Redox Reactions

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Oxidation Numbers

The oxidation numbers of atoms in an ion add up to the charge on the ion.

Oxidation state of S in SO42-?

? – 8 = -2

? = +6

Slide 14

Oxidation Numbers

The oxidation numbers of atoms in an ion add up to the charge on the ion.

Oxidation state of S in S2-?

-2

Slide 15

Oxidation Numbers

The oxidation numbers of atoms in an ion add up to the charge on the ion.

Oxidation state of N in NH4+?

-3

Slide 16

Try Question 2.

Slide 17

H2(g) + ½ O2(g)  H2O(g)

Covalent!

No H+ or OH-

Need a new

definition.

Slide 18

GCSE

Oxidation:

Gain of oxygen

Loss of electrons

Reduction:

Loss of oxygen

Gain of electrons

Increase in

oxidation

number

Decrease in

oxidation

number

Slide 19

H2(g) + ½ O2(g)  H2O(g)

Covalent!

No H+ or OH-

Need a new

definition.

+1

0

-2

0

O

H

Slide 20

H2(g) + ½ O2(g)  H2O(g)

+1

0

-2

0

O

H

Oxidised?

H – increase

in oxidation

number

Reduced?

O – decrease

in oxidation

number

Slide 21

Try Question 3.

Slide 22

Oxidation Numbers and names

To avoid any confusion when an element can have several oxidation numbers, the oxidation number is usually mentioned in the compound’s name. In names like “elementate(X)”, the number refers to “element” and not the associated oxygens.

So if we look at some examples , we get the following names:-

KMnO4 potassium manganate(VII)

NaClO3 sodium chlorate(V)

POCl2F phosphorus(V) oxydichlorofluoride

NaH2PO3 sodium dihydrogenphosphate(III)

K2Cr2O7 potassium dichromate(VI)

Check the

numbers.

Slide 23

Try any 3 in Question 7.

Slide 24

Well done!