Chemistry Unit 2: Group 7 The Halogens
Halogen has 7 electrons in its highest energy level, reacts to gain electron and causes the loss of electron so is a oxidising agent
Halide has full highest energy level, reacts to lose electron and causes the gain of electron so is a reducing agent
Down the group atomic radius increases, boiling point increases due to bigger Van der Waals due to more electrons meaning element has larger temporary dipoles and the electro-negativity decreases as the larger the atom so the more electrons in energy levels meaning more shielding
Chemical Trend: Halogens have high oxidising power so cause oxidation. They themselves are reduced so gain an electron to have a full energy level
Down the group oxidation power decreases since more shielding from the nucleus and there is more distance from the nucleus
Chemical Reactions of the Halogens
The oxidising ability of the halogens increases up the group
Halogens react with metal halides to displace them (If the halogen ismore reactive)
The halides are colourless solutions
Fluorine oxidises Chloride,Bromide and Iodide (Cl2 is faint yellow, Br2 is orange, I2 is red)
Chlorine oxidises Bromide and Iodide (Br2 is yellow, I2 is orange/brown)
Bromine oxidises Iodide (red/brown)
Fluorine cannot be tested in an aqueous solution as it reacts with water
Reactions of Halide Ions
Halides can lose their extra electrons
They act as reducing agents
Reducing power increases down the group as:
Larger atomic radius
Less attraction between nucleus and highest energy level
Due to increased distance there is more shielding
Sodium Chloride and Concentrated Sulfuric Acid
Chloride:White steamy fumes of hydrogen chloride, solid sodium hydrogensulfate formed
Isn't a redox reaction as there is no oxidation state change it is a acid-base reaction
The chloride ion is too weak a reducing agent to reduce the Sulfur
NaCl(s) + H2SO4(l) -> NaHSO4(s) + HCl(g)
Similar reaction with sodium fluorine as the fluorine ion is an even weaker reducing agent
Sodium Bromide and Concentrated Sulfuric Acid
Steamy white fumes of hydrogen bromide formed
Brown fumes of bromine and colourless Sulfur dioxide formed
Doc Brown's Chemistry
A Periodic Table task sheet on the Group 7 The Halogens
Worksheet questions on the properties of elements and compounds and their uses
Revision KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science (revise courses equal to US grades 9-10)
Detailed GCSE revision notes on Group 7 The Halogens * Halogen task sheet answers
Task 1: The properties of the Group 7 Halogen elements
|Symbol||state, colour at room temperature||Melting point oC||Boiling point oC|
|9||Fluorine||F||pale yellow gas||-220||-188|
|35||Br||ends in .7|
|53||I||ends in .7|
|85||Astatine||At||black solid||302||337||ends in .7|
1. (a) Complete the table above.
(b) What is the colour trend down the group?
(c) What is the colour of the vapour (gas) formed on heating (i) bromine, (ii) iodine?
2. (a) What properties in the table are typical of non-metals?
(b) Suggest some other properties the halogens might have compared to metals eg strength of solid, heat and electrical conduction?
3. (a) What is the group trend in melting and boiling point down the group with increase in atomic number?
(b) how does the trend affect their physical state as you go down the group with increase in atomic number?
4. The molecules consist of diatomic molecules. What does this mean?
5. (a) What sort of compounds do they form when combined with metals?
eg sodium chloride NaCl (note (i) the metal keeps its name BUT chlorine becomes chloride, (ii) bromine becomes bromide and iodine becomes iodide in compounds)
(b) What is the charge on the halide ion? Quote the symbol of the ion from chlorine.
HT only Can you explain why its that particular charge?
6. What sort of compounds do they form when combined with non-metals? eg hydrogen chloride HCl
7. Describe simple chemical tests for (a) chlorine, (b) iodine [clue - biological connection]
Task 2: The Reactivity Trend of the Halogens
This may be done by you or demonstrated by the teacher
What you do is add a solution of the halogen in water to the potassium salts of the other halogens, also dissolved in water. If a displacement reaction occurs the solution gets darker (eg more yellow, orange or brown). It also shows whether one halogen is more reactive than another. Note your observations in the table below.
|halogen\salt||Potassium chloride||Potassium bromide||Potassium iodide||Water blank (fair test check)|
|chlorine water (pale green solution)||no change|
|bromine water (orange solution)||no change|
|iodine water (very dark solution of iodine dissolved in potassium iodide solution)||no change (but complicated by the formation of the very darkly coloured I3- ion)|
1. (a) complete the table of results above, either from doing the experiment or working it out from your knowledge of halogen chemistry.
