Chem Factsheet
www.curriculum-press.co.uk Number 204
Chlorine and Chlorates
Chlorine Chloride Chlorate (I)
A group seven element. The monoatomic ion of chlorine. Chlorate (I) ion is a polyatomic ion consisting
Forms through one atom of chlorine gaining one of oxygen and chlorine, ClO .
-
It exists as a diatomic molecule (Cl2).
electron to produce Cl-. Chloride ions are often The (I) included in the name represents the
It is a pale green gas at room temperature and
found as common salts such as NaCl and KCl. positive oxidation state of chlorine in this
pressure. It can be bubbled through water to
These are formed through ionic bonding. compound. i.e. Na(+1) + Cl(+1) + O(-2) = 0
produce chlorine water.
Exam Hint:- Be very careful when answering questions involving Omitting the sodium ions gives the following ionic equation for this
the species above e.g. reaction: Cl2 + 2OH- → Cl- + ClO- + H2O
• Chlorine is an example of a halogen. This reaction is used industrially to produce bleach.
• One Chloride ion reacts with one silver ion to produce silver
chloride, (AgCl). The reaction is an example of a disproportionation reaction.
• A chlorate (I) ion combines with a sodium ion to produce sodium A disproportionation reaction is a reaction where the same species is
chlorate (I), (NaClO). simultaneously oxidised and reduced in the same reaction.
If you do not use the correct terms you will not get the marks! In this reaction two chlorine atoms have undergone redox reactions.
One chlorine atom has been oxidised by losing one electron to form
Chlorine as an Oxidising Agent the chlorate(I) ion. ½Cl2 + 2OH- → ClO- + H2O + e-
Chlorine is a strong oxidising agent.
One chlorine atom has been reduced by gaining one electron to form
An Oxidising agent is a reagent that takes electrons (oxidises) from a chloride ion. ½Cl2 + e- → Cl-
another species
e.g. Cl2 (aq) + 2KI(aq) → 2KCl(aq) + I2(aq) CONDITIONS 2:
Reagents: Concentrated NaOH and chlorine gas
The equation above can also be written as an ionic equation by Temperature: Hot.
removing the spectator ions.
The overall equation is:
i.e. Cl2(aq) + 2I-(aq) → 2Cl-(aq) + I2(aq)
3Cl2(g) + 6NaOH(aq) → NaClO3(aq) + 5NaCl(aq) + 3H2O(l)
A spectator ion is an ion that is not involved in a chemical reaction.
It appears unchanged on the reactant and the product side. Here the This again is an example of a disproportionation reaction.
potassium ion (K+) is the spectator ion. Chlorines atom have been oxidised by losing electrons to form
chlorate(V) ions.
Tip: This reaction is based on the general principle that a more
reactive halogen will displace a less reactive halogen from its halide. ½Cl2 + 6OH- → ClO3- + 3H2O + 5e-
Chlorine atoms have been reduced by gaining electrons to form chloride
Uses of chlorine: ions. ½Cl2 + e- → Cl-
1. Bleach – a substance that removes colour by oxidising “coloured”
molecules to colourless molecules. The Purification of Drinking Water
2. To purify drinking water by killing bacteria by an oxidation process.
3. Killing bacteria in swimming pools. Reagents: Chlorine gas and water
Temperature: Room temperature
The Reaction of Chlorine with Sodium Hydroxide
The compounds produced are dependent on the temperature and the Cl2(g) + H2O(l) ¾ HCl(aq) + HClO(aq) ———— (1)
concentration of the NaOH.
This again is an example of a disproportionation reaction.
CONDITIONS 1:
Reagents: Dilute aqueous NaOH and chlorine gas. One Cl atom has been reduced to produce a Cl- ion : ½Cl2 + e- → Cl-
Temperature: Room temperature (ca. 20°C) One Cl atom has been oxidised to chloric(I) acid (HClO) :
The overall equation is: ½Cl2 + H2O → HClO + H+ + e-
Cl2(g) + 2NaOH(aq) → NaCl(s) + NaClO(aq) + H2O(l) The chloric (I) acid (HClO) slowly decomposes, the reactive oxygen
The products are sodium chloride and sodium chlorate(I) atoms ([O]) killing the bacteria in water. HClO → HCl + [O]
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204. Chlorine and Chlorates Chem Factsheet
Chlorine in an Organic Solvent 4. A solution of chlorine water was added to a solution of potassium
Aqueous chlorine is a very pale green solution because of the presence bromide. The resultant mixture was shaken after an organic solvent
of Cl2 molecules in equilibrium (1). If it is added to an organic solvent had been added. A reaction occurs.
