Redox (OCR A Level Chemistry)

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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

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

 

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:

Worked 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

Answer

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

Electrochemistry Step 1 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

Step 2: Deduce the oxidation state changes

Electrochemistry Step 2 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

Step 3: Balance the oxidation state changes

Electrochemistry Step 3 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

Step 4: Balance the charges

Electrochemistry Step 4 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

Step 5: Finally, balance the atoms

Electrochemistry Step 5 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

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

Worked example

Write complete equations for the following reactions:

  1. Hydrogen iodide reacting with sulfuric acid to form hydrogen sulfide, iodine and one other product
  2. Lead(II) chloride, chlorine and one other product being formed from the reaction of concentrated hydrochloric acid with lead(IV) oxide

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

Examiner Tip

A similar approach can be used to balance more complicated ionic half equations

In this situation, you will have H2O, H+, OH- and electrons available

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.