ES5: Chlorine + Iodine Sodium Thiosulfate Titration

Question 1

What are the risks of chlorine?

Answer 1

• Toxic and corrosive so must be kept away from the skin and eyes
• Dangerous to be inhaled at high concentrations
• Must be kept away from flammable materials

Question 2

Why must chlorine be kept away from flammable materials?

Answer 2

Because it is a powerful oxidising agent

Question 3

Why is inhaling chlorine dangerous?

Answer 3

• Reacts in the lungs to form HCl
• This corrodes lung tissue
• Drowns the person as liquid fills the lungs

Question 4

What is chlorine stored in as it is transported?

Answer 4

• Specially designed pressurised tank containers

- In cylinders

Question 5

How does chlorine travel?

Answer 5

By road or by rail

Question 6

What state is chlorine transported in?

Answer 6

Liquid

Question 7

Why is chlorine transported as a liquid?

Answer 7

More chlorine can be stored in a fixed volume under pressure as a liquid than a gas

Question 8

What do tanks used to transport chlorine have if the temperature/pressure gets too high?

Answer 8

Pressure release devices which vent the tank and release chlorine gas

Question 9

What are the tanks used to transport chlorine made and lined with

Answer 9

Steel

Question 10

What to do the inside of the tanks used to transport chlorine need to be?

Answer 10

Dry

Question 11

Why do the inside of the tanks used to transport chlorine need to be dry?

Answer 11

Because chlorine reacts with water to produce HCl which is corrosive

Question 12

What to the tanks used to transport chlorine have at the top?

Answer 12

Cylindrical protective housing through which all loading and unloading is done

Question 13

What do the tanks used to transport chlorine have in them for safety during unloading?

Answer 13

A scrubber unit which ensures all air being displaced has chlorine removed

Question 14

Name 2 uses of chlorine

Answer 14

• Water treatment

- Bleach

Question 15

How is chlorine used in water treatment?

Answer 15

• To sterilise water for drinking/swimming

Question 16

How is chlorine used in bleach?

Answer 16

• To remove stains from clothing

- To kill bacteria on surfaces

Question 17

What are iodine-sodium thiosulfate titrations used for?

Answer 17

To find the concentration of an oxidising agent

Question 18

Describe a method for an iodine-sodium thiosulfate titration

Answer 18

• Measure out a certain volume of the oxidising agent and pipette into a conical flask
• Add excess potassium iodide + sulfuric acid
• Put sodium thiosulfate in the burette
• After the solution fades to pale yellow, add 2cm3 of starch solution in the flask as an indicator for iodine - turning blue when the iodine is detected
• Add sodium thiosulfate one drop at a time until the blue colour disappears
• Measure the amount of sodium thiosulfate needed to do this
• Repeat

Question 19

What happens after potassium iodide is added to the oxidising agent? Give an equation with chlorate as the oxidising agent

Answer 19

• Oxidising agent oxidises iodide ions to iodine

ClO- + 2I- + 2H+ =====> I2 + H2O + Cl-

Question 20

Give an equation for the reduction of iodine ions by sodium thiosulfate.

Answer 20

I2(aq) + 2S2O3^2- (aq) =====> 2I- (aq) + S4O62- (aq)

Question 21

What are the steps for calculating the concentration of an oxidising agent?

Answer 21

• Convert all cm3 readings to dm3
• Write the 2 ionic equations for the titration and calculate their stoichiometry
• Calculate the moles of sodium thiosulfate
  (moles=conc x volume)
• Calculate moles of iodine
  (moles of thiosulfate/2)
• Calculate the moles of the oxidising agent
  (Use ratio of iodine produced:oxidising agent)
• Calculate the conc. of the oxidising agent
  (conc=moles/volume)

Question 22

What’s the unit for concentration?

Answer 22

moldm-3

Question 23

Name 5 problems that could arise in a iodine-sodium thiosulfate titration

Answer 23

• Contaminated apparatus could make inaccurate results
• Burette could be read incorrectly
• Random errors
• Solutions could react with oxygen in the air
• Adding starch too soon could make iodine “stick to the starch” and not react properly with the sodium thiosulfate

Question 24

What is the solution to the problem of contaminated apparatus making inaccurate results?

Answer 24

• Ensure the burette is very clean

- Rinse with sodium thiosulfate (water could dilute the solution)

Question 25

What is the solution to the problem of the burette being read incorrectly?

Answer 25

• Always read from the bottom of the meniscus

- Keep eyes level with the liquid

Question 26

What is the solution to the problem of random errors?

Answer 26

Repeat experiment until you obtain 3 results within 0.1cm3 of each other then take an average

Question 27

What is the solution to the problem of solutions reacting with oxygen air?

Answer 27

Solutions should be made up freshly

Question 28

What is the solution to the problem of adding starch too soon making iodine “stick to the starch” and not react with the sodium thiosulfate?

Answer 28

Only add the starch when the solution is pale yellow