Group 7 Elements: Redox Reactions

Seite 1

The Group 7 Elements

The Group 7 elements are fluorine, chlorine, bromine, iodine and astatine. These elements are generally known as the halogens.

Physical Properties

The halogens have the following physical properties:

they have low melting and boiling points

the exist as diatomic molecules X2

As you move down the group the number of electrons increases, leading to an increase in the van der Waals' forces between molecules.

the boiling points of the halogens increases down the group

the physical states of the halogens at room temperature and pressure show the classic trend of gas to liquid to solid as you move down the group

Electronic Configuration

The elements in Group 7 have seven electrons in their outer shell. The highest energy electrons are in the p sub-shell and the elements form part of the p-block in the Periodic Table.

Each Group 7 element has:

one electron fewer than the electron configuration of a noble gas

an outer p sub-shell containing 5 electrons

Halogens as oxidsing agents

The halogens are the most reactive non-metals in the Periodic Table and are strong oxidising agetns removing electrons in reactions.

The oxidising power of a halogen is a measure of the strength with which a halogen atom is able to attract and capture an electron to form a halide ion.

In redox reactions each halogen atom gains one electron into a p sub-shell to for a halide ion with a 1- charge.

1/2 Cl2 + e- ----> Cl-

The halogens become less reactive down the group as their oxiding power decreases.

As halogens gain an electron in their reactions, reactivity decreases down the group because:

atomic radius increases

electron shielding increases

the ability to gain an electron into the p sub-shell decreases to form a halide ion

Redox Reactions of the Halogens

Redox reactions can show that the halogens become less able to form halide ions down the group. We can show the decrease in reactivity using redox reactions of:

aqueous solutions of halide ions: Cl-, Br-, and I-

with aqueous solutions of halogens: Cl2, Br2, I2

Each halogen is mixed with aqueous solutions of the different halides. A more-reactive halogen will oxidise and displace a halide of a less-reactive halogen. This is often called a displacement reaction.

Halogens form solutions with different colours, so any change in colour will show whether a redox reaction has taken place. The mixture is usually shaken with an organic solvent, such as cyclohexane, to help distinguish between bromine and iodine.

Cl2 - pale green in water, pale green in cyclohexane

Br2 - orange in water, orange in cyclohexane

I2 - brown in water, violet in cyclohexane

Chlorine oxidises bromine and iodine ions.

Cl2 + 2Br- -------> 2Cl- + Br2 orange in water and in cyclohexane

Cl2 + 2I- ---------> 2Cl- + I2 brown in water, purple in cyclohexane

Changes in oxidation number for reaction with bromine is:

Cl2 + 2Br- --------> 2Cl- + Br2

0x2 2x-1 2x-1 2x0

Bromine oxidises iodine ions only:

Br2 + 2I- --------> 2Br- + I2 brown in water, purple in cyclohexane

Iodine does not oxidise either Cl- or Br- ions.

Disproportionation

Disproportionation is a reaction in which the same element is both oxidised and reduced.

Disproportionation of chlorine in water

Small amounts of chlorine are added to water to kill bacteria and make the water safer to drink.

Chlorine reacts with water, forming a mixture of two acids, hydrochloric acid and chloric (I) acid, HClO.

This is a disproportionation reaction in which chlorine is both oxidised and reduced.

Cl2 + H20 ----------> HCl + HClO

0 +1 chlorine reduced

0 -1 chlorine oxidised

Disproportionation of chlorine in aqueous sodium hydroxide

Chlorine is only slightly soluble in water and has a mild bleaching action. Household bleach is formed when dilute aqueous sodium hydroxide and chlorine react together at room temperature.

This is also a disproportionation reaction because chlorine is both oxidised and reduced.

Cl2 + NaOH -------------> NaCl + NaClO + H2O

0 -1 chlorine reduced

0 +1 chlorine oxidised

New Page