Chapter 18 Electrochemistry

Definition The study of the interchange of chemical and electrical energy. Involve oxidation-reduction reactions

Half-Reaction Method Used to balance redox rxns in aqueous solution One of the rxns is the oxidation rxn Other is the reduction rxn The half reactions are balanced separately and then added Differs in acidic vs. basic solution

In Acidic Solution Separate the oxid. and red. parts of the rxn. For each: a. Balance all elements but H and O b. Balance O using H2O c. Balance H using H+ d. Balance charge using electrons If needed, mult. One or both half-rxns. by a whole # to make electrons equal for both Add half-rxns and cancel same on both sides Make sure elements/charges are balanced

Acronym for Balancing in Acid Big White Hogs Eat Bagels and Cream Cheese B = balance all but H & O W = add water for O’s H = Add H E = Add electrons B = Make sure electrons match CC = Combine and cancel

Example in Acid Balance the following equation in acid solution using the half-reaction method. Cu(s) + HNO3(aq) -> Cu2+(aq) + NO(g) Answer: 3Cu + 2HNO3 + 6H+ -> 3Cu2+ + 2NO + 4H2O Cr2O72-(aq) + NO(g) -> Cr3+(aq) + NO3-(aq) Cr2O72-(aq) + 2NO(g) + 6H+(aq) -> 2Cr3+(aq) + 2NO3-(aq) + 3H2O(l)

In Basic Solution Follow the steps used in acidic solution THEN… Add OH- to match H+ on same side. Add to other side as well Form H2O by adding OH- and H+ Cancel H2O’s on both sides Check for element/charge balance

P4 + 2H2O + 4OH- -> 2PH3 + 2HPO32- Example in Base Balance the following equation in basic solution: P4 -> PH3 + HPO32- Answer P4 + 2H2O + 4OH- -> 2PH3 + 2HPO32-