The document explains redox reactions, focusing on the oxidation of magnesium and the reduction of oxygen to form magnesium oxide (MgO). It describes the transfer of electrons, the roles of oxidizing and reducing agents, and the importance of oxidation numbers in determining which substances are oxidized or reduced. Additionally, it provides rules for assigning oxidation numbers and examples for identifying these in various chemical compounds.

1. Mg ribbon burns with a very
bright white flame when held
in the bunsen flame with a
pair of tongs to form a white
powder.
We say the magnesium has
been oxidised.
2Mg + O2 → 2MgO or
2Mg + O2 → 2Mg2+ + 2O2-
Each Mg loses 2ē & each O gains 2 ē. This is ē transfer.
We say the Mg has been oxidized while the O has been
reduced to form MgO. This is a redox reaction.

2. Mg Mg2+ + 2ē
O + 2ē O
2-
Oxidation
Reduction
Oxidation is the loss of ē, while
Reduction is the gain of ē.
Remember: Oilrig

3. In a redox reaction there must be an oxidising agent
and a reducing agent.
The reducing agent is the substance oxidised while
the oxidising agent is the substance reduced.
2Mg + O2 → 2 MgO
Mg is being oxidised & is thus the reducing agent.
O is being reduced & is thus the oxidising agent.
Mg has lost ē, while O has gained ē.
In redox reactions ē are transferred, while in acid-
base reactions H+ are transferred.

4. Redox reactions can be written in 2 parts.
These are called half-reactions & show the
oxidation half-reaction and the reduction half-
reaction as 2 separate half-reactions.
The MgO reaction can be written as follows:
2Mg → 2Mg + 4ē Oxidation half reaction – loss of ē
O2 + 4ē → 2O2- Reduction half reaction – gain of ē
Mg is the reducing agent & O is the oxidising agent.
There will always be a reducing & an oxidising half-
reaction in every redox reaction.

5. Sometimes it is difficult to identify which
substance is being oxidised or which is reduced.
Consider this reaction:
Mg(s) + Pb(NO3)2(aq) → Pb(s) + Mg(NO3)2(aq)
The concept of oxidation numbers (O.N.) has
been established to help us identify which is
oxidised & which reduced.

6. The oxidation number (O.N.) indicates how many ē
have been lost/gained in a redox reaction.
If the number is negative (–) means the atom gained ē
If + this means the atom lost ē
O.N. may then be used to identify the respective
oxidation and reduction half-reactions.

7. Oxidation numbers
Look at the following oxidation reaction:
Zn → Zn2+ + 2e-
Oxidation numbers: 0 +2
Look at the following reduction reaction:
Cu2+ + 2e- → Cu
Oxidation numbers: +2 0

8. Oxidation Numbers
Oxidation is associated with an increase in
oxidation number.
Reduction is associated with a decrease in
oxidation number.
How do we determine oxidation numbers?

9. Rules for oxidation numbers
 For an element that is not bonded, the oxidation
number is 0. e.g. in Na, Ca, H2, P4 & S8
 The oxidation number for H is +1 except when it is
bonded to a metal (i.e. a metal hydride, KH, then -1).
 The oxidation number for O is -2 except in peroxides
(where it is -1 e.g. in H2O2) and in a bond with F.
 For a monatomic ion (Mg2+) the oxidation number is
equal to the charge on the ion.
 In a neutral substance all numbers add up to 0 and in a
polyatomic ion (PO4)3- it adds up to the charge of the
ion.

10. Examples
1. Determine the oxidation number of C in
H2CO3
2(+1) + ox. num. (C) + 3x(-2) = 0
Oxidation number of C = +4
2. Determine the oxidation number of N in
NO3
-
ox. num. (N) + 3x(-2) = -1
Oxidation number of N = +5

11. Examples
Identify the oxidizing and reducing agent in
the following reaction:
2MnO4
- + 5SO2 + 2H2O → 2Mn2+ + 5SO4
2- + 4H+
There are no peroxides or hydrides in the
reaction, so all H’s are +1 and all O’s are -2.
The rest are awarded as follows:
2MnO4
- + 5SO2 + 2H2O → 2Mn2+ + 5SO4
2- + 4H+
+7 +4 +2 +6
Manganese: decreases from +7 to +2 - MnO4
- is
the oxidizing agent.
Sulphur: increases from +4 to +6 - SO2 is
the reducing agent.

12. Elements such as S, N, P, Cu, C & Fe can have more
than one O.N.
Find the O.N. of the underlined element in each of
the following compounds/substances:
H2S, SO3, SO2, NH3, N, NO2, NO, K2Cr2O7
-2 +6 +4 -3 0 +4 +2 +6

13. Names of compounds in which there are elements
that have several O.N. are usually indicated with the
O.N. indicated between the words in Roman
numerals.
FeCl3 is iron (Ⅲ) chloride
Cu2O is copper (Ⅱ) oxide
However, when it is a non-metal that has different
O.N. then we can use a prefix instead:
NO2 is nitrogen dioxide OR nitrogen (lV) oxide
SO3 is sulphur trioxide OR sulphur (Vl) oxide.