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REDOX (REDUCTION & OXIDATION) II
08 OCTOBER 2013
Lesson Description
In this lesson, we:



Revise Oxidation Numbers.
Write half-reactions to show Reduction and Oxidation.
Combine half-reactions to make net-ionic reactions.
Key Concepts
Revision of Oxidation Numbers:
Oxidation numbers tell us the number of electrons that have been removed.
Oxidation is the loss of Electrons.
In the formula MgO we could see this:
Mg loses 2 electrons.
This means that the oxidation number is +2.
What about oxygen?
O gains two electrons.
This means that the oxidation number is -2.
What are half-reactions?
Half-reactions tell the same story as a normal equation but in two parts. Let's look at the example from
earlier:
2Mg + O2 -> 2MgO
2+
The magnesium atom turned from an element into a Mg ion in the product. It also lost two electrons:
Mg
2+
->
Mg
-
+ 2e
So we can see that electrons came out of the reaction and this is oxidation (electrons are lost!).
Now let's do the same for the oxygen:
-
O2 + 4e -> 2O
2-
Notice how the electrons are on the left? This is because they are going into the reaction. This is
reduction.
Can half-reactions combine to make a whole?
You may have guessed that half-reactions are exactly that, half the story. We can combine them
again.
Step 1: Write out the reactions.
2+
Mg ->
-
O2 + 4e
-
Mg
+ 2e (oxidation)
->
2O (reduction)
2-
Step 2: Balance the electrons to cancel them out.
2+
2Mg ->
2Mg
-
O2 + 4e
-
+ 4e (oxidation)
2-
-> 2O (reduction)
Step 3: Write them together
2Mg + O2 ->
2+
2-
2Mg + 2O (net ionic)
Terminology
Reduction
The gain of electrons.
Oxidation
The loss of electrons.
Half-reaction
The reaction (including electrons) showing the path of only one atom or ion.
Net-ionic reaction
The combined half reactions, excluding the electrons.
Demonstration
If we burn magnesium in air, the white powder that is formed doesn’t resemble the silver magnesium
nor the colourless oxygen gas.
Try to trace out the paths of the magnesium and oxygen into ions.
Questions
Question 1
Write out half-reactions for the following reactions:
2Na + Cl2 -> 2NaCl
2Sr + Br2 -> 2 SrBr2
2Fe + 3O2 -> 2Fe2O3
Question 2
Combine the half-reactions below into net-ionic reactions.
+
-
a.) Na -> Na + e
Cl2 + 2e -> 2Cl
2+
-
b.) Mg -> Mg + 2e
2O2 + 4e -> 2O
3+
c.) Al -> Al + 3e
2S + 2e -> S
-
Question 3
Write complete half –reactions and net-ionic reactions between the following elements:
a.) Oxygen + Iron => Iron(III)oxide
b.) Magnesium + Bromine => Magnesium bromide
c.) Iodine + Calcium => Calcium iodide