3.2.3 Group 7, the halogens

Trend in oxidising ability

Halogens are oxidising agents that react by gaining an electron

Florine is the strongest oxidising agent as it accepts electrons more easily

Iodine is the weakest oxidising agent at it accepts electrons less easily

Oxidising ability decreases down the group:

• Number of shells increase to sheilding increases

• Atom becomes larger hence the outer shell is further away from the nucleus

• Attraction of electrons to the nucleus is weaker

• So atom is less likely to accept electrons

Relative reducing ability of halide ions

Reducing ability increases

• Ionic radius increases down the group as more electron shells are added

• The increasing radius leads to outer shells being further away from the nucleus

• The outer electrons experience more shielding from the inner electrons

• The electrostatic attraction between the outer electron and the nucleus gets weaker

• Increase in ionic radius and shielding is greater than the increase in nuclear charge

• Easier for larger halide ions to become oxidised and for an electron to leave

Trend in boiling points

Increases down the group

• the molecule becomes larger in size and relative mass

• Down the group the halogen molecules have larger electron clouds

• This allows for stronger induced dipole dipole forces between molecules

• These stronger intermolecular forces require increasing amounts of energy to overcome

Trend in electronegativity

Decreases down the group

• Nuclear charge increases down the group as more protons are present

• Atomic radius increases down the group, as more electron shells, greater shielding

• Atomic radius and shielding effect is greater than the nuclear charge effect

• So easier to remove an electron

Halogen halide displacement reaction

The more reactive halogen will displace the less reactive halogen from an aqueous halide solution

The displacing halogen is reduces and gains an electron to form a halide ion

The displaced halide gets oxidised and loses an electron to form a halogen

Outcomes of displacement reactions and observations

Sodium solid Halide ion reactions: why is it reacted with sulfuric acid

React with concentrated sulfuric acid because

• It acts as an acid, forming hydrogen halides

• Can act as an oxidising agent if the hydrogen halide formed is a strong reducing agent

Observations with solid sodium halide and sulfuric acid

In sodium chloride, chloride ions are not strong enough reducing agents to reduce sulfuric acid

Sodium bromide initially forms hydrogen bromide. Hydrogen bromide reduces sulfuric acid to sulfur dioxide and is oxidised to bromine

Sodium iodide forms hydrogen iodide, which is a stronger reducing agents, so reduces sulfuric acid further to different sulfur containing products

Test for halide ions

Reagents: silver nitrate and nitric acid

• Nitric acid is added first to remove ions that interfere with the test such as carbonate and hydroxide ions

• If the halide ion is in solids state, dissolve in water first

• Hydrochloric acid is never used as it forms a white precipitate

• Cannot test for fluoride ions because silver chloride is soluble and does not form a precipitate

Confirmation test for halide ion presence

Reagent: dilute and concentrated ammonia

Chlorine reactions:

• Absence of sunlight- disproportionation reaction

• with sunlight

• With sodium chlorate

Chlorine reactions extra

Water chlorination

chlorine is added to water to kill harmful microorganisms reducing the chance that disease carrying organisms will be carried from a water supply to people

Chlorine is toxic, but the benefits outweigh the possible side effects

Controlled chlorination can greatly reduce public health risk

Adding sodium fluoride to water

May be added to water to increase the fluoride concentration

It reduces tooth decay

Potential harmful effects at different exposure levels

Excess fluoride exposure can cause dental fluorosis

Hazard controls when testing for presence of ions

Group two cation identification

Reagents:

• Add sodium hydroxide to one beaker

• Add sodium sulfate to the other beaker

Ammonium ion identification

Hydroxide ion identification

Sulfate ion identification

All anion and cation tests