© University of South Carolina Board of Trustees Chapt. 18 Electrochemistry Sec. 2 Balancing Redox Reactions

© University of South Carolina Board of Trustees Balancing Redox Reactions Copper dissolves in nitric acid to form Cu 2+. NO (g) is released at the same time. Find the balanced chemical equation. aCu (s) + bNO 3 − (aq) + cH + (aq)  dCu 2+ (aq) + eNO (g) + fH 2 O (l)

© University of South Carolina Board of Trustees Balancing Redox Reactions Acid Solution 1.Assign oxidation states. 2.Identify reduction and oxidation half-reactions. aCu (s) + bNO 3 − (aq) + cH + (aq)  dCu 2+ (aq) + eNO (g) + fH 2 O (l)

© University of South Carolina Board of Trustees Half-Reactions 2a.Write the reduction and oxidation portions of the reaction separately. oxidation states 2b.Use e − to balance oxidation states in each half-reaction. ●e − will cancel out in full reaction

© University of South Carolina Board of Trustees Balancing Redox Reactions Acid Solution 1.Assign oxidation states. 2.Identify reduction and oxidation half-reactions. 3.Balance the electrons, then balance atoms except H and O. 4.Combine half-reactions so no free e − appear. 7.Double check charge balance. 6.Add H + to balance H. 5.Add H 2 O to balance the O.

© University of South Carolina Board of Trustees Student Example Balance the equation (acid solution) Cr 2 O 7 2− + C 2 H 5 OH  Cr 3+ + CO 2

© University of South Carolina Board of Trustees Balance the equation Cu (s) + NO 3 - (aq)  Cu 2+ (aq) + NO (g) if the reaction occurs in basic solution (H 2 O and OH − readily available) Balancing Redox Reactions Basic Solution

© University of South Carolina Board of Trustees Balance the equation Cu (s) + NO 3 - (aq)  Cu 2+ (aq) + NO (g) if the reaction occurs in basic solution (H 2 O and OH − readily available) 1.Solve the problem in acidic solution. Balancing Redox Reactions Basic Solution

© University of South Carolina Board of Trustees Balance the equation Cu (s) + NO 3 - (aq)  Cu 2+ (aq) + NO (g) if the reaction occurs in basic solution (H 2 O and OH − readily available) 1.Solve the problem in acidic solution. 2.Add OH - to each side to eliminate H +. Balancing Redox Reactions Basic Solution

© University of South Carolina Board of Trustees Balance the equation Cu (s) + NO 3 - (aq)  Cu 2+ (aq) + NO (g) if the reaction occurs in basic solution (H 2 O and OH − readily available) 1.Solve the problem in acidic solution. 2.Add OH − to each side to eliminate H Cancel extra H 2 O formed. Balancing Redox Reactions Basic Solution

© University of South Carolina Board of Trustees Student Example Balance the reaction Zn + ClO −  Zn(OH) 4 2− + Cl − in basic solution.

© University of South Carolina Board of Trustees Final Example When O 2 and H 2 are present in reaction they are involved in the redox chemistry Ag + CN - + O 2  Ag(CN) 2 - In this case you must add H 2 O to the reaction. Ag + CN - + O 2  Ag(CN) H 2 O Similar with H 2 H 2 + Pt +  Pt Must add H+ to reaction to be able to solve H 2 + Pt +  Pt + 2H +