Topic 4: Group 7

Question 1

Trend in melting and boiling points?

Answer 1

Both increase down the group.
This is due to the number of instantaneous dipole-induced dipole intermolecular forces which increase in number as the electron cloud gets bigger (due to an increase in electrons down the group)
Therefore the forces increase in strength down the group, meaning more energy is needed to break them.

Question 2

Trend in electronegativity?

Answer 2

Fluorine has the highest, yet all halogens have high electronegativities. This depends on:
1. nuclear charge
2. distance between nucleus and bonding pair of electrons (shorter distance = higher EN)
3. shielding of electrons (fewer energy levels = higher EN)

Question 3

Trend in reactivity?

Answer 3

Decreases down the group.
Due to their high EN, most halogens act as oxidising agents, which gain electrons to form negative or partially negative ions. The strength of the oxidising agent decreases down the group, due to increases shielding and larger atomic radius, therefore it is harder for the halogen to act as an oxidising agent lower in the group.

Question 3

Reactions with G1 and G2 metals:

Answer 3

• Most vigorous reactions happen with elements at the bottom of G1/2 and elements at the top of G7
• Produce salts
• All transfer electrons to the halogen (which acts as the oxidising agent)

Question 3

What is the rule for displacement between halogens?

Answer 3

A more reactive halogen will displace a less reactive halogen.

Cl will displace Br and I
Br will displace I
I does not displace

Question 3

Why is an organic solvent used in displacement reactions?

Answer 3

Cyclohexane!!
Halogens are more soluble in cyclohexane than water, so it will dissolve in the organic upper layer, where the colour can be seen.

Question 4

Colour changes from aqueous in water to cyclohexane

Answer 4

Chorine: Pale green –> Pale green

Bromine: Orange –> Darker orange

Iodine: Dark orange –> Violet

Question 5

What is most reactive, Cl, Br or I?

Answer 5

Chlorine. It has a smaller atomic radius, smallest number of complete energy levels, so the incoming electron experiences less repulsion.

Question 6

Chlorine + cold alkali?

Answer 6

Must be cold, dilute aqueous NaOH.
Disproportionation
Forms sodium chloride (NaCl)
and sodium chlorate I (NaClO) and water

Question 7

What are the uses of NaClO

Answer 7

Sodium chlorate I is also a disinfectant, but mainly used as a bleach –> active ingredient in household bleach.

Question 8

Chlorine + hot alkali?

Answer 8

Must be hot, concentrated NaOH solution.
Forms sodium chlorate V (NaClO3) as well as NaCl and water

Question 9

What are the uses of NaClO3

Answer 9

Bleaching / weed killer