Identifying Redox reactions

Recognizing Redox Reactions

Recall, a redox reaction is one that involves the transfer of one or more electrons from one atom to another. In order to recognize a redox reaction, we must follow the electrons. We can follow the electrons by determining if the oxidation numbers for each element change from reactants to products. If the oxidation numbers of none of the elements change, the reaction is NOT a redox reaction.

LEO says GER

Losing electrons is Oxidation

Gaining electrons is Reduction 

Example 1. Is the reaction SO2 + H2O → H2SO3 a redox reaction?

Solution.

If we assign oxidation numbers to each atom, we find

  • the oxidation number of S remains at +4 in the reactants and in the products,
    • O remains unchanged in the products at –2 and
    • H remains unchanged at +1.

The oxidation numbers do not change, so electrons were not transferred. Therefore, this is not a redox reaction.

Recall, if an atom is oxidized, it will lose electrons. If it loses electrons, it becomes more positively charged and its oxidation number becomes more positive from the reactants to the products.

If an atom is reduced, it gains electrons. If it gains electrons, it becomes more negatively charged and its oxidation number becomes more negative, from reactants to products.

Example 2. Is the following reaction a redox reaction?

Cu(s) + 2 AgNO3(aq) → Cu(NO3)2(aq) + 2 Ag(s)

Solution

Assign oxidation numbers.

  • Cu is zero in the reactants and +2 in the products.
  • Ag is +1 in the reactants and zero in the products.
  • N = +5 in both products and reactants.
  • O = –2 in both products and reactants.

This is redox reaction, since the oxidation numbers of Cu and Ag change.

Cu is oxidized because its oxidation number becomes more positive, indicating it has lost electrons. .

The oxidation number of Ag becomes more negative, indicating a gain of electrons. Ag is reduced.