1. Define oxidation and reduction in terms of oxygen loss /gain, and identify such
    reactions from given information.

Oxidation is the gain of oxygen. When oxygen is added to a compound, it becomes oxidised.

2Mg + O2 ————–> 2MgO      here, the magnesium has been oxidised.

Reduction is the loss of oxygen. When oxygen is removed from a compound, it becomes reduced.

2H2O  —–heat—–>  2H2  + O2      here, water has been reduced.


  1. Define redox in terms of electron transfer, and identify such reactions from given information.

Look at the reaction between copper(II) oxide and hydrogen.

CuO(s) + H2  ————–>  Cu(s) +  H20

Here, both oxidation and reduction have taken place. The copper(II) oxide became just copper, so the copper oxide was reduced. The hydrogen gained oxygen to become water. The hydrogen was oxidised.

This is called a redox reaction. Both reduction and oxidation takes place together.

Here, the hydrogen is called the reductant or reducing agent because it removed the oxygen from the copper. The copper(II) oxide is the oxidant or oxidising agent because it gave the oxygen to the hydrogen.


In electrolysis we defined oxidation as the loss of electrons and reduction as the gain of electrons. So how does that relate to the oxidation and reduction here?

Let’s look at the reaction between zinc and copper sulfate.

Zn(s) +  CuSO4(aq)  ————–>  ZnSO4(aq)  +  Cu(s)

Here, the zinc has been oxidised to form zinc sulfate and copper sulfate has been reduced to copper.

We can see what happens to each element’s electrons through half equations:

Zn(s) – 2e   ———->  Zn2+(aq)

The zinc lost two electrons to form zinc ions and so the zinc has been oxidised.

Cu2+(aq) + 2e  ———–> Cu(s)

The copper ions have gained two electrons to form copper and so the copper has been reduced.

So the zinc has been oxidised, in terms of oxygen as well as electron transfer and the copper sulfate has been reduced in terms of oxygen and electron transfer as well.


So, redox reactions are reactions where both oxidation and reduction take place (could be in terms of oxygen or electron transfer. And since, every reaction involves electron transfer, we can safely assume that every reaction is in fact, redox).



Notes submitted by Lintha

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