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Boardworks AS Chemistry Halogens

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1 Boardworks AS Chemistry Halogens

2 Write down everything you know about the elements in group 7.
Starter Write down everything you know about the elements in group 7.

3 Group 7 Elements: Redox Reactions
Lesson Objectives: • Explain the trend in boiling points of Cl2, Br2 and I2. • Explain the trend in reactivity for the Group 7 elements. • Describe the redox reactions of the Group 7 elements with other halide ions. • Describe and interpret, using oxidation numbers, the reaction of chlorine with water and aqueous sodium hydroxide. Key Words: reactivity, displacement reaction, disproportionation, precipitation reaction

4 Boardworks AS Chemistry Halogens
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

5 Boardworks AS Chemistry Halogens
What are the halogens? Boardworks AS Chemistry Halogens The halogens are the elements in Group 7 of the periodic table. The name halogen comes from the Greek words for salt-making.

6 Group 7 Elements: The Halogens
Physical Properties: Low melting and boiling points Exist as diatomic molecules On descending the group, no. of electrons increases  increasing van der Waals’ forces between molecules. Boiling points increase on descending the group. The physical states of the halogens at room temperature; Gas  Liquid  Solid On descending the group

7 Physical properties of halogens
Boardworks AS Chemistry Halogens

8 Trends in boiling point
Boardworks AS Chemistry Halogens 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. van der Waals forces Teacher notes See the ‘Bonding and Intermolecular Forces’ presentation for more information about van der Waals forces. fluorine atomic radius = 42 × m boiling point = °C iodine atomic radius = 115 × m boiling point = 184 °C

9 Trends in electronegativity
Boardworks AS Chemistry Halogens Electronegativity of the halogens decreases down the group due to an increase in atomic radius. 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. Teacher notes See the ‘Bonding and Intermolecular Forces’ presentation for more information about electronegativity and its relationship to atomic radius and shielding. fluorine atomic radius = 42 × m electronegativity = 4.0 iodine atomic radius = 115 × m electronegativity = 2.5

10 Boardworks AS Chemistry Halogens
Astatine Boardworks AS Chemistry Halogens The name astatine comes from the Greek word for unstable. 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 (210At) has a half-life of only 8 hours. It was first made artificially in 1940, by bombarding 209Bi with a-radiation. What do you predict for these properties of astatine? Teacher notes The square brackets around the mass number for astatine indicate that this is the atomic mass of the most stable isotope. Astatine is the rarest naturally-occurring element. The properties of astatine: black (presumed), solid at room temperature (presumed), Pauling electronegativity of 2.2 colour state at room temperature electronegativity.

11 Halogens: true or false?
Boardworks AS Chemistry Halogens Teacher notes This slide can be used for revision of the material about halogens covered at GCSE.

12 Boardworks AS Chemistry Halogens
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

13 Group 7 Elements: Electron Configuration Electron Configuration
[He]2s22p5 Cl [Ne]3s23p5 Br [Ar]4s24p5 I [Kr]5s25p5 At [Xe]6s26p5 7 electrons in the outer shell, highest energy electrons in a p sub-shell Each element has two fewer electrons than the next noble gas Outer p sub-shell containing 5 electrons (needs 1 more to form a -1 ion)

14 The Halogens: Oxidising Agents
The halogens are the most reactive non-metals in the periodic table Strong oxidising agents (they gain electrons) Oxidising power decreases down the group. Therefore less reactive down the group ½ Cl2(g) + e-  Cl-(g) (-1 oxidation state) Reactivity decreases down the group because; Atomic radius decreases Electron shielding increases Ability to gain an electron decreases

15 Reactions of the halogens
Boardworks AS Chemistry Halogens Halogens react with metals such as sodium and iron: halogen + sodium ® sodium halide They also react with non-metals such as hydrogen: halogen + hydrogen ® hydrogen halide 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: Teacher notes Students could be asked to write balanced symbol equations for these reactions. chlorine + potassium bromide potassium chloride ® bromine +

16 Boardworks AS Chemistry Halogens
Reaction with iron Boardworks AS Chemistry Halogens Teacher notes This virtual experiment compares the reactivity of the halogens with iron wool. It could be used as a precursor to running the practical in the lab, or as a revision exercise. When using this activity it should be made clear that for iodine the reaction takes several minutes of strong heating before it reacts. This is illustrated by the stop watch, but is not run in real time.

