Redox Titrations Flashcards
Magnate (VII) Ion .vs.
Iron (II) Ion
Iodine .vs.
Thiosulphate
What do roman numerals indicate
the oxidation numbers of the transition elements
Redox equation for magnate (VII) Ion vs Iron (II) Ion
MnO4- + 5Fe++ +8H+ -> Mn++ + 4H2O
where does the MnO4- come from
KMnO4
where does the 5Fe++ come from
FeSO4
where does the 8H+ come from?
H2SO4
what colour is KMnO4
purple
what colour is FeSO4
colourless
what colour is H2SO4
colourless
is indicator used in the magnate (VII) Ion vs Iron (II) Ion titration
no
what is KMnO4
a powerful oxidising agent
easily reduced
equation for when KMnO4 is reduced
(+7)___ (+2)
Mn —> Mn++
what colour is Mn
purple
what colour is Mn++
colourless
what happens when the Fe is oxidised in the magnate (VII) Ion vs Iron (II) Ion titration
(+2)____(+3)
Fe++ –> Fe+++
if H2SO4 is not added as H+ ions
(+7)____(+4)
Mn —-> Mn
what colour is Mn (+7)
purple
what colour is Mn (+4)
brown
second function in H2SO4 in the magnate (VII) Ion vs Iron (II) Ion titration
dilute H2SO4 prevents Fe++ reacting with water
what is it called when Fe++ reacts with water to form a new substance
hydrolysis
what is the only substance that can be used to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration
H2SO4
what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration
it would react with KMnO4 producing Cl2 gas
what would happen if you used HNO3 to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration
it is a powerful oxidising agent and would itself oxidise Fe++ instead of KMnO4
2 places where the Fe ++ ions come from
Iron (ii) sulphate or amm. Iron (ii) sulphate
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
- it is obtained pure and can be weighed accurately PRIMARY STANDARD
- not easily oxidised by air
why is KMnO4 not a primary standard 2
not obtainable in 100% purity
it is decomposed by sunlight
where do you keep KMnO4
in a brown bottle
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
because there is a sharp colour change
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
purple to colourless
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
the KMnO4 acts as it’s own indicator
2 reactants in what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration
MnO4- Fe++ and H+
2 products in what would happen if you used HCl to get H+ ions in the magnate (VII) Ion vs Iron (II) Ion titration
Fe++ Mn++ and H2O
what is significant about the first drop of KMnO4 to not turn colourless
All of the other reactants are used up
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
when the first drop of pink colour remains
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
slow to lose their purple colour
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
one of the products, Mn++ acts as an autocatalyst, but it is not present at the start
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
about 0.02 M`
a higher conc. of KMnO4 would: 2
- not be 100% soluble in water
2. too powerful an oxidising agent and would oxidise other substances in storage or in handling
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
from the top of the meniscus
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
there is a very strong purple colour and we cannot see through it
what do you weight out the ammonium iron sulphate crystals on ( magnate (VII) Ion vs Iron (II) Ion titration )
on a clock glass
what do you do with the H2SO4 at the start ( magnate (VII) Ion vs Iron (II) Ion titration )`
dilute it into water
( magnate (VII) Ion vs Iron (II) Ion titration )
what do you add to the H2SO4 and water solution
the weighed out ammonium iron (II) sulphate
( magnate (VII) Ion vs Iron (II) Ion titration )
how do you dissolve the crystals
stir with a glass rod
( magnate (VII) Ion vs Iron (II) Ion titration )
how to finish off making up the standard ammonium iron sulphate solution
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
( magnate (VII) Ion vs Iron (II) Ion titration )
what do you wash the pipette, the burette and the conical flask with
deionised water
( magnate (VII) Ion vs Iron (II) Ion titration )
with what do you wash the burette and pipette but not the conical flask
the liquid it will contain
( magnate (VII) Ion vs Iron (II) Ion titration )
what do you fill the burette with
a KMnO4 solution
( magnate (VII) Ion vs Iron (II) Ion titration )
what goes into the conical flask
ammonium iron sulphate solution
( magnate (VII) Ion vs Iron (II) Ion titration )
getting the ammonium iron sulphate into the conical flask
pour aprox. 100cm cubed into a clean dry beaker and pipette 25 into the conical flask
( magnate (VII) Ion vs Iron (II) Ion titration )
what else do you add to the conical flask
aprox 20 cm cubed of H2SO4 using a pipette
( magnate (VII) Ion vs Iron (II) Ion titration )if you dont add H2SO4
a brown precipitate is formed and the titration is ruined
( magnate (VII) Ion vs Iron (II) Ion titration ) how do you get an average figure
throw out the first number and get an average of the second and third titration
( magnate (VII) Ion vs Iron (II) Ion titration ) what do you do with the glassware when you are done
wash all glassware immediately after the titration is complete
( magnate (VII) Ion vs Iron (II) Ion titration ) why do we have to wash all glassware immediately after the titration is complete
KMnO4 is known to stain glassware
( magnate (VII) Ion vs Iron (II) Ion titration )
2 times that you add H2SO4 in the experiment
when making up the ammonium iron sulphate solution and before carrying out the titration (in the conical flask)
( magnate (VII) Ion vs Iron (II) Ion titration )
why do we add H2SO4 twice?
