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1 of 43© Boardworks Ltd 2009
2 of 43© Boardworks Ltd 2009
3 of 43© Boardworks Ltd 2009 What are the halogens? The halogens are the elements in Group 7 of the periodic table. The name halogen comes from the Greek words for salt-making.
4 of 43© Boardworks Ltd 2009 Physical properties of halogens
5 of 43© Boardworks Ltd 2009 Trends in boiling point Halogen molecules increase in size down the group. This leads to greater van der Waals forces between molecules, increasing the energy needed to separate the molecules and therefore higher melting and boiling points. fluorine atomic radius = 42 × m boiling point = -118 °C iodine atomic radius = 115 × m boiling point = 184 °C van der Waals forces
6 of 43© Boardworks Ltd 2009 Trends in electronegativity Electronegativity of the halogens decreases down the group due to an increase in atomic radius. fluorine atomic radius = 42 × m electronegativity = 4.0 iodine atomic radius = 115 × m electronegativity = 2.5 Increased nuclear charge has no significant effect because there are more electron shells and more shielding. Iodine atoms therefore attract electron density in a covalent bond less strongly than fluorine.
7 of 43© Boardworks Ltd 2009 Astatine The name astatine comes from the Greek word for unstable. It was first made artificially in 1940, by bombarding 209 Bi with -radiation. What do you predict for these properties of astatine? Astatine exists in nature in only very tiny amounts. It is estimated that only 30 grams of astatine exist on Earth at any one time. This is because it is radioactive, and its most stable isotope ( 210 At) has a half-life of only 8 hours. electronegativity. state at room temperature colour
8 of 43© Boardworks Ltd 2009 Halogens: true or false?
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10 of 43© Boardworks Ltd 2009 Reactions of the halogens Halogens react with metals such as sodium and iron: They also take part in displacement reactions with halide ions, such as the reaction that is used to make bromine from potassium bromide in seawater: halogen + hydrogen hydrogen halide They also react with non-metals such as hydrogen: halogen + sodium sodium halide chlorine + potassium bromide potassium chloride bromine +
11 of 43© Boardworks Ltd 2009 Reaction with iron
12 of 43© Boardworks Ltd 2009 Reactions with hydrogen Chlorine and hydrogen explode in bright sunlight but react slowly in the dark. The halogens react with hydrogen gas to product hydrogen halides. For example: Cl 2(g) + H 2(g) 2HCl (g) Iodine combines partially and very slowly with hydrogen, even on heating. Bromine and hydrogen react slowly on heating with a platinum catalyst.
13 of 43© Boardworks Ltd 2009 Redox reactions of halogens
14 of 43© Boardworks Ltd 2009 What is the reactivity of the halogens? The reactions of the halogens with iron and hydrogen show that their reactivity decreases down the group. How do you think fluorine and astatine would react with iron wool and hydrogen? Iron wool burns and glows brightly. Iron wool has a very slight glow. Iron wool glows but less brightly than with chlorine. chlorine bromine iodine HalogenReaction with iron woolReaction with hydrogen Explodes in sunlight, reacts slowly in the dark. Reacts slowly on heating with catalyst. Reacts partially and very slowly.
15 of 43© Boardworks Ltd 2009 Electron structure and reactivity
16 of 43© Boardworks Ltd 2009 Halogen displacement reactions
17 of 43© Boardworks Ltd 2009 Halogen displacement reactions
18 of 43© Boardworks Ltd 2009 Halogen displacement reactions Halogen displacement reactions are redox reactions. Cl 2 + 2KBr 2KCl + Br 2 To look at the transfer of electrons in this reaction, the following two half equations can be written: Chlorine has gained electrons, so it is reduced to Cl - ions. What has been oxidized and what has been reduced? 2Br - Br 2 + 2e - Cl 2 + 2e - 2Cl - Bromide ions have lost electrons, so they have been oxidized to bromine.
19 of 43© Boardworks Ltd 2009 Oxidizing ability of halogens fluorine increasing oxidizing ability iodine bromine chlorine In displacement reactions between halogens and halides, the halogen acts as an oxidizing agent. This means that the halogen: What is the order of oxidizing ability of the halogens? is reduced to form the halide ion. gains electrons oxidizes the halide ion to the halogen
20 of 43© Boardworks Ltd 2009 Oxidizing ability of halogens
21 of 43© Boardworks Ltd 2009 Chlorine and disproportionation
22 of 43© Boardworks Ltd 2009 Reaction of chlorine with water Chlorination of drinking water raises questions about individual freedom because it makes it difficult for individuals to opt out. Chlorine is used to purify water supplies because it is toxic to bacteria, some of which can cause disease. Adding it to water supplies is therefore beneficial for the population. However, chlorine is also toxic to humans, so there are risks associated with gas leaks during the chlorination process. There is also a risk of the formation of chlorinated hydrocarbons, which are also toxic.
