Redox | Chemistry for Grade 12 PDF Download

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Oxidising & Reducing Agents

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
  

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
  

• 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

Redox Equations

• 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:

Example: Writing overall redox reactions
Manganate(VII) ions (MnO₄^- ) react with Fe²⁺ ions in the presence of acid (H⁺) to form Mn²⁺ ions, Fe³⁺ ions and water
Write the overall redox equation for this reaction

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

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Step 4: Balance the charges

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Step 5: Finally, balance the atoms

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Interpreting & Predicting Redox Reactions

• 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

Example: Write complete equations for the following reactions:

(i) Hydrogen iodide reacting with sulfuric acid to form hydrogen sulfide, iodine and one other product

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

(ii) Lead(II) chloride, chlorine and one other product being formed from the reaction of concentrated hydrochloric acid with lead(IV) oxide

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