Group 7 elements: Redox reactions:
These are F, Cl, Br, I and At
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.
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:
Loss of electrons
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.
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|
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 Cl2(aq) + 2Br-(aq) à 2Cl-(aq) + Br2(aq)
Pale green Orange
2 Cl2(aq) + 2I-(aq) à 2Cl-(aq) + I2(aq)
Pale green Brown
3 Br2(aq) + 2I-(aq) à 2Br-(aq) + I2(aq)
Reactions 1 – 3 involve a transfer of an electron from the halide to the halogen.
Cl2(aq) + 2Br-(aq) à 2Cl-(aq) + Br2(aq)
|Element||Cl2(aq)||+||2Br-(aq)||à||2Cl-(aq)||+||Br2(aq)||Change in Ox No||Redox|
The reduction of Cl2(aq) oxidises Br-(aq) à Cl2(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.
This is a reaction when the same element has been both oxidised and reduced
Disproportionation of chlorine in water (Bleach):
Cl2(aq) + 2H2O(l) à HCl(aq) + HClO(aq)
|Change in Ox No||Redox|
Disproportionation of chlorine in aq sodium hydroxide:
Cl2(aq) + 2NaOH(aq) à NaCl(aq) + NaClO(aq) + H2O(l)
0 -1 +1
|+||H2O(l)||Change in Ox No||Redox|
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.
|NaF / SnF2||Toothpaste|
|CaF2 (Fluorite / fluorspar)||Used to make lenses to focus IR light|
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
|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