orange and phenolpthalein

A

methyl orange - red to yellow
phenolpthalein - colourless to pink

44
Q

what indicator should be used in a reaction between a weak acid and a weak base

A

no indicator can be used as there is no sharp pH change, therefore a pH meter must be used

45
Q

explain what a titration curve of a diprotic acid looks like

A

for diprotic acids, they neutralise in two steps. this is because the 2 H+ ions leave separately, they do not leave at the same time. this means that there are two equivalence points, one for each proton that is dissociated

A-level chemistry unit 4 (4 decks)

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