C15 Redox Titration

Question 1

Define reduction

Answer 1

The gain of electrons, loss of oxygen and decrease in oxidation number

Question 2

Define oxidation

Answer 2

The loss of electrons, gain of oxygen and increase in oxidation number

Question 3

What is a redox reaction?

Answer 3

A reaction in which reduction and oxidation both occur

Question 4

What is a reducing agent?

Answer 4

A substance that gets oxidised

Question 5

What is an oxidising agent?

Answer 5

A substance that gets reduced

Question 6

What is a primary standard?

Answer 6

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

Question 7

What is a standard solution?

Answer 7

A solution whoes concentration is accurately known

Question 8

State the physical appearance of KMnO4

Answer 8

Purple solid

Question 9

Explain why it is necessary to use dilute sulfuric acid when preparing the standard solution from hydrtaed ammonium iron(II) sulfate

Answer 9

To prevent oxidation of Fe3+ to Fe3+

Question 10

Explain why it is necessary to add dilute sulfuric acid to potassium permanganate

Answer 10

To provide H+ to ensure MnO4- is reduced fully to Mn2+ (to prevent the formation of MnO2)

Question 11

What colour change occurs when MnO4- is reduced to Mn2+

Answer 11

purple to colourless

Question 12

What colour change occurs when MnO4- is reduced to MnO2

Answer 12

purple to brown

Question 13

Why must KMnO4 be standardised immediately before titration?

Answer 13

It could decompose in light

Question 14

KMnO4 is ‘self indicating’ what does that mean?

Answer 14

Its acts as its own indicator (purple to colourless at end-point)

Question 15

State the end point colour change in a KMnO4 titration

Answer 15

pale pink colour persists

Question 16

How do you read the volume of KMnO4 in a burette?

Answer 16

Read the top of the meniscus

Question 17

What reagent is used to standardise KMnO4?

Answer 17

ammonium iron(II) sulfate

Question 18

State how to calculate molarity

Answer 18

molarity (M) = moles/volume

Question 19

What are the units for molarity

Answer 19

mol/L

Question 20

Why are iron tablets sometimes medically percribed?

Answer 20

To prevent anaemia

Question 21

What iron compound do iron tablets contain?

Answer 21

iron sulfate, FeSO4

Question 22

Describe how to prepare a Fe2+ solution from iron tablets

Answer 22

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

Question 23

Why is it not possible to make a standard solution of iodine by direct weighing?

Answer 23

Iodine sublimes at room temperature

Question 24

What reagents are needed to to make a standard solution of iodine?

Answer 24

KMnO4, excess KI, H2SO4

Question 25

Why is excess KI needed when making a standard solution of iodine?

Answer 25

1) To ensure that all KMnO4 reacts 2) TO keep iodine in solution (KI reacts with I2 to form soluble KI3)

Question 26

Why is sulfuric acid needed when making a standard solution of iodine?

Answer 26

To provide H+ to ensure MnO4- is reduced fully to Mn2+ (to prevent the formation of MnO2)

Question 27

State the physical appearance of sodium thiosulfate

Answer 27

white crystalline solid

Question 28

Why is sodium thiosulfate not a primary standard?

Answer 28

1) It cannot be obtained in a pure state 2) It is efflorescent (loses water to the air)

Question 29

How is sodium thiosulfate standardised?

Answer 29

By titrating it against iodine

Question 30

Name the indicator used during iodine/sodium thiosulfate titrations

Answer 30

Starch

Question 31

When is starch added during a sodium thiosulfate titration?

Answer 31

When the colour becomes pale yellow

Question 32

What is the ngoing colour change in a sodium thiosulfate titration

Answer 32

red-brown–> yellow–> pale yellow

Question 33

What is the end point colour change during a sodium thiosulfate titration?

Answer 33

blue-black–> colourless

Question 34

Desribe how to take a 25cm3 portion of iodine from a 500cm3 volumetric flask

Answer 34

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

Question 35

What compound is found in household bleach?

Answer 35

NaClO

Question 36

Why must bleach be diluted before carrying out the titration?

Answer 36

Undiluted bleach would require the addition of large amount of I- ions

Question 37

Describe how to dilute a 25cm3 bleach in the lab

Answer 37

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

Question 38

Why is distilled water better than deionised water for experiments?

Answer 38

Deionised water only has ions removed so could still contain other substances eg. Chlorine which is an oxidising agent. Distilled water is pure water.