3.3 - Balancing Redox Reactions

Half-Reaction Method There are many ways to balance a redox reaction We are going to learn the most effective and easiest way to do it This is called the half-reaction method

Steps for Balancing Redox Reaction Assign oxidation numbers Split your reaction into 2 half-reactions One for the oxidation One for the reduction Balance mass of the equation In an acid: To balance for H, use H+ To balance for O, use H2O In a base: To balance for H, use H2O To balance for O, use OH- Balance charges with electrons Make electrons in each half-reaction equal by multiplying Add half-reactions together; if necessary, reform compound/return spectator ions

Example 1 Al(s) + Cu2+(aq) → Al3+(aq) + Cu(s) Al(s) → Al3+(aq) The mass is balanced because there are no O’s or H’s Al(s) → Al3+(aq) + 3 e- [Al(s) → Al3+(aq) + 3 e-]*2 2 Al(s) → 2 Al3+(aq) + 6 e- Cu2+(aq) → Cu(s) The mass is balanced because there are no O’s or H’s Cu2+(aq) + 2 e-→ Cu(s) [Cu2+(aq) + 2 e-→ Cu(s)]*3 3 Cu2+(aq) + 6 e-→ 3 Cu(s) 2 Al(s) → 2 Al3+(aq) + 6 e- 3 Cu2+(aq) + 6 e- → 3 Cu(s) 3 Cu2+(aq) + 2 Al(s) → 2 Al3+(aq) + 3 Cu(s)

Cu(s) + AgNO3 (aq) → Cu(NO3)2 (aq) + Ag(s) Example 2 Next example: Cu(s) + AgNO3 (aq) → Cu(NO3)2 (aq) + Ag(s) Identify the elements undergoing oxidation (Cu) and reduction (Ag). The nitrate group (NO3) is a spectator ion which we will not include in our half-reactions. oxidation Cu → Cu+2 + 2 e- reduction Ag+ + 1 e- → Ag

Example 2 Write the balanced half-reactions Balance electrons Step 1 Step 2 Step 3 Write the balanced half-reactions Balance electrons Add half-reactions Cu → Cu+2 + 2 e-   Ag+ + 1 e- → Ag × 2 2 Ag+ + 2e- → 2 Ag Add equations together Cu + 2 Ag+ → Cu+2 + 2 Ag Reform compound/return spectator ions Cu + 2 AgNO3 → Cu(NO3)2 + 2 Ag

MnO4- + Fe2+ + H+ → Mn2+ + Fe3+ + H2O Example 3 Here is a reaction occurring in an acid solution, which accounts for the presence of the H+ions. This example adds a little more complexity to our problem. MnO4- + Fe2+ + H+ → Mn2+ + Fe3+ + H2O In this example, spectator ions have already been removed. Even though hydrogen and oxygen do not undergo changes in oxidation number they are not spectators and we need to work with them in our half-reactions.

Example 3 We determine that Mn undergoes reduction (+7 to +2) while Fe undergoes oxidation (+2 to +3). The iron half-reaction is straight forward, but the manganese reaction is more complex - we must include hydrogen and oxygen in its half-reaction: oxidation Fe2+→ Fe +3 + 1e- reduction MnO4- + 8 H+ + 5 e- → Mn2+ + 4 H2O To balance the manganese half-reaction - first balance for Mn and O atoms. Next balance the H atoms, and finally add enough electrons to balance the charge on both sides of the equation. Be sure you see what has been done so you can do it on your own.

Example 3 Write the balanced half-reactions Add half-reactions Step 1 Step 2 Step 3 Write the balanced half-reactions Add half-reactions Fe2+→ Fe +3 + 1e- × 5 5Fe2+→ 5Fe +3 + 5e- MnO4- + 8H+ + 5e- → Mn2+ + 4H2O MnO4- + 8 H+ + 5e- → Mn2+ + 4 H2O Add equations together MnO4- + 5 Fe2+ + 8 H+ → Mn2+ + 5 Fe3+ + 4H2O

Ce4+ + I– + OH–  Ce3+ + IO3– + H2O Example 4 Here is a reaction occurring in a basic solution, which accounts for the presence of the OH-ions. This example requires a similar approach as our previous example, just in a base instead of an acid. Ce4+ + I– + OH–  Ce3+ + IO3– + H2O In this example, spectator ions have already been removed. Even though hydrogen and oxygen do not undergo changes in oxidation number they are not spectators and we need to work with them in our half-reactions.

Example 4 oxidation I– + 6 OH–  IO3– + 3 H2O + 6 e– reduction Ce4+ + 1 e–  Ce3+

Example 4 Step 1 Step 2 Step 3 Write the two balanced half-reactions, removing any spectator ions: Balance electrons Add the half-reactions, replacing any spectator ions that were removed and/or recombining compounds Ce4+ + 1 e–  Ce3+  6 6Ce4+ + 6 e–  6Ce3+ I– + 6 OH–  IO3– + 3 H2O + 6 e–   added together: 6 Ce4+ + I– + 6 OH–  6 Ce3+ + IO3– + 3 H2O Reform compound/return spectator ions Reform compound/return spectator ions