8.2 The Halogens
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How do the halogens occur on Earth
As stable halide ions Cl- Br- and I-, dissolved in sea water or combined with sodium or potassium as solid deposits such as in salt mines
At RTP, all halogens exist in ______ molecules
Diatomic
In their solid states, the halogens form ______ with ______ molecular structures
Lattices Simple
How many electrons are in the outer shell of a halogen
7
What type of reaction is the most common for the halogens
Redox Halogen is reduced gaining one electron
What do the results of a displacement reaction of halogens with halide ions show
That the reactivity of the halogens decreases down the group
If the halogen added is more reactive than the halide present:
A reaction takes place, the halogen displacing the halide from solution The solution changes colour
How can you tell the difference between iodine and bromine
An organic non-polar solvent such as cyclohexane can be added and the mixture shaken The non-polar halogens dissolve more readily in cyclohexane than in water The colours are much easier to tell apart with iodine being a deep violet
Fluorine gas
Pale yellow gas Reacts with almost any substance it comes in contact with
Why is Astatine extremely rare
It is radioactive and decays rapidly and the element has never been seen
How do halogens react in redox reactions
Gain electrons
Trend in reactivity of halogens
The tendency to gain an electron decreases and they become less reactive going down the group Atomic radius increases, more inner shells so shielding increases, less nuclear attraction to capture an electron from another species
Which of the halogens is the strongest oxidising agent
Fluorine Gains electrons from other species more readily than the other halogens.
What is disproportionation
A redox reaction in which the same element is oxidised and reduced
What name is given to a reaction where the same element is both oxidised and reduced
A disproportionation reaction
Examples of disproportionation reactions
Chlorine and water Chlorine and cold dilute sodium hydroxide
What is chlorine used in
Water purification
What happens when chlorine is added to water
A disproportionation reaction takes place. For each chlorine molecule, one chlorine atom is oxidised and the other is reduced
How does chlorine kill bacteria when added to wter
Disproportionation reaction occurs HClO and HCl are produced The bacteria are killed by chloric acid and chlorate ions rather than by chlorine Chloric acid acts as a weak bleach
Why is the reaction of chlorine with water limited
Due to the low solubility of chlorine in water If the water contains dissolved sodium hydroxide, much more chlorine dissolves and another disproportionation reaction takes place
Chlorine in drinking water can react with ______ ______ such as methane formed by decaying ______. Chlorinated hydrocarbons are formed which are suspected of causing ______. However, the overall risk to health of not adding chlorine is far ______ than the risk posed by the chlorinated hydrocarbons.
Organic Hydrocarbons Vegetation Cancer Greater
What is the test for halide ions
Aqueous halide ions react with aqueous silver ions to form precipitates of silver halides as shown by the general equation Ag+(aq) + X-(aq) -> AgX(s)