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Chemistry (Tim) > Redox Titrations > Flashcards

Redox Titrations Flashcards

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1
Q

Magnate (VII) Ion .vs.

A

Iron (II) Ion

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2
Q

Iodine .vs.

A

Thiosulphate

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3
Q

What do roman numerals indicate

A

the oxidation numbers of the transition elements

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4
Q

Redox equation for magnate (VII) Ion vs Iron (II) Ion

A

MnO4- + 5Fe++ +8H+ -> Mn++ + 4H2O

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5
Q

where does the MnO4- come from

A

KMnO4

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6
Q

where does the 5Fe++ come from

A

FeSO4

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7
Q

where does the 8H+ come from?

A

H2SO4

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8
Q

what colour is KMnO4

A

purple

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9
Q

what colour is FeSO4

A

colourless

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10
Q

what colour is H2SO4

A

colourless

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11
Q

is indicator used in the magnate (VII) Ion vs Iron (II) Ion titration

A

no

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12
Q

what is KMnO4

A

a powerful oxidising agent

easily reduced

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13
Q

equation for when KMnO4 is reduced

A

(+7)___ (+2)

Mn —> Mn++

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14
Q

what colour is Mn

A

purple

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15
Q

what colour is Mn++

A

colourless

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16
Q

what happens when the Fe is oxidised in the magnate (VII) Ion vs Iron (II) Ion titration

A

(+2)____(+3)

Fe++ –> Fe+++

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17
Q

if H2SO4 is not added as H+ ions

A

(+7)____(+4)

Mn —-> Mn

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18
Q

what colour is Mn (+7)

A

purple

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19
Q

what colour is Mn (+4)

A

brown

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20
Q

second function in H2SO4 in the magnate (VII) Ion vs Iron (II) Ion titration

A

dilute H2SO4 prevents Fe++ reacting with water

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21
Q

what is it called when Fe++ reacts with water to form a new substance

A

hydrolysis

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22
Q

what is the only substance that can be used to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

H2SO4

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23
Q

what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

it would react with KMnO4 producing Cl2 gas

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24
Q

what would happen if you used HNO3 to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

it is a powerful oxidising agent and would itself oxidise Fe++ instead of KMnO4

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25
Q

2 places where the Fe ++ ions come from

A

Iron (ii) sulphate or amm. Iron (ii) sulphate

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26
Q

2 reasons why amm. iron sulphate is better than iron sulphate for the what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A
  • it is obtained pure and can be weighed accurately PRIMARY STANDARD
  • not easily oxidised by air
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27
Q

why is KMnO4 not a primary standard 2

A

not obtainable in 100% purity

it is decomposed by sunlight

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28
Q

where do you keep KMnO4

A

in a brown bottle

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29
Q

why is no indicator necessary in the what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

because there is a sharp colour change

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30
Q

what is the colour change in the what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

purple to colourless

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31
Q

what is the indicator in the what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

the KMnO4 acts as it’s own indicator

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32
Q

2 reactants in what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

MnO4- Fe++ and H+

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33
Q

2 products in what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

Fe++ Mn++ and H2O

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34
Q

what is significant about the first drop of KMnO4 to not turn colourless

A

All of the other reactants are used up

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35
Q

what is the actual end point of the what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

when the first drop of pink colour remains

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36
Q

first few drops of KMnO4 in what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

slow to lose their purple colour

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37
Q

why does the colour change speed up in the what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

one of the products, Mn++ acts as an autocatalyst, but it is not present at the start

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38
Q

what conc is the KMnO4 used in the what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

about 0.02 M`

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39
Q

a higher conc. of KMnO4 would: 2

A
  1. not be 100% soluble in water

2. too powerful an oxidising agent and would oxidise other substances in storage or in handling

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40
Q

where do you read the meniscus in the what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

from the top of the meniscus

41
Q

why do we read from the top of the meniscus in the what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration

A

there is a very strong purple colour and we cannot see through it

42
Q

what do you weight out the ammonium iron sulphate crystals on ( magnate (VII) Ion vs Iron (II) Ion titration )