17 Reactions with hydrogen
Boardworks AS Chemistry Halogens The halogens react with hydrogen gas to product hydrogen halides. For example: Cl2(g) + H2(g) ® 2HCl(g) Chlorine and hydrogen explode in bright sunlight but react slowly in the dark. Bromine and hydrogen react slowly on heating with a platinum catalyst. Teacher notes Substances can obtain the activation energy they need to react from (sun)light. Halogens can also react with other non-metals. For example, fluorine is so reactive that it can even form bonds with the noble gases such as xenon. Xenon(II) fluoride (XeF2) is made by the reaction of xenon and fluorine, using heat or electricity to provide the activation energy. It is used to etch patterns on silicon chips. If XeF2 is heated, it reacts to form xenon(VI) fluoride, XeF6. XeF4 also exists. The xenon fluorides are often used in exam question to test knowledge of molecular shapes. XeF2 is linear, XeF4 is square planar, and XeF6 is octahedral. See the ‘Structure and Shape’ presentation for more information about molecular shapes. Iodine combines partially and very slowly with hydrogen, even on heating.

18 Redox reactions of halogens
Boardworks AS Chemistry Halogens Teacher notes Students should remember the mnemonic OILRIG – Oxidation Is Loss of electrons; Reduction Is Gain of electrons. See the ‘Redox reactions’ presentation for more information about redox reactions and half equations.

19 What is the reactivity of the halogens?
Boardworks AS Chemistry Halogens The reactions of the halogens with iron and hydrogen show that their reactivity decreases down the group. Halogen Reaction with iron wool Reaction with hydrogen chlorine Iron wool burns and glows brightly. Explodes in sunlight, reacts slowly in the dark. bromine Iron wool glows but less brightly than with chlorine. Reacts slowly on heating with catalyst. Teacher notes Fluorine and hydrogen explode even in the dark at temperatures as low as -200°C. Fluorine and iron wool ignite immediately and burn more vigorously than chlorine and iron wool. The reactions of astatine would be expected to be similar to those of iodine but less vigorous. iodine Iron wool has a very slight glow. Reacts partially and very slowly. How do you think fluorine and astatine would react with iron wool and hydrogen?

20 Electron structure and reactivity
Boardworks AS Chemistry Halogens

21 Halogen displacement reactions
Boardworks AS Chemistry Halogens

22 Halogen displacement reactions
Boardworks AS Chemistry Halogens

23 Halogen displacement reactions
Boardworks AS Chemistry Halogens Halogen displacement reactions are redox reactions. Cl2 + 2KBr ® 2KCl + Br2 To look at the transfer of electrons in this reaction, the following two half equations can be written: Cl2 + 2e-  2Cl- 2Br-  Br2 + 2e- What has been oxidized and what has been reduced? Teacher notes See the ‘Redox Reactions’ presentation for more information about redox reactions. Chlorine has gained electrons, so it is reduced to Cl- ions. Bromide ions have lost electrons, so they have been oxidized to bromine.

24 Oxidizing ability of halogens
Boardworks AS Chemistry Halogens In displacement reactions between halogens and halides, the halogen acts as an oxidizing agent. fluorine This means that the halogen: oxidizes the halide ion to the halogen chlorine increasing oxidizing ability gains electrons Teacher notes Students could be encouraged to see the relative oxidizing ability of the halogens as their ability to accept electrons, relating it to the size, nuclear charge and shielding. Also they could connect this trend to the electronegativity values and with the electron structures of the ions. bromine is reduced to form the halide ion. What is the order of oxidizing ability of the halogens? iodine

25 The Halogens: Recognising them
The halogens form solutions with different colours. A colour change will show if a reaction has taken place. Halogen Water cyclohexane Cl2 Pale-green Br2 Orange I2 Brown violet If you shake the reaction mixture with an organic solvent it can help distinguish between halogens that have reacted

26 Oxidizing ability of halogens
Boardworks AS Chemistry Halogens

27 Chlorine and disproportionation
Boardworks AS Chemistry Halogens Teacher notes The reaction of chlorine and water shown here is used in water purification: see slide 22 of this presentation. The reaction of sodium hydroxide and water shown here is used to make bleach: see slide 23 of this presentation.

28 Reaction of chlorine with water
Boardworks AS Chemistry Halogens 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. Photo credit: Danvlchenko Iaroslav / Shutterstock.com Teacher notes Addition of fluoride to drinking water is also carried out in certain areas and there are debates about extending the practice. The main benefit is reduction of tooth decay. However, fluoride can cause dental fluorosis (flecks of white in the teeth which can turn brown) and critics have linked it to other, more serious, health problems. Chlorination of drinking water raises questions about individual freedom because it makes it difficult for individuals to opt out.

