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Halogens.

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Presentation on theme: "Halogens."— 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 • exist as separate diatomic molecules… eg Cl2
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
Boardworks AS Chemistry Halogens Physical properties of halogens

6 Trends in boiling point
Boardworks AS Chemistry Halogens 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 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

7 Trends in electronegativity
Boardworks AS Chemistry Halogens 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. 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

8 Boardworks AS Chemistry Halogens
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? 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.

9 Appearance Fluorine F Chlorine Cl Bromine Br Iodine I Halogen Symbol
\ Symbol State Colour Colour of vapour Fluorine F Chlorine Cl Bromine Br Iodine I

10 What would you predict about the appearance of astatine?
Halogen \ Symbol State Colour Colour of vapour Fluorine F gas Pale yellow Yellow Chlorine Cl Pale green Green Bromine Br liquid Orange / brown Orange Iodine I solid Grey-black crystals Purple What would you predict about the appearance of astatine?

11 What happens to the physical properties as you go down Group VII?
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 AS Chemistry - Halogens (BEL - May 2002)

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
Boardworks AS Chemistry Halogens Electron structure and reactivity

16 Halogen displacement reactions
Boardworks AS Chemistry Halogens Halogen displacement reactions

17 Halogen displacement reactions
Boardworks AS Chemistry Halogens Halogen displacement reactions

18 Halogen displacement reactions
Boardworks AS Chemistry Halogens 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: 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.

19 Oxidizing ability of halogens
Boardworks AS Chemistry Halogens 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 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

20 Oxidizing ability of halogens
Boardworks AS Chemistry Halogens Oxidizing ability of halogens

21 Displacement Reactions
Writing an ionic equation for a displacement reaction: Br I Br- + 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: Cl Br Cl- + Br2 Cl I Cl- + I2 However, bromine can only oxidise iodide ions: Br Cl no reaction Br I Br- + 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 Fluoride (F-) Chloride (Cl-) Bromide (Br-) Iodide (I-)
Reducing ability of the halides Fluoride (F-) Chloride (Cl-) Bromide (Br-) Iodide (I-) Increasing reducing ability

25  attraction drops off as distance increases
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 This is demonstrated by reacting the halogens with other halide ions.
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
Halide Colour of precipitate Formula of precipitate Solubility in ammonia Fluoride no ppt. soluble Chloride white AgCl Bromide cream AgBr partially soluble Iodide yellow AgI insoluble 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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