C15 Redox Titration Flashcards
Define reduction
The gain of electrons, loss of oxygen and decrease in oxidation number
Define oxidation
The loss of electrons, gain of oxygen and increase in oxidation number
What is a redox reaction?
A reaction in which reduction and oxidation both occur
What is a reducing agent?
A substance that gets oxidised
What is an oxidising agent?
A substance that gets reduced
What is a primary standard?
A substance that can be obtained as a stable, pure and soluble solid form so that it can be weighed out and dissolved in water to give a solution of accurately-known concentration
What is a standard solution?
A solution whoes concentration is accurately known
State the physical appearance of KMnO4
Purple solid
Explain why it is necessary to use dilute sulfuric acid when preparing the standard solution from hydrtaed ammonium iron(II) sulfate
To prevent oxidation of Fe3+ to Fe3+
Explain why it is necessary to add dilute sulfuric acid to potassium permanganate
To provide H+ to ensure MnO4- is reduced fully to Mn2+ (to prevent the formation of MnO2)
What colour change occurs when MnO4- is reduced to Mn2+
purple to colourless
What colour change occurs when MnO4- is reduced to MnO2
purple to brown
Why must KMnO4 be standardised immediately before titration?
It could decompose in light
KMnO4 is ‘self indicating’ what does that mean?
Its acts as its own indicator (purple to colourless at end-point)
State the end point colour change in a KMnO4 titration
pale pink colour persists
How do you read the volume of KMnO4 in a burette?
Read the top of the meniscus
What reagent is used to standardise KMnO4?
ammonium iron(II) sulfate
State how to calculate molarity
molarity (M) = moles/volume
What are the units for molarity
mol/L
Why are iron tablets sometimes medically percribed?
To prevent anaemia
What iron compound do iron tablets contain?
iron sulfate, FeSO4
Describe how to prepare a Fe2+ solution from iron tablets
1) Crush tablets and dissolve in distilled water in a beaker 2) Transfer solution with washing to a volumetric flask using a funnel 3) Bring bottom of meniscus to the mark using distilled water, slowly near the end point using a dropper 4) Stopper and invert 20 times
Why is it not possible to make a standard solution of iodine by direct weighing?
Iodine sublimes at room temperature
What reagents are needed to to make a standard solution of iodine?
KMnO4, excess KI, H2SO4
Why is excess KI needed when making a standard solution of iodine?
1) To ensure that all KMnO4 reacts 2) TO keep iodine in solution (KI reacts with I2 to form soluble KI3)
Why is sulfuric acid needed when making a standard solution of iodine?
To provide H+ to ensure MnO4- is reduced fully to Mn2+ (to prevent the formation of MnO2)
State the physical appearance of sodium thiosulfate
white crystalline solid
Why is sodium thiosulfate not a primary standard?
1) It cannot be obtained in a pure state 2) It is efflorescent (loses water to the air)
How is sodium thiosulfate standardised?
By titrating it against iodine
Name the indicator used during iodine/sodium thiosulfate titrations
Starch
When is starch added during a sodium thiosulfate titration?
When the colour becomes pale yellow
What is the ngoing colour change in a sodium thiosulfate titration
red-brown–> yellow–> pale yellow
What is the end point colour change during a sodium thiosulfate titration?
blue-black–> colourless
Desribe how to take a 25cm3 portion of iodine from a 500cm3 volumetric flask
1) Pour some iodine solution into a clean, dry beaker 2) Rinse pipette with distilled water 3) Rinse pipette with a small amount of the iodine solution 4) Fill pipette using pipette filler until bottom of meniscus is on the mark 5) Read at eye level
What compound is found in household bleach?
NaClO
Why must bleach be diluted before carrying out the titration?
Undiluted bleach would require the addition of large amount of I- ions
Describe how to dilute a 25cm3 bleach in the lab
1) Use funnel to transfer 25cm3 bleach to volumetric flask 2) Rinse beaker into the volumetric flask using distilled water 3) Bring bottom of meniscus to the mark using distilled water, slowly near the end point using a dropper 4) Stopper and invert 20 times
Why is distilled water better than deionised water for experiments?
Deionised water only has ions removed so could still contain other substances eg. Chlorine which is an oxidising agent. Distilled water is pure water.