A

on a clock glass

43
Q

what do you do with the H2SO4 at the start ( magnate (VII) Ion vs Iron (II) Ion titration )`

A

dilute it into water

44
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

what do you add to the H2SO4 and water solution

A

the weighed out ammonium iron (II) sulphate

45
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

how do you dissolve the crystals

A

stir with a glass rod

46
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

how to finish off making up the standard ammonium iron sulphate solution

A

transfer the solution into a clean volumetric flask, pour washings of everything in and bring up to the mark with water
stopper and invert 20 times

47
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

what do you wash the pipette, the burette and the conical flask with

A

deionised water

48
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

with what do you wash the burette and pipette but not the conical flask

A

the liquid it will contain

49
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

what do you fill the burette with

A

a KMnO4 solution

50
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

what goes into the conical flask

A

ammonium iron sulphate solution

51
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

getting the ammonium iron sulphate into the conical flask

A

pour aprox. 100cm cubed into a clean dry beaker and pipette 25 into the conical flask

52
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

what else do you add to the conical flask

A

aprox 20 cm cubed of H2SO4 using a pipette

53
Q

( magnate (VII) Ion vs Iron (II) Ion titration )if you dont add H2SO4

A

a brown precipitate is formed and the titration is ruined

54
Q

( magnate (VII) Ion vs Iron (II) Ion titration ) how do you get an average figure

A

throw out the first number and get an average of the second and third titration

55
Q

( magnate (VII) Ion vs Iron (II) Ion titration ) what do you do with the glassware when you are done

A

wash all glassware immediately after the titration is complete

56
Q

( magnate (VII) Ion vs Iron (II) Ion titration ) why do we have to wash all glassware immediately after the titration is complete

A

KMnO4 is known to stain glassware

57
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

2 times that you add H2SO4 in the experiment

A

when making up the ammonium iron sulphate solution and before carrying out the titration (in the conical flask)

58
Q

( magnate (VII) Ion vs Iron (II) Ion titration )

why do we add H2SO4 twice?

A

the iron ion only needs a little but the MnO4 ion needs a lot, so more is added.
it is impractical to add it all into the FeSO4 solution

59
Q

m/l =

A

g/l x Mr

60
Q

Iodine vs. Thiosulphate titration equation

A

2S2O3 -2 + I2 —> S4O6 -2 + 2I-

61
Q

S2O3

A

thiosulphate

62
Q

I2

A

iodine

63
Q

Iodine vs. Thiosulphate titration

source of thiosulphate

A

Na2S2O3

64
Q

Na2S2O3

A

sodium Thioulphate

65
Q

Iodine vs. Thiosulphate titration

source of iodine

A

I2

66
Q

Iodine vs. Thiosulphate titration

what colour is 2S2O3 -2

A

colourless

67
Q

Iodine vs. Thiosulphate titration

what colour is I2

A

red/brown

68
Q

Iodine vs. Thiosulphate titration

what colour is S4O6-2

A

colourless

69
Q

Iodine vs. Thiosulphate titration

what colour is 2I-

A

colourless

70
Q

Iodine vs. Thiosulphate titration

the usual souce of S2O3-2

A

Na2S2O3

71
Q

Iodine vs. Thiosulphate titration

describe Na2S2O3

A

exists as colourless crystals

72
Q

Iodine vs. Thiosulphate titration

describe I2

A

exists as red/brown crystals

73
Q

Iodine vs. Thiosulphate titration

in this titration, what do I2 and Na2S2O3 have in common

A

they are both in an aqueous solution

74
Q

Iodine vs. Thiosulphate titration

4 colours that may be seen in the conical flask during the titration

A

red/brown
straw yellow
blue/black
colourless

75
Q

Iodine vs. Thiosulphate titration

why might red/brown be seen in the conical flask?

A

due to a very high concentration of I2 in the conical flask at the start of the titration

76
Q

Iodine vs. Thiosulphate titration

why might straw yellow be seen in the conical flask?

