Skip to main content
Chemistry LibreTexts

18.2: Balancing Redox Reactions By Inspection

  • Page ID
    177454
  • Learning Objectives

    • Learn to balanced simple redox reactions by inspection.
    • Learn to balance complex redox reactions by the half reaction method.
    • Use the solvent, or parts of it, as a reactant or a product in balancing a redox reaction.

    Balancing simple redox reactions can be a straightforward matter of going back and forth between products and reactants. For example, in the redox reaction of Na and Cl2:

    \[\ce{Na + Cl2 → NaCl}\]

    it should be immediately clear that the Cl atoms are not balanced. We can fix this by putting the coefficient 2 in front of the product:

    \[\ce{Na + Cl2 → 2NaCl}\]

    However, now the sodium is unbalanced. This can be fixed by including the coefficient 2 in front of the Na reactant:

    \[\ce{2Na + Cl2 → 2NaCl}\]

    This reaction is now balanced. That was fairly straightforward; we say that we are able to balance the reaction by inspection. Many simple redox reactions can be balanced by inspection.

    Example \(\PageIndex{1}\):

    Balance this redox reaction by inspection.

    SO2 + O2 → SO3

    Solution

    There is one S atom on both sides of the equation, so the sulfur is balanced. However, the reactant side has four O atoms while the product side has three. Clearly we need more O atoms on the product side, so let us start by including the coefficient 2 on the SO3:

    SO2 + O2 → 2SO3

    This now gives us six O atoms on the product side, and it also imbalances the S atoms. We can balance both the elements by adding coefficient 2 on the SO2 on the reactant side:

    2SO2 + O2 → 2SO3

    This gives us two S atoms on both sides and a total of six O atoms on both sides of the chemical equation. This redox reaction is now balanced.

    Exercise \(\PageIndex{1}\)

    Balance this redox reaction by inspection.

    Al + O2 → Al2O3

    Answer

    4Al + 3O2 → 2Al2O3

    The first thing you should do when encountering an unbalanced redox reaction is to try to balance it by inspection.