Group 7 elements:  Redox reactions:

• These are F, Cl, Br, I and At

Physical properties:

• The melting and boiling point of the halogens increases with atomic number due to increased van der Waals. 

• This can be seen by their physical states at room temperature.

• Iodine sublimes to a purple vapour.

Electronic configuration:

Each Group 7 element has 7 electrons in the outer shell. Each have s2 p5 configuration, and known are p block elements. The require 1 electron to complete their outer shell

Halogens as oxidising agents:

• These elements gain 1 electron when they react.

• This means that what ever they react with must lose electrons.

• Losing electrons is an oxidation reaction:

Oxidation

            Is

            Loss of electrons

            Reduction

            Is

            Gain of electrons

• As the Group 7 elements cause the oxidation of other compounds or elements we say it is a good Oxidising agent.

• Reactivity decreases as you go down the Group.  This means they gain their electrons less readily.

• This means as you go down Group 7, their oxidising power decreases.

Explanation:

• There are more electron shells which increases shielding between nucleus and outer electron shell.

• As there are more electron shells, the distance between nucleus and outer shell increases.

• This means that attraction between the nucleus and outer electrons decreases.

• This means that the incoming electron is not captured as easily:

Redox reactions of the halogens:

• Redox reactions can show how the Halogens ability to form ions reduces as you go down the Group (ie its reactivity)

• By competing the Halogens (Cl₂, Br₂, and I₂) with the Halides (Cl^-, Br^-, and I^-)

• Each Halogen is mixed with the Halides.

• The more reactive Halogen will oxidise and displace the Halide of a less reactive Halogen.

• Halogens are coloured in solution, this can indicate whether a redox reaction has occurred:

	Halogen	In water	In cyclohexane
	Cl2	Pale green	Pale Green
	Br2	Orange	Orange
	I2	Brown	Violet

Interpretation:

• Reactivity decreases as you go down group 7.

• It becomes harder to capture an electron with increased distance and shielding.  Ie the opposite of group 2 elements.

• The reactions are:-

1                       Cl_2(aq)         +          2Br^-_(aq)         à         2Cl^-_(aq)            +             Br_2(aq)

                       Pale green                                                                                            Orange

                                                                                                                                       Orange (CH)

2                       Cl_2(aq)         +          2I^-_(aq)            à         2Cl^-_(aq)            +             I_2(aq)

                      Pale green                                                                                             Brown

                                                                                                                                       Purple (CH)

3                       Br_2(aq)         +          2I^-_(aq)            à          2Br^-_(aq)            +             I_2(aq)

                         Orange                                                                                                 Brown

                                                                                                                                        Violet (CH)

• Reactions 1 – 3 involve a transfer of an electron from the halide to the halogen.

                       Cl_2(aq)         +          2Br^-_(aq)         à         2Cl^-_(aq)            +             Br_2(aq)

• The reduction of Cl_2(aq) oxidises Br^-_(aq)             à   Cl_2(aq) is an Oxidising agent

• In each reaction the halogen is an oxidising agent and the halide is a reducing agent.

• As chlorine is the most reactive it is the strongest oxidising agent.

Disproportionation

            This is a reaction when the same element has been both oxidised and reduced

Disproportionation of chlorine in water (Bleach):

       Cl_2(aq)      +       2H₂O_(l)       à        HCl_(aq)           +       HClO_(aq)

• In this reaction, the Cl has been oxidised and reduced

Disproportionation of chlorine in aq sodium hydroxide:

Cl_2(aq)      +       2NaOH_(aq)       à        NaCl_(aq)           +       NaClO_(aq)         +          H₂O_(l)

          0                                                            -1                            +1

• The Cl has been oxidised and reduced in this reaction too.

Questions 1 - 2  P 93

Group 7 elements:  Uses and halide tests

Properties of Group 7 elements and compounds:

• Because of Periodicity we only have to learn the Chemistry for one of the elements in Group 7.

• They all form diatomic covalent molecules, X will be used to represent any halogen.

• All the elements in Group 7 will react in the same way (but with different vigour).

• Group 7 elements form have Van Der Waals forces of attraction - increasing down the Group - more electrons

• As you go down Group 7: the elements become less reactive.

• As you go down Group 7: the elements become a weaker oxidising agent.

• Group 7 elements form ionic halides with metals.

Fluorine:

• Is the most reactive element on the Periodic Table.
• It burns virtually anything in its path.
• Many Chemists have been killed in the explosion when using fluorine.

Halides:

• Usually have a 1- charge, X^1-

Testing for Halide ions:

• A simple test tube test can be done to identify halide ions in compounds.

• Dissolve a small amount of compound in water

• Add silver nitrate, AgNO₃.

• The silver ions, Ag⁺ combines with the Halide ions, X^- to form a silver halide precipitate

• The silver halide precipitates are coloured depending upon the halide present.

• Sometimes it is difficult to judge the exact colour.

• Ammonia can be added as the different silver halides as they have different solubility's in ammonia

Interpretation:

	Chloride:	Ag+(aq)	+	Cl-(aq)	à	AgCl(s)	White precipitate	Soluble in dilute NH3
	Bromide:	Ag+(aq)	+	Br-(aq)	à	AgBr(s)	Cream precipitate	Soluble in conc NH3
	Iodide:	Ag+(aq)	+	I-(aq)	à	AgI(s)	Yellow precipitate	Insoluble in conc NH3

• The solubility of the precipitates decreases down the group.

Question 1 - 3  P95  /  5,6,7,8,10 and 14  P97  /  5,6 and 7  P99