Group 7 elements

Question 1

State and explain the trends in:

1. Melting and boiling temperatures and physical state at room temperature
2. Electronegativity
3. Reactivity down the group

of group 7 elements.

Answer 1

1. Melting and boiling temperatures increase as you go down the group.

All halogens are diatomic and so their melting and boiling points are compared using intermolecular forces. As you go down the group, the number of electrons increases and this means that the London forces are stronger. So more energy required to overcome London forces.

2. Electronegativity decreases down the group. Fluorine is the most electronegative atom.

Electronegativity is a measure of how well an atom attracts electrons in a covalent bond.

It decreases down Group 7 due to the increase in the number of inner electron shells and the increase in distance between the nucleus and the bonding electrons.

Halogens have the highest electronegativities.

3. Reactivity decreases down the group:

Halogen atoms usually react by gaining an electron in their outer p subshell, which means they’re reduced. As they’re reduced, they oxidise another substance (it’s a redox reaction) — so they’re oxidising agents.

As you go down the group, the atoms become larger, so their outer electrons are further from the nucleus.
The outer electrons are also shielded more from the attraction of the positive nucleus, because there are
more inner electrons. This makes it harder for larger atoms to attract the electron needed to form an ion.
So larger atoms are less reactive and reactivity decreases down the group.

Question 2

Explain, in terms of changes in oxidation number, the following reactions of the halogens:

1. oxidation reactions with Group 1 and 2 metals
2. the disproportionation reaction of chlorine with water and the use of chlorine in water treatment
3. the disproportionation reaction of chlorine with cold, dilute aqueous sodium hydroxide to form bleach
4. the disproportionation reaction of chlorine with hot alkali

Answer 2

1. They get reduced, so they oxidise other substances.

Reactions are most vigorous between elements at the bottom of g1 and 2 and elements at the top of g7.

2. Chlorine is both oxidised and reduced when reacted with water. The reaction produces HCl and Chloric acid (HClO). Both acids are colourless.

Reaction:
Cl2 + H2O —> HCl + HClO

Chlorine is used to treat water because it kills the pathogens responsible for water-borne diseases such as cholera.

3. Chlorine is oxisided and reduced. The reaction produces NaCl and NaClO salts.

Reaction:
Cl2 + 2NaOH —> NaCl + NaClO + H2O

Sodium chlorate is used as bleach and a disinfectant.

4. Chlorine is oxidised and reduced. The reaction produces NaCl and NaClO3.

Reaction:
3Cl2 + 6NaOH —> 5NaCl + NaClO3 + 3H2O

Sodium chlorate is also used as a bleach and a weed killer.

Question 3

Explain the trend in reactivity of Group 7 elements in terms of the redox reactions of Cl2, Br2 and I2 with halide ions in aqueous solution

Answer 3

Reactivity decreases down the group, which means elements higher up can displace elements lower than them in the group.

The element that is displacing the other is reduced, the element that is being displaced is oxidised.

If a reaction takes place, a colour change will be seen:

-If bromide is displaced and bromine (Br2) is formed,
the reaction mixture will turn orange.

-If iodide is displaced and iodine (I2) is formed,
the reaction mixture will turn brown.

You can make these changes easier to see by shaking
the reaction mixture with a non-polar organic solvent like hexane. The halogen that’s present will dissolve in the organic solvent, which settles out as a distinct layer above the aqueous solution.