A

now there is very little I2 left in the conical flask as the s2O3-2 has converted most of it to I-

77
Q

Iodine vs. Thiosulphate titration

why might blue/black be seen in the conical flask?

A

starch indicator is now added

although there is very little I2 left, it is still enough to give the blue black colour

78
Q

Iodine vs. Thiosulphate titration

why might colourless be seen in the conical flask?

A

at this point all of the I2 reactant is used up so that the tiny amount of starch appears colourless

79
Q

Iodine vs. Thiosulphate titration

end point colour change

A

blue/black -> colourless

80
Q

Iodine vs. Thiosulphate titration

why is an indicator even necessary for the titration as there is a colour change without it

A

it would be difficult to note the exact point where straw yellow turns to colourless, but starch is added to give a sharp colour change from blue / black to colourless

81
Q

Iodine vs. Thiosulphate titration

why not add starch in at the start, why do we wait until near the end to add it?

A

if it was added when there was a very high concentration of I2 in the conical flask, it would not disappear immediately at end point, it will persist for a short time afterwards causing inaccuracy

82
Q

Iodine vs. Thiosulphate titration

can you use tap water to make up the thiosulphate solution? why?

A

no because it contains dissolved chlorine and it would react with the iodine

83
Q

Iodine vs. Thiosulphate titration

formula for if chlorine reacted with iodine

A

Cl2 + 2I- –> 2Cl- + I2

84
Q

Iodine vs. Thiosulphate titration

why is the chlorine reacting with the iodine a problem

A

the titration is converting I2 to I- and the chlorine would be turning I- into I2, the opposite
would cause major inaccuracy

85
Q

Iodine vs. Thiosulphate titration

what is also added to the conical flask with the iodine

A

KI crystals

86
Q

Iodine vs. Thiosulphate titration

2 functions of adding KI crystals

A
  • help I2 dissolve in water as it is non polar

- to give the maximum amount of I2 available for reaction

87
Q

To Determine the % of NaClO in Household Bleach Titration

first step in the titration

A

pipette 25cm cubed of household bleach into a 250cm cubed volumetric flask

88
Q

To Determine the % of NaClO in Household Bleach Titration

diluting the bleach

A

add deionised water to the volumetric flask and make up to the mark. stopper and invert 20 times

89
Q

To Determine the % of NaClO in Household Bleach Titration

how much is the bleach diluted

A

diluted to 1 tenth the strength of the original solution

90
Q

To Determine the % of NaClO in Household Bleach Titration

what do you fill the burette with

A

Na2S2O3 solution,

91
Q

To Determine the % of NaClO in Household Bleach Titration

getting diluted bleach into the conical flask

A

pour approx. 100cm cubed into a beaker and pipette 25cm cubed across from there

92
Q

To Determine the % of NaClO in Household Bleach Titration

2 things you add to the conical flask in addition to the diluted bleach

A

approx. 20cm cubed of H2SO4

approx. 10cm cubed of KI

93
Q

To Determine the % of NaClO in Household Bleach Titration

after adding the three components to the conical flask, what can be noted?

A

the red/brown colour of the I2 produced

94
Q

To Determine the % of NaClO in Household Bleach Titration

what is the active ingredient in bleach which reacts with KI to make I2

A

NaClO

95
Q

To Determine the % of NaClO in Household Bleach Titration

when you starch the titration, you keep going until what happens?

A

until a pale straw yellow colour appears

96
Q

To Determine the % of NaClO in Household Bleach Titration

when a pale straw yellow colour appears

A

add a few drops of starch indicator until a blue/black colour is observed

97
Q

To Determine the % of NaClO in Household Bleach Titration

when are you at end point

A

as soon as the blue/black colour disappears to give a colourless solution

98
Q

how else can FeSO4 and Na2S2O3 be found

A

in their hydrated crystalline forms
FeSO4 . XH2O
Na2S2O3 . XH2O

Chemistry (Tim) (8 decks)

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