Oxidation numbers

Define ‘oxidation number’

Measure of the number of electrons lost or gained to form an ion or part of a compound

Why are oxidation numbers hypothetical?

Can’t be derived experimentally

What is always the oxidation number for uncombined elements?

• Zero

• E.g in H2, O2, P4, S8, Na & Fe

How do you work out oxidation numbers?

• Sum of the oxidation numbers = total charge

What are the oxidation numbers for transition elements?

• Transition elements form ions with different charges. E.g iron (III) chloride

• Oxidation numbers represented by roman numerals

How do you work out oxidation numbers for molecular ions?

• The sum of oxidation states adds up to the overall charge on the ion

What are oxyanions?

• Negative ions that contain an element along with oxygen

• E.g SO4²-, NO3-, CO3²-

Give 2 rules for compounds & ions

• Each atom in a compound has an oxidation number

• An oxidation number has a sign placed before the number

Oxidation number for oxygen

-2

Oxidation number for hydrogen

+1

Oxidation number for fluorine

-1

Oxidation number for sodium & potassium

+1

Oxidation number for calcium & magnesium

+2

Oxidation number for chlorine, bromine, iodine

-1

What are the special cases for oxygen?

• O in peroxides → -1 E.g H2O2

• O bonded to fluorine → +2 E.g F2O

What is the special case for hydrogen?

• H in metal hydrides → -1 E.g NaH

Define oxidation

• Loss of electrons & gain of oxygen

Define reduction

• Gain of electrons & loss of electrons

Define oxidising agent

• Substance that oxidises another species & gains electrons

Define reduction agent

A substance that reduces another species by donating electrons

Describe redox in terms of oxidation number

• Reduction is a decrease in oxidation number

• Oxidation is an increase in oxidation number

Representing redox reactions

• Mg is oxidised as its oxidation number increases from 0 to 2+

• Chlorine is reduced as its oxidation number decreases from 0 to -1

What is meant by a disproportionate reaction?

A redox reaction in which the same element is simultaneously oxidised and reduced in a single chemical reaction.