29 Bleach and the chlorate(I) ion
Boardworks AS Chemistry Halogens Household bleach commonly contains the chlorate(I) ion, ClO-, in the form of sodium chlorate(I), NaOCl. The chlorate(I) ion behaves as an oxidizing agent. It oxidizes the organic compounds in food stains, bacteria and dyes. ClO- + H2O ® Cl- + 2OH- 2e- How many electrons are needed to balance this equation? Has the chlorine been oxidized or reduced in the reaction? Teacher notes The chlorate(I) ion reacts with acids to form chlorine. Care must therefore be taken to avoid bleach coming into contact with acids in the home because chlorine gas is toxic. The chlorate(I) ion is unstable. It decomposes to form chloride ions and the chlorate(V) ion, ClO3-: 3ClO- → 2Cl- + ClO3- This is another disproportionation reaction, in which the chlorine atoms are both oxidized from oxidation state +1 to +5 in ClO3-, and reduced to -1 in Cl-. The chlorate(V) ion is an oxidizing agent: ClO3- + 6H+ + 6e- → Cl- + 3H2O Potassium and sodium chlorate, KClO3 and NaClO3, are used in weedkillers. They can also be used to oxidize the sucrose molecules in jelly babies in a common demonstration. The chlorine has been reduced because it has gained electrons. Its oxidation state has decreased from +1 in ClO- to –1 in Cl-.

30 Redox reactions of chlorate ions
Boardworks AS Chemistry Halogens

31 Boardworks AS Chemistry Halogens
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

32 Uses and Halide Tests Lesson Objectives:
• Interpret and make predictions from the chemical and physical properties of the Group 7 elements/compounds. • Contrast the benefits and risks of chlorine’s use as a water treatment. • Describe the precipitation reactions of aqueous anions Cl–, Br– and I– with aqueous silver ions, followed by aqueous ammonia. • Recognise the use of these precipitation reactions as a test for different halide ions. Key Words: reactivity, displacement reaction, disproportionation, precipitation reaction

33 Boardworks AS Chemistry Halogens
Halides Boardworks AS Chemistry Halogens When halogens react with metals, they form compounds called halides. Many naturally-occurring halides have industrial, household and medical applications. Halide Formula Uses caesium chloride CsCl Extraction and separation of DNA sodium hexafluoroaluminate NaAlF6 Electrolysis of aluminium oxide titanium(IV) chloride Teacher notes Sodium hexafluoroaluminate is commonly known as cryolite. See the ‘Redox Reactions’ presentation for more information about the electrolysis of aluminium oxide and the extraction of titanium. TiCl4 Extraction of titanium lithium iodide LiI Electrolyte in batteries potassium bromide KBr Epilepsy treatment in animals

34 Identifying halide ions
Boardworks AS Chemistry Halogens Halides can be identified by their reaction with acidified silver nitrate solution to form silver halide precipitates. potassium chloride silver chloride + potassium nitrate silver nitrate KCl(aq) + AgNO3(aq) ® KNO3(aq) AgCl(s) Photo credit: Martyn F. Chillmaid / Science Photo Library Teacher notes The equation shows the reaction behind the detection of chloride ions in potassium chloride. Silver chloride has a low solubility in water, so it forms a white precipitate: the positive result in the test for chloride ions.

35 Identifying halide ions
Boardworks AS Chemistry Halogens Teacher notes Silver fluoride is soluble in water so this test cannot be used to identify fluoride ions.

36 Identifying halide ions
Boardworks AS Chemistry Halogens

37 Uses of halides in photography
Boardworks AS Chemistry Halogens Silver halides are used in photography. 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. Ag+ + e- ® Ag light silver precipitate mask paper coated in silver halide white paper under mask

38 Boardworks AS Chemistry Halogens
William Fox Talbot Boardworks AS Chemistry Halogens 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. 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. 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. Teacher notes The picture shown is an image by Talbot of a latticed window in Lacock Abbey in It is a print from the oldest photographic negative in existence. Images produced using Daguerre’s method are called daguerrotypes. Fox Talbot’s Pencil of Nature (1844) was the first book to be illustrated with photographs. Fox Talbot also made major contributions to the development of photography as an artistic medium.

