Oxidation-Reduction Volumetric Analysis Flashcards
Name an oxidising agent used in Oxidation-Reduction titrations
KMnO4/
Potassium Permanganate/
Potassium Manganate (VII)
Describe KMNO4
- Purple coloured solid
- Not a primary standard as it cannot be obtained in a state of high purity
- Unstable and decomposes in the presence of sunlight and heat
Write the half equations in relation to KMnO4 where KMnO4 acts as an oxidising agent.
MnO4- + 8H+ +5e- -> Mn2+ + 4H20
Why is it necessary to add an acid, and which acid, to KMnO4
To provide H+ ions for Mn7+ to reduce to Mn2+ (purple to colourless, otherwise it would be Mn4+, a brown colour)
Sulfuric acid is used because Nitric acid would interfere (it’s an strong oxidising agent) and in HCl, Cl- would oxidise to Cl2.
Does an indicator need to be used in the titration involving KMnO4?
No, Potassium Permanganate is self-indicating
Why is KMnO4 read from the top of the meniscus and not from the bottom?
because of it’s intense colour, the bottom would be difficult to see.
What primary standard solution is used in Oxidation-Reduction titrations?
Ammonium Iron (II) Sulfate
Write the half equation of Iron from Fe2+ to Fe3+
Fe2+ -> Fe3+ + e-
What is the molecular formula for ammonium Iron (II) Sulfate
(NH4)2SO4.FeSO4.6H20
What is necessary to add when making up a solution of ammonium Iron (II) Sulfate and why
A small amount of Sulfuric acid is added to prevent the oxidation of Fe2+ to Fe3+
What is haemoglobin
It is the molecule that is responsible for the transport of oxygen in our blood
Why are iron tablets sometimes medically prescribed
To treat Amaenia
Name reducing agent used in oxidation-reduction reactions
Sodium thiosulfate/ Na2S2O3
What are some uses of Sodium thiosulfate
Analysing the amount of certain substances in water, extracting gold, antidote to cyanide poisoning, for developing photographs by chemical means
Describe Na2S2O3
- colourless crystalline
- common formula Na2S203.5H20
What is oxidised to the tetrathionate ion in the reaction between Iodine and a thiosulfate ion
the thiosulfate
Why is thiosulfate not a primary standard?
It cannot be obtained in a sufficiently pure state and the crystals are efflorescent
Why is Iodine not a primary standard?
because iodine slightly vaporises even at room temp. it is also completely insoluble in water
Give two reasons why excess potassium iodide is added to the reaction between acidified potassium ions with iodide ions
- all the KMnO4 reacts completely with the I- ions to form the maximum amount of I2
- Iodine is only sparingly soluble in water but is readily soluble in potassium iodide solution. When iodine reacts with the potassium iodide ion it forms tri-iodide ions which have the same chemical behaviour as Iodine
What is the colour change of Iodine in the reaction between thiosulfate and iodine. (without the indicator)
Reddish brown to colourless
What indicator is used in the reaction of thiosulfate and iodine, describe how its used and its colour change
strach
add it once most of the iodine has disappeared
black-blue to colourless
Why is the starch indicator only added when most of the iodine has reacted?
If the starch were added at an earlier stage the iodine present would become strongly adsorbed on to the starch and this would make the titration less accurate since the iodine would be desorbed slowly to react with the thiosulfate.
In the experiment to standardise a solution of sodium thiosulfate, why is it not necessary to know the precise mass of the sodium thiosulfate crystals?
The purpose of the experiment is to standardise the sodium thiosulfate, from the titration results we will be able to find the origional mass. Also it is not a primary substance.
