Q1. * Determination of the % of hypochlorite in bleach

Question 1

What form does chlorine take in bleach?

Answer 1

The form of sodium hypochlorite NaClO

Question 2

What is the formula for sodium hypochlorite?

Answer 2

NaClO

Question 3

What is the name of NaClO?

Answer 3

Sodium hypochlorite

Question 4

What is contained in sodium hypochlorite?

Answer 4

The hypochlorite ion, ClO-

Question 5

What is the formula for the hypochlorite ion?

Answer 5

CIO-

Question 6

What is CIO-?

Answer 6

A hypochlorite ion

Question 7

What is the more modern name for the hypochlorite ion?

Answer 7

Chlorate (I) ion

Question 8

What is the hypochlorite ion?

2

Answer 8

It is an oxidising agent ]

It converts iodide ions to iodine

Question 9

Is the hypochlorite ion an reducing or oxidising agent?

Answer 9

An oxidising agent

Question 10

What does the hypochlorite ion convert into iodine?

Answer 10

Iodine ions

Question 11

What does the hypochlorite ion convert iodine ions into?

Answer 11

Iodine

Question 12

Give the equation for the conversion of iodide ions to iodine using hypochlorite as an oxidising agent.

Give the equation for the reaction that occurs during this experiment.

Answer 12

ClO- + 2l- +2H+ –> Cl- + I2 + H2O

Question 13

In general, how is the concentration of sodium hypochlorite in household bleach determined?
(2)

Answer 13

A sample of the bleach is reacted with a solution of iodide ions

The liberated iodine is then titrated against a solution of sodium thiosulfate of known concentration

Question 14

Why must the bleach be diluted?

2

Answer 14

As household bleach is usually fairly diluted

If not diluted a very large volume of sodium thiosulfate would be required to react with the large amount of I2 liberated

Question 15

What is added to the conical flask of diluted bleach?

Answer 15

Dilute sulfuric acid

Question 16

Why is dilute sulfuric acid added to the dilute bleach?

2

Answer 16

To ensure the acid is in excess

To supply sufficient H+ ions so that all of the hypochlorite ions, ClO- are completely reduced to chloride ions Cl-

Question 17

What colour is the solution of sulfuric acid and dilute bleach?

Answer 17

Colourless

Question 18

What affect does adding the dilute sulfuric acid have on the colour of the bleach solution?

Answer 18

It has no affect on the colour - remains colourless unless coloured by the bleach’s brand

Question 19

What does the sulfuric acid supply?

Answer 19

H+ ions

Question 20

Why are sufficient H+ ions needed?

Answer 20

To ensure all of the hypochlorite ions, ClO- are completely reduced to chloride ions Cl-

Question 21

What ensures all of the hypochlorite ions, ClO- are completely reduced to chloride ions Cl-?

Answer 21

The H+ ions supplied by the sulfuric acid

Question 22

What is added to the solution of dilute bleach and sulfuric acid?

Answer 22

Potassium iodide solution in excess

Question 23

What colour change can be seen when the potassium iodide solution is added to the solution of bleach and sulfuric acid?

Answer 23

Changes from colourless to reddish-brown

Question 24

What causes the colour change from colourless to reddish-brown?

Answer 24

The formation of iodine

Question 25

Why is the potassium iodide solution added in excess? | 2

Answer 25

To ensure all of the bleach has reacted to produce the maximum amount of iodine To keep the iodine produced in solution

Question 26

What colour change takes place when the solution of bleach, sulfuric acid and potassium iodide is titrated against a solution of sodium thiosulfate of known concentration? (2)

Answer 26

Changes from reddish-brown to pale yellow Brown colour becomes less intense and gradually changes to a yellow colour then a pale yellow colour

Question 27

Why is there a colour change from reddish-brown to pale yellow?

Answer 27

As the iodine becomes used up which results in the reddish-brown colour becoming less intense and fading to pale yellow

Question 28

When is the indicator added?

Answer 28

When the solution in the conical flask becomes pale yellow or straw coloured

Question 29

What indicator is used?

Answer 29

Starch

Question 30

What colour change takes place when the titration is carried out in the presence of starch?

Answer 30

Blue-black to colourless

Question 31

What colour does the solution become when starch is added?

Answer 31

Pale yellow to blue-black

Question 32

What does the pale yellow colour indicate?

Answer 32

It indicates that there is little iodine left in solution

Question 33

What causes the blue-black colour?

Answer 33

Starch reacting with the small amount of iodine left in solution

Question 34

What does the fact that the blue-black colour disappears indicate? (2)

Answer 34

there is no more iodine left in solution i.e. all of the iodine has reacted with the thiosulfate

Question 35

What formula is used for the calculations?

Answer 35

Vo x Mo/No = Vr x Mred/Nr

Question 36

What is the oxidising agent in this experiment?

Answer 36

Hypochlorite / ClO-

Question 37

What is the reducing agent in this experiment?

Answer 37

Thiosulfate S4O6^2-

Question 38

What is the formula for thiosulfate?

Answer 38

S4O6^2-