5.5.1 Redox

Oxidising agent

• An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons
• An oxidising agent itself gets reduced – gains electrons
• Therefore, the ox. no. of the oxidising agent decreases

Example of an oxidising agent in a chemical reaction

Reducing agent

• A reducing agent is a substance that reduces another atom or ion by causing it to gain electrons
• A reducing agent itself gets oxidised – loses/donates electrons
• Therefore, the ox. no. of the reducing agent increases

Example of a reducing agent in a chemical reaction

• For a reaction to be recognised as a redox reaction, there must be both an oxidising and reducing agent
• Some substances can act both as oxidising and reducing agents
• Their nature is dependent upon what they are reacting with and the reaction conditions

Oxidising & Reducing Agents Table

• Balancing equations using redox principles is a useful skill and is best illustrated by following an example
• It is important to follow a methodical step-by-step approach so that you don't get lost:

Answer

Step 1: Write the unbalanced equation and identify the atoms which change in oxidation state

Step 2: Deduce the oxidation state changes

Step 3: Balance the oxidation state changes

Step 4: Balance the charges

Step 5: Finally, balance the atoms

• All of the species involved in a chemical reaction might not be known
• You can be asked to interpret the information given to you and predict any other chemicals involved in the reaction
  • You can use the method shown in the Redox Equations section above
  • The worked example below is a method that works for balancing ionic half equations but can also be used to help predict redox equations

Answers

Answer 1:

Step 1: Start with what you know:

HI + H₂SO₄ → H₂S + I₂ 

Step 2: Consider any unaccounted for elements

The only element that is not currently considered is oxygen

Step 3: Make a common and appropriate suggestion for the missing product

Most of these questions are in solution so there is always H₂O, H⁺ and OH^- available

Missing product suggestion = 4H₂O

HI + H₂SO₄ → H₂S + I₂ + 4H₂O

Step 4: Balance the remaining chemicals

8HI + H₂SO₄ → H₂S + 4I₂ + 4H₂O

Answer 2:

Step 1: Start with what you know:

PbO₂ + HCl → PbCl₂ + Cl₂ 

Step 2: Consider any unaccounted elements

The only element that is not currently considered is oxygen

Step 3: Make a common and appropriate suggestion for the missing product

Missing product suggestion = 2H₂O

PbO₂ + HCl → PbCl₂ + Cl₂ + 2H₂O

Step 4: Balance the remaining chemicals

PbO₂ + 4HCl → PbCl₂ + Cl₂ + 2H₂O