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*Chemistry: Experiments > Q1. * Determination of the % of hypochlorite in bleach > Flashcards

Q1. * Determination of the % of hypochlorite in bleach Flashcards

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1
Q

What form does chlorine take in bleach?

A

The form of sodium hypochlorite NaClO

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2
Q

What is the formula for sodium hypochlorite?

A

NaClO

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3
Q

What is the name of NaClO?

A

Sodium hypochlorite

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4
Q

What is contained in sodium hypochlorite?

A

The hypochlorite ion, ClO-

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5
Q

What is the formula for the hypochlorite ion?

A

CIO-

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6
Q

What is CIO-?

A

A hypochlorite ion

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7
Q

What is the more modern name for the hypochlorite ion?

A

Chlorate (I) ion

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8
Q

What is the hypochlorite ion?

2

A

It is an oxidising agent ]

It converts iodide ions to iodine

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9
Q

Is the hypochlorite ion an reducing or oxidising agent?

A

An oxidising agent

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10
Q

What does the hypochlorite ion convert into iodine?

A

Iodine ions

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11
Q

What does the hypochlorite ion convert iodine ions into?

A

Iodine

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12
Q

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.

A

ClO- + 2l- +2H+ –> Cl- + I2 + H2O

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13
Q

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

A

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

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14
Q

Why must the bleach be diluted?

2

A

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

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15
Q

What is added to the conical flask of diluted bleach?

A

Dilute sulfuric acid

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16
Q

Why is dilute sulfuric acid added to the dilute bleach?

2

A

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-

17
Q

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

A

Colourless

18
Q

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

A

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

19
Q

What does the sulfuric acid supply?

A

H+ ions

20
Q

Why are sufficient H+ ions needed?

A

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

21
Q

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

A

The H+ ions supplied by the sulfuric acid

22
Q

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

A

Potassium iodide solution in excess

23
Q

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

A

Changes from colourless to reddish-brown

24
Q

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

A

The formation of iodine

25
Q

Why is the potassium iodide solution added in excess?

2

A

To ensure all of the bleach has reacted to produce the maximum amount of iodine

To keep the iodine produced in solution

26
Q

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)

A

Changes from reddish-brown to pale yellow

Brown colour becomes less intense and gradually changes to a yellow colour then a pale yellow colour

27
Q

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

A

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

28
Q

When is the indicator added?

A

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

29
Q

What indicator is used?

A

Starch

30
Q

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

A

Blue-black to colourless

31
Q

What colour does the solution become when starch is added?

A

Pale yellow to blue-black

32
Q

What does the pale yellow colour indicate?

A

It indicates that there is little iodine left in solution

33
Q

What causes the blue-black colour?

A

Starch reacting with the small amount of iodine left in solution

34
Q

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

A

there is no more iodine left in solution

i.e. all of the iodine has reacted with the thiosulfate

35
Q

What formula is used for the calculations?

A

Vo x Mo/No = Vr x Mred/Nr

36
Q

What is the oxidising agent in this experiment?

A

Hypochlorite / ClO-

37
Q

What is the reducing agent in this experiment?

A

Thiosulfate S4O6^2-

38
Q

What is the formula for thiosulfate?

A

S4O6^2-

*Chemistry: Experiments (15 decks)

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