SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 3)

July 24, 2010

As promised in the previous post in this series, let Berry Berry Easy present all Berry Readers with Part 3 of the SPM Form 5 Chemistry notes on “Oxidation and Reduction”. The previous two parts were focused on the concepts of redox reactions and oxidation numbers. (You ought to understand how to write a redox reaction from the two half reaction components by now, else do revise from the beginning before starting this part) Now, we are going to show all of you how oxidation and reduction can be understood in terms of changes in oxidation numbers. (Do remember that you can also view it through the gain and loss of electrons, but that’s for another time).

SPM Chemistry Form 5 – Terminology and Concepts: Oxidation and Reduction (Part 3)

Oxidation and Reduction in Terms of Changes in Oxidation Numbers

Redox reactions – oxidation number of all elements change.

Rusting of iron, combustion, displacement of metal from its salt solution, displacement of halogen from its halide solution and electrolysis are redox reaction.

-10 …. -3  -2  -1  0  +1  +2  +3  …  +10

<———-  Reduction || Oxidation ———->

• H₂ (g) + CuO (s) –> H₂O (l) + Cu (s)
  Hydrogen: 0 –> +1 (Oxidised to water & Hydrogen is a reducing agent)
  Copper oxide: +2 –> 0 (Reduced to copper & Copper oxide is a oxidising agent)

• 2Zn (s) + O₂ (g) –> 2ZnO (s)
  Zinc: 0 –> +2 (Oxidised to zinc ion & Zinc is a reducing agent)
  Oxygen: 0 –> -2 (Reduced to oxide ion & Oxygen is an oxidising agent)

• 2Mg (s) + CO₂ (g) –> 2MgO (s) + C (s)
  Magnesium: 0 –> +2 (Oxidised to magnesium ion & Magnesium is a reducing agent)
  Carbon dioxide: +4 –> 0 (Reduced to carbon & Carbon dioxide is an oxidising agent)

• Br₂ (l) + 2HI (aq) –> 2HBr (aq) + I₂ (s)
  Hydroiodic acid / Hydrogen iodide: -1 –> 0 (Oxidised to iodine & Hydroiodic acid is a reducing agent)
  Bromine: 0 –> -1 (Reduced to hydrobromic acid & Bromine is a oxidising agent)

Non-redox reactions – oxidation number of all elements remain unchanged.

Precipitation, decomposition and neutralisation are not redox reaction (non-redox reaction)

Precipitation:

• AgNO₃ (aq) + NaCl (aq) –> AgCl (s) + NaNO₃ (aq)
  +1 +5 3(-2)      +1  -1              +1  -1        +1 +5  3(-2)

No change in the oxidation numbers.

Decomposition:

• ZnCO₃ (s) –> ZnO (s) + CO₂ (g)
  +2 +4  3(-2)    +2 -2       +4  2(-2)

No change in the oxidation numbers.

Neutralisation:

• NaOH (aq) + HCl (aq) –> NaCl (aq) + H₂O (l)
  +1 -2 +1          +1 -1             +1  -1             2(+1)  -2

No change in the oxidation numbers.

Revision time:

Questions to ask yourself at this point:

1. Do you understand what is a redox reaction?
2. Can you write the two half reactions out of a redox reaction?
3. Do you understand the concept of oxidation number?
4. Give three examples of an oxidising agent and the example of the reaction involved.
5. Give three examples of a reducing agent and the example of the reaction involved.
6. Can you differentiate a redox reaction with a non-redox reaction?
7. What is the characteristics of a non-redox reaction?

If you can answer all these questions, it shows that you understand the basics of oxidation and reduction.

Please also view the other berry related posts:

1. SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 1)
2. SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 10)
3. SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 11)
4. SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 4)
5. SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 7)

Posted in: Berry Reference (Notes)