23 of 43© Boardworks Ltd 2009 Bleach and the chlorate(I) ion Household bleach commonly contains the chlorate(I) ion, ClO -, in the form of sodium chlorate(I), NaOCl. ClO - + H 2 O + Cl - + 2OH - The chlorine has been reduced because it has gained electrons. Its oxidation state has decreased from +1 in ClO - to –1 in Cl -. How many electrons are needed to balance this equation? The chlorate(I) ion behaves as an oxidizing agent. It oxidizes the organic compounds in food stains, bacteria and dyes. Has the chlorine been oxidized or reduced in the reaction? 2e -
24 of 43© Boardworks Ltd 2009 Redox reactions of chlorate ions
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26 of 43© Boardworks Ltd 2009 Halides When halogens react with metals, they form compounds called halides. Many naturally-occurring halides have industrial, household and medical applications. caesium chloride sodium hexafluoroaluminate titanium(IV) chloride lithium iodide potassium bromide HalideFormulaUses CsCl NaAlF 6 TiCl 4 LiI KBr Extraction and separation of DNA Electrolysis of aluminium oxide Extraction of titanium Electrolyte in batteries Epilepsy treatment in animals
27 of 43© Boardworks Ltd 2009 Identifying halide ions Halides can be identified by their reaction with acidified silver nitrate solution to form silver halide precipitates. Silver chloride has a low solubility in water, so it forms a white precipitate: the positive result in the test for chloride ions. KCl (aq) + AgNO 3(aq) KNO 3(aq) + AgCl (s) potassium chloride silver chloride + potassium nitrate silver nitrate +
28 of 43© Boardworks Ltd 2009 Identifying halide ions
29 of 43© Boardworks Ltd 2009 Identifying halide ions
30 of 43© Boardworks Ltd 2009 Uses of halides in photography Silver halides are used in photography. Ag + + e - Ag Photographic film coated with a silver halide is exposed to light, causing the halide to decompose to form silver. This appears as a black precipitate on the photographic film. light mask paper coated in silver halide silver precipitate white paper under mask
31 of 43© Boardworks Ltd 2009 William Fox Talbot William Fox Talbot (1800–1877) was a British scientist and mathematician. He was one of the key figures in the development of the use of silver halides in photography. Fox Talbot adapted the process by removing any unreacted silver halide by washing with sodium thiosulfate solution. This meant that the print could be used repeatedly in the way that photographic negatives can be today. A French scientist called Louis Daguerre developed the use of silver halides on copper plates. These were effective at producing prints, but could only be used once.
32 of 43© Boardworks Ltd 2009 Hydrogen halides The hydrogen halides are colourless gases at room temperature. Hydrogen fluoride has an unexpectedly high boiling point compared to the other hydrogen halides. This is due to hydrogen bonding between the H–F molecules. Hydrogen halideBoiling point (°C) HF HCl HBr HI
33 of 43© Boardworks Ltd 2009 Halides as reducing agents A substance that donates electrons in a reaction (i.e. is oxidized) is a reducing agent because it reduces the other reactant. fluoride increasing reducing ability iodide bromide chloride The larger the halide ion, the easier it is for it to donate electrons and therefore the more reactive it is. This is because its outermost electrons are further from the attraction of the nucleus and more shielded from it by other electrons. The attraction for the outermost electrons is therefore weaker.
34 of 43© Boardworks Ltd 2009 Halides: true or false?
35 of 43© Boardworks Ltd 2009 Sodium halides and sulfuric acid The sodium halides react with concentrated sulfuric acid. The reactions of sodium halides with concentrated sulfuric acid demonstrate the relative strengths of the halide ions as reducing agents. During this reaction two things can happen to the sulfuric acid. It can act as an acid. be reduced
36 of 43© Boardworks Ltd 2009 Sodium halides and sulfuric acid
37 of 43© Boardworks Ltd 2009 Sodium halides and sulfuric acid
38 of 43© Boardworks Ltd 2009 Oxidation states
39 of 43© Boardworks Ltd 2009 Sodium halides and sulfuric acid
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41 of 43© Boardworks Ltd 2009 Glossary
42 of 43© Boardworks Ltd 2009 What’s the keyword?
43 of 43© Boardworks Ltd 2009 Multiple-choice quiz
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1 of 22© Boardworks Ltd 2011 Extracting Metals by Electrolysis.
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