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Halogens

Halogens. 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. Specification... Where this comes from. GROUP PROPERTIES.

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Halogens

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  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 Cl2 • all have the electronic configuration ... ns2 np5 TRENDS•appearance • boiling point • electronic configuration • electronegativity • atomic size • ionic size • oxidising power

  5. Physical properties of halogens

  6. 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. van der Waals forces fluorineatomic radius = 42 × 10-12mboiling point = -118°C iodine atomic radius = 115 × 10-12mboiling point = 184°C

  7. Trends in electronegativity 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. fluorineatomic radius = 42 × 10-12melectronegativity = 4.0 iodineatomic radius = 115 × 10-12melectronegativity = 2.5

  8. Astatine 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? • colour • state at room temperature • electronegativity.

  9. Appearance

  10. Appearance What would you predict about the appearance of astatine?

  11. Physical Properties • Fluorine (F2) • Chlorine (Cl2) • Bromine (Br2) • Iodine (I2) What happens to the physical properties as you go down Group VII?

  12. Trends in Physical Properties • Fluorine (F2) • Chlorine (Cl2) • Bromine (Br2) • Iodine (I2) • 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 (F2) • Chlorine (Cl2) • Bromine (Br2) • Iodine (I2) • 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. Electron structure and reactivity

  16. Halogen displacement reactions

  17. Halogen displacement reactions

  18. Cl2 + 2e- 2Cl- 2Br- Br2 + 2e- Halogen displacement reactions 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: What has been oxidized and what has been reduced? • Chlorine has gained electrons, so it is reduced to Cl- ions. • Bromide ions have lost electrons, so they have been oxidized to bromine.

  19. Oxidizing ability of 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 bromine • is reduced to form the halide ion. What is the order of oxidizing ability of the halogens? iodine

  20. Oxidizing ability of halogens

  21. Displacement Reactions Writing an ionic equation for a displacement reaction: Br2 + 2I- 2Br-+ I2 • 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: • Cl2 + 2Br- 2Cl- + Br2 • Cl2 + 2I- 2Cl- + I2 However, bromine can only oxidise iodide ions: • Br2 + 2Cl- no reaction • Br2 + 2I- 2Br- + I2 So, chlorine is a stronger oxidising agent than bromine.

  23. Oxidising ability of the halogens • Fluorine (F2) • Chlorine (Cl2) • Bromine (Br2) • Iodine (I2) 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 ELECTRONEGATIVITY F Cl Br I Electronegativity 4.0 3.5 2.8 2.5 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

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

  27. GROUP TRENDS OXIDISING POWER • 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

  28. GROUP TRENDS OXIDISING POWER • 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.

  29. Reactions with Silver Nitrate How is AgNO3 used to test for halide ions? Which silver halide precipitate is formed most quickly? Can you suggest a use for the silver halides?

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