REDOX

Burning Magnesium in air

Oxidation is gain of oxygen When magnesium reacts with oxygen in

the air:

2Mg + O2 2MgO

It is easy to see that the magnesium has gained oxygen and an oxidation reaction has occurred.

The magnesium has been OXIDISED.

Reduction of lead(II)oxide

Reduction is loss of oxygen When lead is extracted from lead (II)

oxide , carbon is used to remove the oxygen

2PbO + C 2Pb + CO2

The oxygen has been removed from the lead (II) oxide

The lead (II) oxide has been REDUCED.

Lets look at this further… What actually happens when the

magnesium joins with oxygen?

The magnesium atom LOSES ELECTRONS to become a magnesium ion

We can write what happens like this:

Mg Mg2+ + 2e-

Oxidation is loss of electrons

So, when magnesium takes part in an oxidation reaction it loses electrons.

When magnesium reacts with chlorine it also loses electrons.

Mg + Cl2 MgCl2

This is an oxidation reaction too!

Reduction is gain of electrons

In the compound lead (II) oxide the lead is an ion, Pb2+

In order to form lead the Pb2+ ion has to GAIN 2 electrons

We can write this as follows:

Pb2+ + 2e- Pb

Redox

You may have noticed that when the magnesium loses its electrons they are gained by the oxygen

Oxidation (loss of electrons) and reduction (gain of electrons) happen simultaneously.

We call these REDOX reactions

OILRIG

Oxidation IsLossReductionIsGain

(of electrons)

Oxidation Numbers

When magnesium lost electrons it got more POSITIVE

Mg Mg2+ + 2e- (0 to +2)

When lead gained electrons it became more NEGATIVE

Pb2+ + 2e- Pb (+2 to 0)

Oxidation Numbers

For a reaction to be a redox reaction, different reactants have to be oxidised or reduced.

To identify these reactants, OXIDATION NUMBERS are assigned to the reactants and products

These are also called OXIDATION STATES

Assigning oxidation numbers Rule 1

The oxidation number of an element is 0

Zn + CuSO4 ZnSO4 + Cu

The oxidation numbers of Zn and Cu in this reaction are 0

Rule 2

The oxidation number of a monatomic ion equals the charge on the ion

Mg + Cl2 MgCl2

In this example the oxidation number of Mg2+ and Cl- are +2 and -1 respectively

Rule 3

The oxidation number of oxygen is -2 (except in certain compounds called peroxides)

MgOFe2O3 Na2O

In each case the O2- ion is formed and the oxidation number is +2

Rule 4

The oxidation number of hydrogen is +1, except in metal hydrides when it is -1

H2O HCl

In both cases hydrogen forms H+ ions and has an oxidation number of +1

In NaH is has an oxidation number of -1

Rule 5

The oxidation number of fluorine is always -1 and the oxidation number of the other halogens -1 (except for a few compounds)

NaCl LiBr CaF2

In each case the halide ion has a charge of -1 (e.g. Cl-) and the oxidation number is -1

Rule 6

The oxidation number of the atoms in a compound is zero

The sum of the oxidation numbers of the atoms in a polyatomic ion equals the charge in the ion

Rule 6 Examples

e.g. in the hydroxide ion (OH-) the oxidation number of oxygen is -2 and the oxidation number of hydrogen is +1

-2 + (+1) = -1

The overall charge on the hydroxide ion is -1

See C.I pg 195

Practice

1. What is the oxidation number of oxygen in O2?

2. What is the oxidation number of sodium in NaCl?

3. What is the oxidation number of iron in Fe2O3?

4. What is the oxidation number of chromium in CrO3?

5. What is the oxidation number of gallium in Ga(OH)3?

6. What is the oxidation number of phosphorous in PO4

3-

7. What is the oxidation number of sulphur in H2SO4?

Recognising oxidation and reduction in reactions Look at the equation Write out the oxidation numbers of

the atoms involved When the oxidation number of an

atom increases (gets more positive) oxidation has occurred

When the oxidation number of an atom decreases (gets more negative) reduction has occurred.

Example H2 + Cl2 2HCl Hydrogen and chlorine both have oxidation

numbers of o in H2 and Cl2 as they are elements

Hydrogen forms the H+ ion so has an oxidation number of +1

Chlorine forms the Cl- ion and has an oxidation number of -1

The oxidation number of hydrogen has changed from 0 to +1 so hydrogen has been oxidised

The oxidation number of chlorine has changed from 0 to -1 so chlorine has been reduced

Now try these:

In the reactions below, which element is being oxidised and which reduced?

1. 2Ca + O2 2CaO

2. NaBr + Cl2 NaCl + Br2

3. 2Al + Fe2O3 2Fe + Al2O3

Pg 197 Q 1-8 for HL

Oxidising and Reducing Agents A substance which donates electrons

easily and causes another substance to be reduced is called a REDUCING AGENT

A substance which accepts electrons easily and helps another substance to become oxidised is called an OXIDISING AGENT

Think of some examples for each!