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
m/l =
g/l x Mr
Iodine vs. Thiosulphate titration equation
2S2O3 -2 + I2 —> S4O6 -2 + 2I-
S2O3
thiosulphate
I2
iodine
Iodine vs. Thiosulphate titration
source of thiosulphate
Na2S2O3
Na2S2O3
sodium Thioulphate
Iodine vs. Thiosulphate titration
source of iodine
I2
Iodine vs. Thiosulphate titration
what colour is 2S2O3 -2
colourless
Iodine vs. Thiosulphate titration
what colour is I2
red/brown
Iodine vs. Thiosulphate titration
what colour is S4O6-2
colourless
Iodine vs. Thiosulphate titration
what colour is 2I-
colourless
Iodine vs. Thiosulphate titration
the usual souce of S2O3-2
Na2S2O3
Iodine vs. Thiosulphate titration
describe Na2S2O3
exists as colourless crystals
Iodine vs. Thiosulphate titration
describe I2
exists as red/brown crystals
Iodine vs. Thiosulphate titration
in this titration, what do I2 and Na2S2O3 have in common
they are both in an aqueous solution
Iodine vs. Thiosulphate titration
4 colours that may be seen in the conical flask during the titration
red/brown
straw yellow
blue/black
colourless
Iodine vs. Thiosulphate titration
why might red/brown be seen in the conical flask?
due to a very high concentration of I2 in the conical flask at the start of the titration
Iodine vs. Thiosulphate titration
why might straw yellow be seen in the conical flask?
now there is very little I2 left in the conical flask as the s2O3-2 has converted most of it to I-
Iodine vs. Thiosulphate titration
why might blue/black be seen in the conical flask?
starch indicator is now added
although there is very little I2 left, it is still enough to give the blue black colour
Iodine vs. Thiosulphate titration
why might colourless be seen in the conical flask?
at this point all of the I2 reactant is used up so that the tiny amount of starch appears colourless
Iodine vs. Thiosulphate titration
end point colour change
blue/black -> colourless
Iodine vs. Thiosulphate titration
why is an indicator even necessary for the titration as there is a colour change without it
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
Iodine vs. Thiosulphate titration
why not add starch in at the start, why do we wait until near the end to add it?
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
Iodine vs. Thiosulphate titration
can you use tap water to make up the thiosulphate solution? why?
no because it contains dissolved chlorine and it would react with the iodine
Iodine vs. Thiosulphate titration
formula for if chlorine reacted with iodine
Cl2 + 2I- –> 2Cl- + I2
Iodine vs. Thiosulphate titration
why is the chlorine reacting with the iodine a problem
the titration is converting I2 to I- and the chlorine would be turning I- into I2, the opposite
would cause major inaccuracy
Iodine vs. Thiosulphate titration
what is also added to the conical flask with the iodine
KI crystals
Iodine vs. Thiosulphate titration
2 functions of adding KI crystals
- help I2 dissolve in water as it is non polar
- to give the maximum amount of I2 available for reaction
To Determine the % of NaClO in Household Bleach Titration
first step in the titration
pipette 25cm cubed of household bleach into a 250cm cubed volumetric flask
To Determine the % of NaClO in Household Bleach Titration
diluting the bleach
add deionised water to the volumetric flask and make up to the mark. stopper and invert 20 times
To Determine the % of NaClO in Household Bleach Titration
how much is the bleach diluted
diluted to 1 tenth the strength of the original solution
To Determine the % of NaClO in Household Bleach Titration
what do you fill the burette with
Na2S2O3 solution,
To Determine the % of NaClO in Household Bleach Titration
getting diluted bleach into the conical flask
pour approx. 100cm cubed into a beaker and pipette 25cm cubed across from there
To Determine the % of NaClO in Household Bleach Titration
2 things you add to the conical flask in addition to the diluted bleach
approx. 20cm cubed of H2SO4
approx. 10cm cubed of KI
To Determine the % of NaClO in Household Bleach Titration
after adding the three components to the conical flask, what can be noted?
the red/brown colour of the I2 produced
To Determine the % of NaClO in Household Bleach Titration
what is the active ingredient in bleach which reacts with KI to make I2
NaClO
To Determine the % of NaClO in Household Bleach Titration
when you starch the titration, you keep going until what happens?
until a pale straw yellow colour appears
To Determine the % of NaClO in Household Bleach Titration
when a pale straw yellow colour appears
add a few drops of starch indicator until a blue/black colour is observed
To Determine the % of NaClO in Household Bleach Titration
when are you at end point
as soon as the blue/black colour disappears to give a colourless solution
how else can FeSO4 and Na2S2O3 be found
in their hydrated crystalline forms
FeSO4 . XH2O
Na2S2O3 . XH2O
