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.
(a)Write an equation for this equilibrium.
(b)For each chlorine-containing species in the equation, write
the oxidation state of chlorine below the species.
1. Cl2 + H2O -> -< HOCl + HCl
2. Cl2 =0, HOCl = +1 and HCl=−1
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.
1. The hydroxide reacts with the acids
2. The equilibrium shifts to the right
Explain why chlorine is used to kill bacteria in swimming pools,
even though chlorine is toxic.
1. Only used in small amounts
2. The health benefits outweigh the risks
, 1. Br– ions are bigger than Cl– ions
2. Therefore Br– ions more easily oxidised / lose an electron
more easily (than Cl– ions)
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.
Stage 1: formation of precipitates
1. Add silver nitrate
2. to form precipitates of AgCl and AgBr
3. AgNO3 + NaCl → AgCl + NaNO3
4. AgNO3 + NaBr → AgBr + NaNO3
Stage 2: selective dissolving of AgCl
1. Add excess of dilute ammonia to the mixture of
precipitates
2. the silver chloride precipitate dissolves
3. AgCl + 2NH3 → Ag(NH3)2+ + Cl−
,Stage 3: separation and purification of AgBr
4. Filter off the remaining silver bromide precipitate
5. Wash to remove soluble compounds
6. Dry to remove water
Colourless to yellow
Write an equation for the reaction between chlorine and cold,
dilute, aqueous sodium hydroxide.
Give a major use for the solution that is formed by this reaction.
Give the IUPAC name of the chlorine-containing compound
formed in this reaction in which chlorine has an oxidation state of
, +1.
1. Bleach and kills bacteria
2. Sodium chlorate(I)
1. Write an equation for the equilibrium reaction that occurs
when chlorine gas reacts with cold water.
2. Give one reason why chlorine is used for the treatment of
drinking water even though the gas is very toxic.
1. Cl2 +H2O -> -< HClO+HCl
2. The (health) benefit outweighs the risk
3. Once it has done its job, little of it remains
4. Used in (very) dilute concentrations / small amounts / low
doses
State how you could test a sample of water to show that it
contains chloride ions.
In your answer, give a reagent, one observation and the simplest
ionic equation for the reaction with the reagent.
1. Add Silver nitrate/ AgNO3 (with or without nitric acid)
2. White precipitate formed
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.
(a)Write an equation for this equilibrium.
(b)For each chlorine-containing species in the equation, write
the oxidation state of chlorine below the species.
1. Cl2 + H2O -> -< HOCl + HCl
2. Cl2 =0, HOCl = +1 and HCl=−1
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.
1. The hydroxide reacts with the acids
2. The equilibrium shifts to the right
Explain why chlorine is used to kill bacteria in swimming pools,
even though chlorine is toxic.
1. Only used in small amounts
2. The health benefits outweigh the risks
, 1. Br– ions are bigger than Cl– ions
2. Therefore Br– ions more easily oxidised / lose an electron
more easily (than Cl– ions)
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.
Stage 1: formation of precipitates
1. Add silver nitrate
2. to form precipitates of AgCl and AgBr
3. AgNO3 + NaCl → AgCl + NaNO3
4. AgNO3 + NaBr → AgBr + NaNO3
Stage 2: selective dissolving of AgCl
1. Add excess of dilute ammonia to the mixture of
precipitates
2. the silver chloride precipitate dissolves
3. AgCl + 2NH3 → Ag(NH3)2+ + Cl−
,Stage 3: separation and purification of AgBr
4. Filter off the remaining silver bromide precipitate
5. Wash to remove soluble compounds
6. Dry to remove water
Colourless to yellow
Write an equation for the reaction between chlorine and cold,
dilute, aqueous sodium hydroxide.
Give a major use for the solution that is formed by this reaction.
Give the IUPAC name of the chlorine-containing compound
formed in this reaction in which chlorine has an oxidation state of
, +1.
1. Bleach and kills bacteria
2. Sodium chlorate(I)
1. Write an equation for the equilibrium reaction that occurs
when chlorine gas reacts with cold water.
2. Give one reason why chlorine is used for the treatment of
drinking water even though the gas is very toxic.
1. Cl2 +H2O -> -< HClO+HCl
2. The (health) benefit outweighs the risk
3. Once it has done its job, little of it remains
4. Used in (very) dilute concentrations / small amounts / low
doses
State how you could test a sample of water to show that it
contains chloride ions.
In your answer, give a reagent, one observation and the simplest
ionic equation for the reaction with the reagent.
1. Add Silver nitrate/ AgNO3 (with or without nitric acid)
2. White precipitate formed
