Video: Rahmi's final assignment (Group 7-The Halogens)

  Group 7-The Halogens

Please watch my video:
https://youtu.be/Fx7bkOz9Uko

Group 7 – the halogens

The Group 7 elements are called the halogens. They are placed in the vertical column, second from the right, in the periodic table.

Chlorine, bromine and iodine are the three common Group 7 elements. Group 7 elements form salts when they react with metals. The term ‘halogen’ means 'salt former'.

Properties and uses of the halogens
This table summarises some of the properties and uses of three halogens:

Element	Properties	Typical use
Chlorine	Green gas	Sterilising water
Bromine	Orange liquid	Making pesticides and plastics
Iodine	Grey solid	Sterilising wounds

Iodine forms a purple vapour when it is warmed.

Predicting properties

The halogens show trends in physical properties as you go down the group.

Melting point and boiling point

The halogens have low melting points and low boiling points. This is a typical property of non-metals. Fluorine has the lowest melting and boiling points. The melting and boiling points then increase as you go down the group.

Melting and boiling points of Group 7 elements

State at room temperature

Room temperature is usually taken as being 25°C. At this temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. There is therefore a trend in state from gas to liquid to solid as you go down the group.

Colour

The halogens become darker as you go down the group. Fluorine is very pale yellow, chlorine is yellow-green, and bromine is red-brown. Iodine crystals are shiny purple - but easily turn into a dark purple vapour when they are warmed up.

Predictions

When we can see a trend in the properties of some of the elements in a group, it is possible to predict the properties of other elements in that group. Astatine is below iodine in Group 7. The colour of these elements gets darker as you go down the group. Iodine is purple, and astatine is black.

Reactivity of halogens

The non-metal elements in Group 7 - known as the halogens - get less reactive as you go down the group. This is the opposite trend to that seen in the alkali metals in Group 1 of the periodic table.

Fluorine is the most reactive element of all in Group 7.

You can see the trend in reactivity if you react the halogens with iron wool.

Halogen	Reaction with iron wool
Fluorine	Reacts with almost anything instantly. Very few scientists handle fluorine because it is so dangerous.
Chlorine	Reacts with heated iron wool very quickly.
Bromine	Has to be warmed and the iron wool heated. The reaction is faster.
Iodine	Has to be heated strongly and so does the iron wool. The reaction is slow.

Halogen displacement reactions

The reactivity of the halogens – the Group 7 elements - decreases as you move down the group. This can be shown by looking at displacement reactions.

Example

When chlorine (as a gas or dissolved in water) is added to sodium bromide solution, the chlorine takes the place of the bromine. Because chlorine is more reactive than bromine, it displaces bromine from sodium bromide.

The solution turns brown. This brown colour is the displaced bromine. The chlorine has gone to form sodium chloride.

In this equation, the Cl and Br have swapped places:

chlorine + sodium bromide → sodium chloride + bromine


Cl₂(aq) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq)

This type of reaction happens with all the halogens. A more reactive halogen displaces a less reactive halogen from a solution of one of its salts.

Reactivity series

 

If you test different combinations of the halogens and their salts, you can work out a reactivity series for Group 7: 

the most reactive halogen displaces all of the other halogens from solutions of their salts, and is itself displaced by none of the others
the least reactive halogen displaces none of the others, and is itself displaced by all of the others

It doesn’t matter whether you use sodium salts or potassium salts – it works the same for both types.

The slideshow shows what happens when chlorine, bromine and iodine are added to various halogen salts:

Redox reactions involve both oxidation (loss of electrons) and reduction (gain of electrons). You could remember it as: OIL RIG – Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

Halogen displacement reactions are redox reactions because the halogens gain electrons and the halide ions lose electrons.

When we consider one of the displacement reactions, we can see which element is being oxidised and which is being reduced.

bromine + potassium iodide → iodine + potassium bromide 

Br₂ + 2KI → I₂ + 2KBr

As an ionic equation (ignoring the ‘spectator’ potassium ions):

Br₂ + 2I- → I₂ + 2Br-

We can see that the bromine has gained electrons, so it has been reduced. The iodide ions have lost electrons, so they have been oxidised.

Hydrogen chloride
When hydrogen reacts with chlorine, hydrogen chloride is formed. Hydrogen chloride is a gas, and has the formula HCl(g).

When hydrogen chloride dissolves in water, hydrochloric acid is formed. This has the same formula, but you can tell the difference because of the state symbol (aq), which stands for ‘aqueous’. The formula is written as HCl(aq).
 

Hydrogen chloride is made from molecules. The hydrogen atom and the chlorine atom are joined by a covalent bond. When hydrogen chloride forms hydrochloric acid, the molecules split into ions.

HCl(aq) → H+(aq) + Cl-(aq)

The H+ ions make this aqueous solution acidic. The solution also conducts electricity because it contains ions that are free to move.

However, when hydrogen chloride gas dissolves in a solvent called methylbenzene, the molecules do not split up. A solution of HCl in methylbenzene does not contain hydrogen ions, so it is not acidic. The solution also has a low electrical conductivity.