Redox (OCR A Level Chemistry A): Revision Note
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
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
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:
Hydrogen iodide reacting with sulfuric acid to form hydrogen sulfide, iodine and one other product
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 Tips and Tricks
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
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?