(b) What do you think the purpose of the water blank is?
2. (a) Which halogens does chlorine displace?
(b) Which halogens does bromine displace?
(c) Which halogens does iodine displace?
3. (a) What is the reactivity rule for displacement reactions?
(b) From your observations in 2., what is the reactivity trend for chlorine, bromine and iodine?
(c) From (b) what is the Group trend rule for chemical reactivity down the group with increase in atomic number?
(d) How does their reactivity compare to Noble Gases? Why the difference?
4. From your observations in 2. write word and symbol equations for the other displacement reactions that happened.
e.g. chlorine + potassium bromide ==> potassium chloride + bromine
Cl2(aq) + KBr(aq) ==> KCl(aq) + Br2(aq) (not balanced!)
5. is higher GCSE students really and involves using redox and electronic theory
5. Write word and ionic equations for the reactions and explain them in terms of oxidation and reduction (electron loss or gain). The equations only involve the halogen molecules and halide ions. Note the potassium ion K+, does not take part in the reaction and is called a 'spectator ion'. The first equation is given BUT not fully balanced ... and then write out the others where a reaction took place
eg chlorine molecule + bromide ion ==> chloride ion + displaced bromine molecule
Cl2(aq) + Br-(aq) ==> Cl-(aq) + Br2(aq) (not balanced!)
6. Why do elements in the same group have similar chemical properties?
7. How, in electronic terms, do you explain the reactivity trend?
Task 3: The Electrolysis of Sodium chloride Solution (Brine)
Sodium chloride (NaCl, common salt) is a compound of an alkali metal and a halogen. It is found in large quantities in the sea and in underground deposits. It is a most important raw material for the chemical industry. The cheapest sources are (i) evaporation of seawater and (ii) pumping water down into salt deposits to dissolve it, and then evaporating the pumped out solution (called brine). It is far to costly to make the salt by neutralisation.
From salt lots of products can be made.
The electrolysis of sodium chloride solution (brine) is an important industrial process.
In can be demonstrated in the laboratory using a simple electrolysis cell (shown in the diagram).
1. (a) Would the water be a good conductor without the salt in it?
(b) What are present in the salt solution that makes it a good conductor?
2. What do you see when the current is witched (i) ON and (ii) OFF?
3. The gases and solutions can be tested in various ways ...
(a) test gas 1 with a lit splint, observation?, conclusion?
(b) test gas 2 with damp blue litmus, observation? conclusion?
(c) what colour did the universal indicator turn? what sort of chemical was formed? what might its name be?
(d) (i) What is a simple test for iodine in solution? (clue: think of a biology food test!) and (ii) could it be formed by electrolysis? - suggest a method-idea if think so?
higher GCSE students need to do the electrode equations - balancing and redox theory
The ions present in the solution are Na+ and Cl- from the salt and traces of H+ and OH- from water.
From your electrolysis experiment observations and using redox theory ....
4. (a) which ions will move towards the negative electrode? which one must have changed to form the gas?
(b) write out a balanced equation involving the molecule, the ion and electrons to show the formation of this gas
(c) which ions will move towards the positive electrode? which one must have changed to form the gas?
(d) write out a balanced equation involving the molecule, the ion and electrons to show the formation of this gas
(e) what ions are left in solution? what useful compound do they form?
Task 4: The Uses of the Halogens and their compounds
1. Which halogen is used for treating domestic water supplies and swimming pool water? What does it do?
2. Find out four cases where chlorine is changed to another useful compound or material, and briefly describe the uses of them.
3. Find out two uses of hydrogen gas in the manufacture of other products.
4. Find out three products that use sodium hydroxide in their manufacture
5. (a) Which compounds of the halogens are used for photographic film?
(b) What sorts of radiation affect the film?
(c) What is formed from the silver salt when it is hit by the radiation?
6. (a) What is formed on combining chlorine and hydrogen? and describe the product.
(b) What does it form when dissolved in water? and what pH will the solution have?
(c) What sort of chemical is it? and what sort of chemicals react with the solution and what sort of compounds are formed?
(d) What would you expect to form if you combined (i) hydrogen and bromine?, (ii) hydrogen and iodine?
(e) What physical and chemical properties might you expect them to have? Just quote a few ideas based on?!