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Halogens. Objectives Be able to... Recall the main properties and trends of the Halogens Explain trends using knowledge of intermolecular bonding and.

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Presentation on theme: "Halogens. Objectives Be able to... Recall the main properties and trends of the Halogens Explain trends using knowledge of intermolecular bonding and."— Presentation transcript:

1 Halogens

2 Objectives Be able to... Recall the main properties and trends of the Halogens Explain trends using knowledge of intermolecular bonding and redox Write half equations for displacement reactions and electrolysis

3 Specification... Where this comes from...

4 GROUP PROPERTIES GENERAL non-metals exist as separate diatomic molecules… eg Cl 2 all have the electronic configuration... ns 2 np 5 TRENDS appearance boiling point electronic configuration electronegativity atomic size ionic size oxidising power

5 5 of 43© Boardworks Ltd 2009 Physical properties of halogens

6 6 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

7 7 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.

8 8 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

9 Appearance Halogen \ SymbolStateColour Colour of vapour FluorineF Chlorine Cl Bromine Br IodineI

10 Appearance Halogen \ SymbolStateColour Colour of vapour FluorineFgas Pale yellow Yellow Chlorine Cl Cl gas Pale green Green Bromine Brliquid Orange / brown Orange Iodine I solid Grey-black crystals Purple What would you predict about the appearance of astatine?

11 Physical Properties Fluorine (F 2 ) Chlorine (Cl 2 ) Bromine (Br 2 ) Iodine (I 2 ) What happens to the physical properties as you go down Group VII?

12 Trends in Physical Properties Fluorine (F 2 ) Chlorine (Cl 2 ) Bromine (Br 2 ) Iodine (I 2 ) Melting points increase Boiling points increase All more soluble in organic solvents than water Make sure you can use intermolecular forces to EXPLAIN each of these trends

13 Reactivity of the Group 7 elements Fluorine (F 2 ) Chlorine (Cl 2 ) Bromine (Br 2 ) Iodine (I 2 ) Decreasing reactivity (This is due to it getting less easy for the atoms to form negative ions by gaining electrons) Make sure you can EXPLAIN this trend in terms of atomic size, shielding and nuclear attraction

14 Displacement Reactions Which of the following reactions will take place? Chlorine + potassium bromide Bromine + potassium chloride Iodine + potassium chloride Bromine + potassium iodide What has to be true for a displacement reaction to take place?

15 15 of 43© Boardworks Ltd 2009 Electron structure and reactivity

16 16 of 43© Boardworks Ltd 2009 Halogen displacement reactions

17 17 of 43© Boardworks Ltd 2009 Halogen displacement reactions

18 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 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 20 of 43© Boardworks Ltd 2009 Oxidizing ability of halogens

21 Displacement Reactions Writing an ionic equation for a displacement reaction : Br 2 + 2I - 2Br - + I 2 Which species is oxidised in this reaction? Which species is the oxidising agent? Which species is the reductant?

22 Displacement Reactions Chlorine can oxidise both bromide and iodide ions: Cl 2 + 2Br - 2Cl - + Br 2 Cl 2 + 2I - 2Cl - + I 2 However, bromine can only oxidise iodide ions: Br 2 + 2Cl - no reaction Br 2 + 2I - 2Br - + I 2 So, chlorine is a stronger oxidising agent than bromine.

23 Oxidising ability of the halogens Fluorine (F 2 ) Chlorine (Cl 2 ) Bromine (Br 2 ) Iodine (I 2 ) Decreasing oxidising ability Decreasing reactivity

24 Reducing ability of the halides Fluoride (F - ) Chloride (Cl - ) Bromide (Br - ) Iodide (I - ) Increasing reducing ability

25 GROUP TRENDS DECREASES down Group the increasing nuclear charge due to the greater number of protons should attract electrons more, but there is an... an increasing number of shells;  more shielding and less pull on electrons an increasing atomic radius  attraction drops off as distance increases FClCl BrI ELECTRONEGATIVITY Electronegativity

26 GROUP TRENDS halogens are oxidising agents they need one electron to complete their octet the oxidising power gets weaker down the group OXIDISING POWER

27 GROUP TRENDS halogens are oxidising agents they need one electron to complete their octet the oxidising power gets weaker down the group the trend can be explained by considering the nucleus’s attraction for the incoming electron which is affected by the... increasing nuclear charge which should attract electrons more but this is offset by INCREASED SHIELDING INCREASING ATOMIC RADIUS OXIDISING POWER

28 GROUP TRENDS halogens are oxidising agents they need one electron to complete their octet the oxidising power gets weaker down the group the trend can be explained by considering the nucleus’s attraction for the incoming electron which is affected by the... increasing nuclear charge which should attract electrons more but this is offset by INCREASED SHIELDING INCREASING ATOMIC RADIUS This is demonstrated by reacting the halogens with other halide ions. OXIDISING POWER

29 Reactions with Silver Nitrate Can you suggest a use for the silver halides? Which silver halide precipitate is formed most quickly? How is AgNO 3 used to test for halide ions? Halide Colour of precipitate Formula of precipitate Solubility in ammonia Fluorideno ppt. soluble Chloride whiteAgCl soluble BromidecreamAgBr partially soluble IodideyellowAgI insoluble


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