(usually cyclohexane) two layers form, an inorganic layer and an (a) State what would be seen at the end of the experiment.
organic layer. Due to the non polar nature of the chlorine molecules it (b) Write a full equation including state symbols for the reaction
dissolves more readily in the organic layer producing a more distinct that takes place.
pale yellow colour, and reducing the pale green aqueous colour (c) Write an ionic equation for the reaction that takes place.
(d) What type of reaction has occurred?
Bromine and iodine produce similarly enhanced colours when they (e) Show what has been oxidised and reduced in the above equation.
preferentially dissolve in an organic solvent. Bromine goes from orange
in aqueous to red in organic. Iodine changes from brown to purple. Answers
1. (a) Acidify the sample with dilute nitric acid and then add a few
Chloride Ion drops of silver nitrate solution. Chloride is confirmed by a white
A chloride ion is formed by one chlorine atom gaining one electron. precipitate.
(b) NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)
This reduction half equation can be written as: ½Cl2 + e- → Cl- (c) Ag+ (aq)+ Cl- → AgCl(s)
A chloride ion can be identified using silver nitrate and nitric acid. 2. (a) To kill bacteria to aid purification.
Adding AgNO3 and HNO3 to a solution containing Cl- produces a (b) Cl2(g) + H2O(l) ¾ HCl(aq) + HClO(aq)
white precipitate. (c) Disproportionation.
(d) Cl2 reduced to Cl- by gain of electrons ; Cl2 oxidised to HClO
XCl(aq) + AgNO3(aq) → AgCl(s) + XNO3(aq) where X = Na, K etc by loss of electrons.
The ionic equation for this reaction is: Ag+ (aq) + Cl- (aq) → AgCl(s) 3. (a) A reaction where the same species is simultaneously oxidised
and reduced in the same reaction.
The silver chloride is the white precipitate that is seen. (b) Cl2(g) + 2NaOH(aq) → NaCl(s) + NaClO(aq) + H2O(l)
(c) Oxidation of chlorine : ½Cl2 + 2OH- → ClO- + H2O + e-
The nitric acid is essential to remove any OH- or CO32- ions from the test Reduction of chlorine : ½Cl2 + e- → Cl-
solution. These produce interfering precipitates if they are not removed. (d) Bleach.
(e) 3Cl2(g) + 6NaOH(aq) → NaClO3(aq) + 5NaCl(aq) + 3H2O(l)
Practice Questions (f) NaClO4
1. Three unknown sodium halide solutions were found in a laboratory.
The chemist needed to find out which solution contained chloride 4. (a) Two layers. An almost colourless aqueous layer and a red organic
ions. layer.
(a) Describe a simple test the chemist could carry out to identify (b) 2KBr(aq) + Cl2(aq) → Br2(aq) + 2KCl(aq)
the solution containing the chloride ions. (c) 2Br-(aq) + Cl2(aq) → Br2(aq) + 2Cl-(aq)
(b) Write the full equation of the reaction that has occurred in this (d) Redox
test with state symbols. (e) Br-(aq) is oxidised to Br2 by loss of electrons. Cl2(aq) is reduced
(c) Write the ionic equation that has occurred when performing to Cl-(aq) by gain of electrons.
this test with state symbols.
2. The introduction of chlorine to drinking water has meant that
such diseases as cholera and other water-borne diseases have
been minimised in such places as the United States and the United
Kingdom.
(a) Why is chlorine added to drinking water?
(b) Write an equation to show what process has taken place when
chlorine is added to drinking water.
(c) State what type of reaction is occurring in the above reaction.
(d) State what is being oxidised and reduced in terms of electron
transfer.
3. (a) State what is meant by the term disproportionation reaction.
(b) The reaction of chlorine with cold dilute aqueous sodium
hydroxide is an example of a disproportionation reaction. Write
a full equation including states symbols for this reaction.
(c) Show, in terms of electron transfer, how this reaction is an
Acknowledgements: This Factsheet was researched and written by Stuart
example of a disproportionation reaction. Prendergast. Curriculum Press, Bank House, 105 King Street, Wellington,
(d) State a use for one of the products for the above reaction. Shropshire, TF1 1NU. ChemistryFactsheets may be copied free of charge by
(e) Another reaction occurs when chlorine is bubbled through hot teaching staff or students, provided that their school is a registered subscriber.
concentrated sodium hydroxide. Write a full equation including No part of these Factsheets may be reproduced, stored in a retrieval system, or
state symbols for this reaction. transmitted, in any other form or by any other means, without the prior permission
of the publisher. ISSN 1351-5136
(f) Write the chemical formula for the compound sodium chlorate (VII).
