Chapter 8.4 - Uses Of Chlorine And Chlorate

Question 1

For many years, swimming pool water has been treated with chlorine gas. The chlorine is added to kill any harmful bacteria unintentionally introduced by swimmers. Pool managers are required to check that the chlorine concentration is high enough to kill the bacteria without being a health hazard to the swimmers.

When chlorine reacts with water in the absence of sunlight, the chlorine is both oxidised and reduced and an equilibrium is established.

Write an equation for this equilibrium.
For each chlorine-containing species in the equation, write the oxidation state of chlorine below the species (2)

Answer 1

Cl2 + H2O = HOCl + HCl
Cl2 = 0,
HOCl = +1,
HCl = -1

Question 2

The pool manager maintains the water at a pH slightly greater than 7.0

Explain how this affects the equilibrium established when chlorine is added to water (2)

Answer 2

• alkali ions react with the acids,

- equilibrium moves to the right

Question 3

Explain why chlorine is used to kill bacteria in swimming pools, even though chlorine is toxic (2)

Answer 3

• only used in small amounts,

- the health benefits outweighs the risks

Question 4

A colourless solution contains a mixture of sodium chloride and sodium bromide.

Using aqueous silver nitrate and any other reagents of your choice, develop a procedure to prepare a pure sample of silver bromide from this mixture.

Explain each step in the procedure and illustrate your explanations with equations, where appropriate (6)

Answer 4

Stage 1:

• add silver nitrate,
• To form precipitate of AgCl and AgBr,
• AgNO3 + NaCl = AgCl + NaNO3

Stage 2:
-Add excess of dilute ammonia to the mixture of precipitates,
-the silver chloride precipitate dissolves
AgCl + 2NH3 = Ag(NH3)2 + Cl-

Stage 3:

• filter off the remaining silver bromide precipitate,
• wash and dry

Question 5

Write an ionic equation for the reaction between chlorine and cold dilute sodium hydroxide solution.

Give the oxidation state of chlorine in each of the chlorine-containing ions formed (2)

Answer 5

Cl2 + 2NaOH = NaCl + NaOCl + H2O —> the normal equation
Cl2 +2HO– ——> OCl– +Cl– +H2O. —> simplified equation
OCl– is +1
Cl– is –1

Question 6

Write the simplest ionic equation for the reaction between chlorine and aqueous potassium bromide.

State what is observed when this reaction occurs. (2)

Answer 6

Cl2 + 2Br- —> 2Cl- + Br2

Solution goes orange

Question 7

Write an equation for the equilibrium reaction that occurs when chlorine gas reacts with cold water. (1)

Answer 7

Cl2 + H2O = HCLO + HCl

Question 8

Give one reason why chlorine is used for the treatment of drinking water even though the gas is very toxic (1)

Answer 8

The health benefit outweighs the risk

Question 9

Chlorine displaces iodine from aqueous potassium iodide

Give one observation that you would make when this reaction occurs (1)

Answer 9

Solution turns colorless to brown

Question 10

Explain why chlorine has a lower boiling point than bromine (2)

Answer 10

• chlorine has fewer electrons compared to bromine,

- chlorine has weak van der waals forces between molecules

Question 11

Chlorine is toxic to humans. This toxicity does not prevent the large-scale use of chlorine in water treatment.

Explain why the toxicity of chlorine does not prevent this use. (1)

Answer 11

The health benefit outweighs the risk

Question 12

Chlorine is toxic to humans. This toxicity does not prevent the large-scale use of chlorine in water treatment.

Give one reason why water is treated with chlorine.

Answer 12

To kill bacteria

Question 13

Write an equation for the reaction of chlorine with cold water. (1)

Answer 13

Cl2 + H2O = HCLO + HCl

Question 14

Deduce what happens to this equilibrium as the HClO reacts with bacteria in the water supply. Explain your answer. (2)

Answer 14

• Equilibrium shifts to the right to produce more HCLO

- to oppose the loss of HCLO

Question 15

Explain why chloride ions are weaker reducing agents than bromide ions. (2)

Answer 15

• chlorine ions are smaller than bromide ions,

- electron lost from a chloride ion is more strongly held by the nucleus compared with bromide

Question 16

State why chlorine is added to the water supply and give a reason why the amount of chlorine must be carefully monitored. (2)

Answer 16

• kills bacteria,

- chlorine is a toxic substance

Question 17

Write an equation for the reaction of chlorine with water. (1)

Answer 17

Cl2 + H2O = HCLO + HCL

Question 18

Write an equation for the reaction between chlorine gas and dilute aqueous sodium hydroxide.

Name the two chlorine-containing products of this reaction and give the oxidation state of chlorine in each of these products

Equation:
Name of product 1:
Oxidation state of chlorine in product 1:
Name of product 2: (4)

Answer 18

Cl2 + 2NaOH = NaCl + NaOCl + H2O
Sodium chloride,
-1,
Sodium chlorate or bleach 
\+1

Question 19

Write an equation for the reaction between chlorine and cold, dilute aqueous sodium hydroxide. Give two uses of the resulting solution.

Equation:
Use 1:
Use 2 : (3)

Answer 19

Equation : Cl2 + 2NaOH = NaCl + NaOCl + H2O
Use 1 : bleach,
Use 2 : kills bacteria

Question 20

Write an equation for the reaction that occurs when chlorine is added to cold water. State whether or not the water is oxidised and explain your answer (3)

Answer 20

Cl2 + H2O = HCLO + HCl,
Water is not oxidised,
Oxidation state of O= -2 and H= +1