Ad Banner Placeholder

Cambridge IGCSE Chemistry · 0620

Chapter 6: Chemical Reactions — Part 5

Topic 6.4 · Redox reactions

Basic Definitions

  • Redox reactions involve simultaneous oxidation and reduction.
  • Oxidation is the gain of oxygen; reduction is the loss of oxygen.
  • Roman numerals (e.g., Iron(III)) indicate the oxidation number of an element in a compound.

Extended Definitions and Oxidation Numbers

  • Oxidation is the loss of electrons and an increase in oxidation number.
  • Reduction is the gain of electrons and a decrease in oxidation number.
  • Mnemonic: OILRIG (Oxidation Is Loss, Reduction Is Gain of electrons).
  • Oxidation Number Rules:
    1. Uncombined elements = 0.
    2. Monatomic ions = their charge.
    3. Sum in a neutral compound = 0.
    4. Sum in a polyatomic ion = the charge of the ion.

Exam Traps

  • Do not confuse OILRIG with oxygen transfer — at Extended level, oxidation is defined by electron loss and oxidation number increase.

Identification of Redox Species

  • Redox reactions can be identified using colour changes:
    • Acidified aqueous potassium manganate(VII): Tests for reducing agents; purple (Mn7+) ? colourless (Mn2+).
    • Aqueous potassium iodide: Tests for oxidising agents; colourless (I-) ? brown (I2).
  • Oxidising Agent: A substance that oxidises another by gaining electrons and is itself reduced.
  • Reducing Agent: A substance that reduces another by losing electrons and is itself oxidised.
  • Redox species are often identified by splitting full equations into ionic half-equations to track electron transfer.

Exam Traps

  • Do not say manganate(VII) tests for oxidising agents — it is itself an oxidising agent and is reduced by a reducing agent.
  • An oxidising agent is reduced (gain of electrons); a reducing agent is oxidised — do not reverse these roles.

0/15

Ad Banner Placeholder