Balancing Redox Reaction Equations

A. Balancing Redox Reaction Equations There are two ways to balance redox reaction equations methods, namely the half-reaction method and the method of changing the oxidation number. 1. Half Reaction Method How to balance redox reaction equations using half reactions, namely by looking at the electrons that are accepted or released. Balancing is done by equalizing the number of electrons. This method is preferred for reactions where the reaction atmosphere is known. How to balance: Stage 1: Write down the half reactions for the two substances that will be reacted. Stage 2: Balance the elements that experience changes in oxidation state. Stage 3: Add one molecule of H2 O to: - Acidic atmosphere: to one that lacks O atoms. - Alkaline atmosphere: where there is an excess of O atoms. Stage 4: Balance the hydrogen atoms by: - Acidic conditions: by adding H+ ions - Basic conditions: by adding OH– ions . Stage 5: Balance the charges by adding electrons. Stage 6: Equate the number of electrons received with those released, then add them up. Example : a. Balance the reaction: M N + Cl– bo Mn = +7 Cl = –1 Mn2+ + C12 (acid) Mn = +2 Cl = 0 Stage 1 : Mn Mn2+ Cl– C12 Stage 2 : Mn Mn2+ 2 Cl– C12 (Cl atoms balanced ×2) Stage 3 : Mn Mn2+ + 4 H2 O (O atoms are balanced) Cl2 Cl2– Stage 4 : 8 H+ + Mnq Mn2+ + 4 H2 O (H atoms are balanced) 2 C1– Cl2 Stage 5 : 5 e– + 8 H+ + Mn Mn2+ + 4 H2 O (number of charges left = right) 2 C1– Cl2 + 2 e– Stage 6 : Reaction I × 2 : 10 e– + 16 H+ + 2 MnO4 – 2 Mn2+ + 8 H2 O Reaction II × 5 : 10 Cl– 5 C12 + 10 e– 16 H+ + 2 MnO4 – + 10 Cl– 2 Mn2+ + 8 H2 O + 5 C12 B. How to Change the Oxidation Number The way to balance reaction equations is by changing the oxidation number, namely by looking at the change in the oxidation number. Balancing is done by equalizing changes in oxidation numbers. In this way the reaction atmosphere is not so influential, though reaction conditions are not yet known, balancing can be done. Stage 1: Balance the elements that experience changes in oxidation state. Stage 2: Determine the oxidation state of each element that experiences a change in oxidation state. Stage 3: Determine the change in oxidation state. Stage 4: Equalize the two changes in oxidation state. Stage 5: Determine the amount of charge on the left side and on the right side. Stage 6: Balance the charges by: - If the charge on the left is more negative, then add it H+ ions. This means the reaction is in an acidic environment. - If the charge on the left is more positive, then – OH ions are added. This means the reaction is in an alkaline environment. Stage 7: Balance the hydrogen by adding H2O . Example a. Balance the reaction: MnO + PbO2 Mn+ Pb2+ (sour) Answer : Stage 1: The element that undergoes oxidation changes is equivalent, namely Mn and Pb: MnO + PbO2 Mn + Pb2+ Stage 2, 3 : Determine the Mn and Pb oxidation states: Mn + Pb2+ +2 +5 +4 MnO + PbO2 +7 +2 –2 Stage 4 : MnO × 2 2 MnO + 5 PbO2 2 Mn + 5 Pb2+ PbO2 × 5 Stage 5 : 2 MnO + 5 PbO2 2 Mn + 5 Pb2+ 0 +8 Stage 6 : On the left side there is more negative charge (0), so that the number of left charges is the same as the right charge, then the left side plus 8 H+ ions means, sour atmosphere. 2 MnO + 5 PbO2 + 8 H+ 2 Mn + 5 Pb2+ Stage 7 : Balance the H atoms by adding H2 O on the right side. 2 MnO + 5 PbO2 + 8 H+ 2 Mn + 5 Pb2+ + 4 H2 O Check the number of O atoms, if it is equal it means the reaction is correct.