halogens part 2 chemical properties

1 of 16 © Boardworks Ltd 2009


Reactions of the halogens

Halogens react with metals such as sodium and iron:

They also take part in displacement reactions with halide ions, such as the reaction that is used to make bromine from potassium bromide in seawater:

halogen + hydrogen hydrogen halide

They also react with non-metals such as hydrogen:

halogen + sodium sodium halide

chlorine +potassiumbromide

potassiumchloride

bromine +


Reaction with iron


Reactions with hydrogen

Chlorine and hydrogen explode in bright sunlight but react slowly in the dark.

The halogens react with hydrogen gas to product hydrogen halides. For example:

Cl2(g) + H2(g) 2HCl(g)

Iodine combines partially and very slowly with hydrogen, even on heating.

Bromine and hydrogen react slowly on heating with a platinum catalyst.


Redox reactions of halogens


What is the reactivity of the halogens?

The reactions of the halogens with iron and hydrogen show that their reactivity decreases down the group.

How do you think fluorine and astatine would react with iron wool and hydrogen?

Iron wool burns and glows brightly.

Iron wool has a very slight glow.

Iron wool glows but less brightly than with chlorine.

chlorine

bromine

iodine

Halogen Reaction with iron wool

Reaction with hydrogen

Explodes in sunlight, reacts slowly in the dark.

Reacts slowly on heating with catalyst.

Reacts partially and very slowly.


Electron structure and reactivity


Halogen displacement reactions



Halogen displacement reactions are redox reactions.

Cl2 + 2KBr 2KCl + Br2

To look at the transfer of electrons in this reaction, the following two half equations can be written:

Chlorine has gained electrons, so it is reduced to Cl- ions.

What has been oxidized and what has been reduced?

2Br- Br2 + 2e-Cl2 + 2e- 2Cl-

Bromide ions have lost electrons, so they have been oxidized to bromine.


Oxidizing ability of halogens

fluorine

incr

easi

ng

oxi

diz

ing

ab

ility

iodine

bromine

chlorine

In displacement reactions between halogens and halides, the halogen acts as an oxidizing agent.

This means that the halogen:

What is the order of oxidizing ability of the halogens?

is reduced to form the halide ion.

gains electrons

oxidizes the halide ion to the halogen


Oxidizing ability of halogens


Chlorine and disproportionation


Reaction of chlorine with water

Chlorination of drinking water raises questions about individual freedom because it makes it difficult for individuals to opt out.

Chlorine is used to purify water supplies because it is toxic to bacteria, some of which can cause disease. Adding it to water supplies is therefore beneficial for the population.

However, chlorine is also toxic to humans, so there are risks associated with gas leaks during the chlorination process. There is also a risk of the formation of chlorinated hydrocarbons, which are also toxic.


Bleach and the chlorate(I) ion

Household bleach commonly contains the chlorate(I) ion, ClO-, in the form of sodium chlorate(I), NaOCl.

ClO- + H2O + Cl- + 2OH-

The chlorine has been reduced because it has gained electrons. Its oxidation state has decreased from +1 in ClO- to –1 in Cl-.

How many electrons are needed to balance this equation?

The chlorate(I) ion behaves as an oxidizing agent. It oxidizes the organic compounds in food stains, bacteria and dyes.

Has the chlorine been oxidized or reduced in the reaction?

2e-


Redox reactions of chlorate ions

halogens part 2 chemical properties

Technology

hydrogen chlorine

halogens halogens

hydrogen hydrogen halide

reaction of chlorine

redox reactions of halogens

hydrogen gas

iron wool reaction

sodium sodium halide