SCH4U - Chemistry 12 (2024-25) - A

Summary

Balncing Redox reactions using the oxidation numbers method

• When balancing redox reactions, the electrons being gained or lost must also be balanced
• The oxidation number method balances reaction equations by balancing the change in oxidation numbers on either side of the reaction arrow
• Steps for balancing redox reactions using the oxidation numbers method:

1. 1. Write the unbalanced chemical equation from the information you are given.
   2. Determine the oxidation numbers for each atom in the equation
   3. Identify which if any of the atoms have changes in their oxidation numbers and indicate how many electrons have been gained or lost for each.
   4. Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropriate coefficients.
   5. Balance the rest of the equation by inspection. If necessary, add H₂O to balance oxygen and balance hydrogen by add H⁺ or OH^-.
   6. Check your answer and write the completed balanced equation

Balncing Redox reactions using the half-reaction method

• When balancing redox reactions, the electrons being gained or lost must also be balanced
• The half-reaction method of balancing equations breaks the full redox reaction down into its respective half-reactions and than balances each, followed by a balancing of the total number of electrons.
• Steps to balance redox reactions by the half-reaction method

1. 1. 1. Write the unbalanced equation in ionic form
      2. Write the separate half-reactions for oxidation and reduction
      3. Balance the oxygen atoms by adding water molecules
      4. Balance the hydrogen atoms by adding hydrogen ions (H⁺).  If the reaction takes place in a basic solution, add an equal number of hydroxide ions (OH^-) on each side of the reaction as the number of H⁺ ions that were added.  The OH^- and H+ that are on the same side of the equation will bond to form H₂O.
      5. Balance the charges of each reaction by adding electrons.
      6. Balance the half-reactions so that each one has the same number of free electrons
      7. Add the 2 reactions together and cancel out any identical species on opposite sides of the arrow. Check your answer to see if you produced the full balanced equation