39 Boardworks AS Chemistry Halogens
Hydrogen halides Boardworks AS Chemistry Halogens The hydrogen halides are colourless gases at room temperature. Hydrogen halide Boiling point (°C) HF 20 HCl -85 HBr -67 HI -35 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. Teacher notes The hydrogen-fluorine bond is highly polar because fluorine is much more electronegative than hydrogen, and each fluorine atom in HF has three lone pairs of electrons. The hydrogen atoms therefore have a partial positive charge and the fluorine atoms have a partial negative charge. A lone pair of electrons on the fluorine atom in one molecule is attracted to the δ+ hydrogen atom in a neighbouring molecule. This increases the strength of intermolecular forces, which increases the boiling point. The bonds in the other hydrogen halides are less polar because the other halogen atoms are less electronegative than fluorine. The lone pairs on the other halogen atoms are also in higher energy levels so their charge is less concentrated. Students could be asked to predict the boiling point of HAt. See the ‘Bonding and Intermolecular Forces’ presentation for more information about hydrogen bonding.

40 Halides as reducing agents
Boardworks AS Chemistry Halogens A substance that donates electrons in a reaction (i.e. is oxidized) is a reducing agent because it reduces the other reactant. fluoride The larger the halide ion, the easier it is for it to donate electrons and therefore the more reactive it is. chloride increasing reducing ability 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. Teacher notes See the ‘Trends in Period 3’ presentation for more information about shielding. bromide iodide

41 Boardworks AS Chemistry Halogens
Halides: true or false? Boardworks AS Chemistry Halogens

42 Sodium halides and sulfuric acid
Boardworks AS Chemistry Halogens The sodium halides react with concentrated sulfuric acid. During this reaction two things can happen to the sulfuric acid. It can be reduced act as an acid. Teacher notes Here the simplest definition of ‘acid’ to use is the Bronsted–Lowry definition of an acid as a proton (H+) donor. The reactions of sodium halides with concentrated sulfuric acid demonstrate the relative strengths of the halide ions as reducing agents.

43 Sodium halides and sulfuric acid
Boardworks AS Chemistry Halogens

44 Sodium halides and sulfuric acid
Boardworks AS Chemistry Halogens Teacher notes See the ‘Redox Reactions’ presentation for more information about oxidation states, oxidation and reduction.

45 Boardworks AS Chemistry Halogens
Oxidation states Boardworks AS Chemistry Halogens

46 Sodium halides and sulfuric acid
Boardworks AS Chemistry Halogens

47 Boardworks AS Chemistry Halogens
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

48 Boardworks AS Chemistry Halogens
Glossary Boardworks AS Chemistry Halogens Teacher notes acid – A species capable of donating an H+ ion (a proton) to another species; for example, sulfuric acid in its reaction with a sodium halide. (This is the Brønsted–Lowry definition of an acid). displacement reaction – A reaction in which an element is displaced out of a compound by a more reactive element; for example, bromine displaces iodine from potassium iodide: Br2 + 2KI  2KBr + I2. disproportionation – A reaction in which the same element is both oxidized and reduced; for example, chlorine in: Cl2 + H2O > HCl + HOCl. electronegativity – The power of an atom to withdraw electron density and attract the pair of bonding electrons in a covalent bond. half equation – An equation describing either the oxidation or reduction in a redox reaction in terms of electron loss and electron gain. For example, Cl2 + 2e-  2Cl- shows the reduction of chlorine to chloride ions. halide – A salt formed by the reaction of a halogen with a metal. halide ion – A negative ion formed by the addition of an electron to an atom of a halogen. halogen – A group 7 element. hydrogen bonding – A special example of a permanent dipole–dipole intermolecular force that occurs when hydrogen is bonded to nitrogen, oxygen or fluorine. A hydrogen bond is the attraction between a lone pair on a nitrogen, oxygen or fluorine atom on one molecule, and the d+ hydrogen atom (bonded to nitrogen, oxygen or fluorine) on a neighbouring molecule. oxidation state – The charge a particular atom in a compound would have if the compound consisted entirely of separate ions. oxidation – The process during which an atom, molecule or ion loses one or more electrons and increases in oxidation state. oxidizing agent – A species that accepts electrons, thus oxidizing another species while it is itself reduced. precipitate – A solid that forms in a solution during a chemical reaction. redox – Short for reduction / oxidation. A chemical reaction in which the reacting species have their oxidation states changed. A redox reaction is one in which both oxidation and reduction take place. reduction – The process in which an atom, molecule or ion gains one or more electrons and decreases in oxidation state. reducing agent – A species that donates electrons, thus reducing another species while it is itself oxidized. van der Waals forces – Weak intermolecular forces of attraction that occur when temporary dipoles cause induced dipoles in neighbouring atoms.

49 Boardworks AS Chemistry Halogens
What’s the keyword? Boardworks AS Chemistry Halogens

50 Boardworks AS Chemistry Halogens
Multiple-choice quiz Boardworks AS Chemistry Halogens


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