Oxidation numbers

Before we see how we can use oxidation numbers, let's see whether you have remembered how to assign oxidation numbers. Complete the following revision activity to test your skills of assigning oxidation numbers (or oxidation states).

 

worksheet icon

You can access the rules for assigning oxidation numbers here Opens a 174kBMicrosoft® Word document in a new browser window..

If you think you need more practice with these, complete the questions here Opens a 199kBMicrosoft® Word document in a new browser window..

Using oxidation numbers

Oxidation numbers are values that are assigned to atoms to help with working out what is happening to substances during redox reactions. Once you have worked out the oxidation number of an element, the following rules can be applied.

An increase in oxidation number means that the element is being oxidised.

A decrease in oxidation number means that the element is being reduced.

To practise using this idea, complete the following questions based on two redox reactions.

[+] Expand all items
Reaction A: Combustion of methane

methane gas burning

Equation:

CH4 + 2 O2 → CO2 + 2 H2O


5

How does the oxidation number of carbon change during this reaction?

a)
b)
c)
d)
Correct.Incorrect, the oxidation number of hydrogen is +1 throughout this process.Your answer has been saved.
Check your answer

6

How does the oxidation number of oxygen change during this reaction?

a)
b)
c)
d)
Correct, well done.Incorrect, remember that the oxygen starts as an element and finishes as part of an oxide.Your answer has been saved.
Check your answer

7

Which element is being reduced in this reaction?

a)
b)
c)
Correct, well done.Incorrect, the oxidation number of the oxygen changes from 0 to -2.Your answer has been saved.
Check your answer
Reaction B: Acid and magnesium

video icon

Text alternative to the acid and magnesium video

Click here to open the video in a new window. opens in a new window

Equation:

2 HCl + Mg → MgCl2 + H2

or 2 H+ + Mg → Mg2+(aq) + H2(g)

8

How does the oxidation number of magnesium change during this reaction?

a)
b)
c)
d)
Correct, well done.Incorrect, the oxidation number of an element is 0 until it reacts to form a compound.Your answer has been saved.
Check your answer

9

How does the oxidation number of chlorine change during this reaction?

a)
b)
c)
d)
Correct, well done.Incorrect, chlorine is not directly involved in the redox reaction.Your answer has been saved.
Check your answer

10

Which element is being reduced in this reaction?

a)
b)
c)
Correct, well done.Incorrect, the oxidation number of the hydrogen changes from +1 to 0.Your answer has been saved.
Check your answer

reflection icon – click to open reflection questions relating to information in this section







Text Reference: Read Section 9.2 and answer Review Exercise 9.2