Section 2 c) Summary

The group 7 elements, also known as halogens, are F, fluorine, Cl, chlorine, Br, bromine, I, iodine, and At, astatine. They all have antimicrobial properties, but in larger doses are toxic.

F - Fluorine - a pale yellow gas
Cl - Chlorine - a green gas
Br - Bromine - an orange liquid
I - Iodine - a purple solid
At - Astatine - a black solid

The halogens are in the same group, so have similar properties that show trends. As you go down the group, the elements get darker in colour, less reactive, and have a higher melting and boiling point.
The reason for this is because as the molecules increase in size, the distance between the valence electrons and the nucleus increases, weakening the forces of attraction and making it more difficult for the atom to attract another electron. Additionally, the increase in molecule size means the attraction between the molecules is more difficult to break, causing the increase in melting and boiling point.

This topic focuses primarily on fluorine, chlorine and bromine. Their reactivity series can be determined by combining a metal halide and aqueous halide, and seeing if a reaction takes place. By adding methylbenzene, we can see which molecules are present (purple is iodine, yellow is bromine)

This shows that chlorine is the most reactive of the three, and iodine the least. These are displacement reactions, where the less reactive halogen is replaced by the more reactive halogen. The more reactive one is reduced, it gains electrons, and the less reactive one is oxidised, it loses electrons.

Reactions 

 The hydrogen halides formed in reaction with hydrogen can be bubbled through water, which is a polar substance. This causes the molecules to dissociate, and the H+ and halide- ions split, the H+ ions being acidic. This creates an acid, e.g. hydrochloric acid with chlorine, hydrobromic acid with bromine. This only works because water is a polar substance; it has charged ends. Non-polar substances, such as methylbenzene will not cause the compound to dissociate as the charge is evenly distributed.

Section 2 c) Key Words

Displacement reaction: A reaction in which a less reactive molecule is replaced with a more reactive molecule.

Dissociation: the splitting of a molecule into smaller molecules, atoms, or ions, especially by a reversible process

Group 7: The seventh group of the periodic table. Elements in this group have 7 valence electrons and are known as halogens. They share similar properties due to their similar electronic configurations.

Halogen: A group 7 element. Examples include chlorine and bromine.

Non-polar: A substance in which the electrons are shared equally between the nuclei, resulting in an even distribution of charge.

Oxidation: Loss of electrons

Polar: A substance in which the molecules are arranged so one end has a positive charge and on has a negative charge.

Reactivity series: The order of reactivity

Redox Reaction: A reaction in which electron(s) are transferred from one molecule to another. The molecule losing an electron is oxidised, and the one gaining is reduced.

Reduction: Gain of electrons

Section 2 c) Specification

2.9 recall the colours and physical states of the elements at room temperature

Fluorine, F : Pale yellow gas
Chlorine, Cl : Pale green gas
Bromine, Br : Orange liquid
Iodine, I : Purple solid
Astatine, At : Black solid

2.10 make predictions about the properties of other halogens in this group

Based on the information we know about fluorine, chlorine and bromine we can assume that as we travel down the group

• Reactivity decreases
• Melting and boiling point increase
• Elements get darker in colour

They all have antimicrobial properties in small doses, but are toxic in large doses.

2.11 understand the difference between hydrogen chloride gas and hydrochloric acid

Hydrogen chloride gas is the product of the reaction between hydrogen and chlorine. This can be dissolved in water to make it aqueous, causing the ions to be dissociated - detatched. This creates Cl - ions, as well as acidic H + ions, creating hydrochloric acid.

2.12 explain, in terms of dissociation, why hydrogen chloride is acidic in water but not in methylbenzene

Water is a polar substance, and methylbenzene is non-polar. Polar substances cause ionic bonding to be separated (dissociated), as the anions are attracted to the positive poles of the molecules, and the cations are attracted to the negative poles. This causes them to split. Non-polar substances aren't able to dissociate compounds in this way.

2.13 describe the relative reactivities of the elements in Group 7

They become less reactive as you go down the group, so larger elements are less reactive because the force of attraction between the nucleus and the valence electrons is weaker, so it is less able to attract another electron.

2.14 describe experiments to demonstrate that a more reactive halogen will displace a less reactive halogen from a solution of one of its salts

A more reactive halogen will displace a less reactive halogen bonded as a salt. For example, a sodium halide solution could be created for each of the halogens (potassium fluoride, potassium chloride, potassium bromide, etc.), then reacted with a halide solution, to discover the reactivity series:

Methylbenzene turns purple in presence of iodine, and yellow in presence of bromine. It can be added as a layer of indicator, showing us which substances are present (when bonded, does not affect indicator).
This series of experiments shows us that chlorine is the most reactive of these halogens, and iodine is the least, as chlorine water had the most displacements and iodine the least.

2.15 understand these displacement reactions as redox reactions.

Redox reactions are reactions that involve the loss and gain of electrons:
Oxidation
Is
Loss of electrons
Reduction
Is
Gain of electrons

When one more reactive halogen displaces a less reactive halogen, the more reactive halogen is reduced, and the less reactive one is oxidised.

As you can see in the above equations, more reactive chlorine reacts with sodium bromide. Bromine is less reactive than chlorine, so it displaces it. In this process, the bromine loses an electron and bonds with itself to form diatomic molecules. The chlorine molecules bond with sodium, and gain an electron, reducing them and giving them a negative charge.