UNIT12:TRENDS OF CHEMICAL PROPERTIES OF GROUP 17 ELEMENTS AND THEIR COMPOUNDS
UNIT 12: TRENDS OF CHEMICAL PROPERTIES OF GROUP
17 ELEMENTS AND THEIR COMPOUNDS
Key unit Competence:
Compare and contrast the chemical properties of the Group 17 elements and their
compounds in relation to their position in the Periodic Table.
Learning objectives
By the end of this unit, students should be able to:
• Prepare and test halogens.
• Perform experiments to prepare and test chlorine, bromine and iodine.
• Relate the oxidizing power of Group17 elements to their reactivity.
• Relate the acidity strength of oxoacids to the number of oxygen atoms
combined with the halogen.
• Compare the reactions of the halogens with cold dilute sodium hydroxide
and hot concentrated sodium hydroxide solutions.
• State the uses and hazards of halogens and their compounds.
• Test for the presence of halides ions in aqueous solutions
• State the natural occurrence of halogens
• Describe the extraction methods of halogens
• Explain the trends of physical and chemical properties of Group 17 elements
down the group
• Describe the trends in strength acidity, volatility and reducing power of
halogens hydrides
• Describe the chemical properties of chlorates, iodates, perchlorates andperiodates
Checking up 12.1
1. State 2 locations where chlorine can be found in nature.
2. Write the chemical formulae of the compounds of halogens in nature.
3. Give the name of one lake in Rwanda where salt is abundant in water.
4. Explain the separation method you can use to get the salt crystals from the
water.
5. Fluoride ion is the most difficult to oxidise into fluorine, whereas iodide ion isthe easiest. Explain why.
12.2. Preparation methods of halogens
Repeat procedures steps 1 to 7 but this time use solutions of KBr or NaBr instead
of KI in the boiling tube to prepare bromine and chlorine.
Activity 12.2 (b) Preparation of bromine and iodine
1. Put 0.5 gram of MnO2
in a round bottomed flask.
2. Pour concentrated NaBr solution (5 ml of a 0.1 mol/litre) in the round bottomed
flask.
3. Pour 5 ml of 1 mol/litre HCl solution in the round bottomed flask mixture.
4. Connect the apparatus to a delivery tube using a rubber stopper.
5. Heat the round bottomed flask mixture.
6. Direct the delivery tube in a solution of KI in a test tube.7. Note the observable changes.
Activity 12.2. (c): Electrolysis of concentrated NaCl solution
a) Put 1 g of NaCl in a beaker.
b) Add water and stir using a glass rod until all the salt dissolves.
c) Pour the solution in an electrolyser.
d) Connect the electrolyser to the source of direct current and switch on.
e) Dip a test tube full of water in the NaCl solution in inverted position from above
each electrode
f) Put 2 drops of phenolphthalein indicator in the solution under each test tube.
g) Record the observations that take place for 5 minutes.Apparatus set-up: Electrolysis of concentrated NaCl solution
12.2.1. Chlorine
Most commercial chlorine is obtained by electrolysis of chloride ions in aqueous
solutions of sodium chloride or molten NaCl.The reactions that take place are shown by the following chemical equations:
Interpretations
Chlorine is liberated by the reaction between 2M hydrochloric acid and potassium
permanganate solution. Chlorine displaces iodine from potassium iodide solution,which dissolves in water to give a dark-red solution, and turns starch indicator dark blue. The greyish-black residue is due to the formation of Iodine solid.
12.4. Preparation of Hydrogen halides
Activity 12.4.(a)
Laboratory preparation of Chlorine
Reactants:Sodium Chloride and sulfuric acid
Rocedure:
1.Put 50g of NaCl in round bottomed flask
2.Pour conc sulfuric acid throuth the filter fannel and heat
3.The liberated gas is pased throuth the concentrated sulfuric acid
4.Collect the gas by the Downward delivery
Note:The lower end of the thistle funnel must be dippen in acid,or you can use the funnelwith Syphon
12.5. Trends in strength of acidity, volatility and reducing
power of hydrogen halides
12.5.1. Acid strength
The acid strength is a measure of how an acid dissociates in water into its ions.
Strong acids dissociate completely into their ions, whereas a weak acid dissociatespartially into its ions.
bonds. It is liquid at room temperature while other hydrogen halides are gases.
The trend in volatility:
12.5.4. Tests for halide ions in aqueous solution
Test of substance X with an unknown anion
Activity 12.5
Identification of ions:
i) You are provided with a solution of X substance.
ii) Put 1 ml of X solution in each of the 4 test tubes.
iii) Add in each test tube the reagent solutions as indicated in the table below.iv) Note down the observations for interpretation later in each test.
12.6. Chemical properties of chlorates, iodates, perchlorates
and periodates
12.7. Uses of halogens and their compounds
Activity 12.7
1. When you want to eat food, salt is dissolved in it. Indicate the chemical composition
of table salt and its natural occurrence.
2. Chlorine compounds are used in the treatment of water. Explain how chlorine reacts to be a good disinfectant in water treatment.
3. a)Write the observations of the phenomenon that takes place when electrolysis of
a concentrated solution of chlorine is carried out in the laboratory. b) Deduce the
product of reaction that is formed at the anode.
12.7.1. Uses of HalogensHalogens and their compounds have many applications and uses in different
12.7.2. Hazards caused by group 17 elements
Bromine effects
• On heating, toxic fumes are formed.
• Reacts violently (explosively) with many compounds.
• Attacks plastics, rubber and coatings.
Chlorine effects
• It reacts violently with many compounds like ammonia and may cause fire
and explosion.
• It attacks many metals in the presence of water.
• It attacks plastics, rubber and coatings.
Chlorine oxide effects
• It may explosively decompose when it encounters shock and friction then
it may explode on heating.
• It reacts violently with mercury, phosphorus, sulphur, etc causing fire and
explosion hazard.
Fluorine effects
• It reacts violently with water to produce toxic and corrosive vapours: ozone
and hydrogen fluoride.
• It reacts violently with ammonia, metals, oxidants, etc, to cause fire and
explosion.
Hydrogen bromide effects
• It reacts violently with strong oxidants and many organic compounds to
cause fire and explosion.
• It attacks many metals forming flammable hydrogen gas.
Hydrogen fluoride effects
• It reacts violently with many compounds causing fire and explosion.
• On contact with air, it emits corrosive fumes which are heavier than air.
• It attacks glass and other silicon-containing compounds.
SF6 effects
• The substance decomposes in a fire to produce toxic fumes of sulphur
oxides and hydrogen fluoride
• When it is heated, there is formation of toxic fumes.
12.8. End of unit assessment
