Balancing Redox Reactions Chapter 20: Day 2 and 3 To play the movies and simulations included, view the presentation in Slide Show Mode.

Review of Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species; increase in oxidation number. REDUCTION—gain of electron(s); decrease in oxidation number. OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized.

Redox: Electron transfer TRANSFER REACTIONS Atom/Group transfer (not) HCl + H2O ---> Cl- + H3O+ Redox: Electron transfer Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)

Copper + Silver Ion

OXIDATION-REDUCTION REACTIONS Direct Redox Reaction Oxidizing and reducing agents in direct contact. Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s) Why 2?

Cu + Ag+ --give--> Cu2+ + Ag Need to Balance BOTH mass and CHARGE Balancing Equations Cu + Ag+ --give--> Cu2+ + Ag Need to Balance BOTH mass and CHARGE Step 1: Divide into half-reactions: one for oxidation and the other for reduction. Ox Cu ---> Cu2+ Red Ag+ ---> Ag

Already done in this case. charge by adding electrons. Balancing Equations Step 2: Balance each for mass. Already done in this case. Step 3: Balance each half-reaction for charge by adding electrons. Ox Cu ---> Cu2+ + 2e- Red Ag+ + e- ---> Ag

Balancing Equations Step 4: Multiply each half-reaction by a factor to have the electrons lost equal to number gained Cu ---> Cu2+ + 2e- 2 Ag+ + 2 e- ---> 2 Ag Step 5: Add to give the overall equation. Cu + 2 Ag+ ---> Cu2+ + 2Ag The equation is now balanced for BOTH charge and mass.

5+ 2- 4+ 3- 2+ 2- 2+ 2- 6+ 4- 4+ 4+ 2+ 3+ 3+ 5+ 3- 3- 2-

Ready for more complex reactions? YES!

Acid / Base Redox Reactions Some redox reactions have equations that must be balanced by special techniques. If reactions occur in the “presence of” acid or base MnO4- + 5 Fe2+ + 8 H+ ---> Mn2+ + 5 Fe3+ + 4 H2O

Step 1: Divide into half-reactions Permanganate and iron(II) ions are reacted in an acidic solution MnO4- + Fe2+ ---> Mn2+ + Fe3+ Step 1: Divide into half-reactions Ox Fe2+ ---> Fe3+ Red MnO4- ---> Mn2+ Need to balance MASS

Step 2: Balance each for mass. Already done for Iron Fe2+ ---> Fe3+ Need to have “O” on both sided: Add water MnO4- ---> Mn2+ + H2O Never add O2, O atoms, or O2- to balance oxygen. MnO4- ---> Mn2+ + 4H2O

Step 2: Balance each for mass. MnO4- ---> Mn2+ + 4H2O Need to have “H” on both sided. Told in an acidic solution: need H+ 8 H+ + MnO4- ---> Mn2+ + 4H2O Never add H2 or H atoms to balance hydrogen.

charge by adding electrons. 8 H+ + 5 e- +MnO4- ---> Mn2+ + 4H2O Step 3: Balance each half-reaction for charge by adding electrons. 8 H+ + 5 e- +MnO4- ---> Mn2+ + 4H2O 5Fe2+ ---> 5Fe3+ + 5e- Multiply each half-reaction by a factor to have the electrons lost equal to number gained

Acid / Base Redox Reactions Step 5: Add to obtain the overall equation MnO4- + 5 Fe2+ + 8 H+ ---> Mn2+ + 5 Fe3+ + 4 H2O Check by adding charges on both sides and by counting atoms The equation is now balanced for BOTH charge and mass.

Balancing Equations Never add O2, O atoms, or O2- to balance oxygen. Never add H2 or H atoms to balance hydrogen. Be sure to write the correct charges on all the ions. Check your work at the end to make sure mass and charge are balanced. PRACTICE!

Reduction of VO2+ with Zn

Balancing Equations Balance the following in acid solution— VO2+ + Zn ---> VO2+ + Zn2+ Step 1: Write the half-reactions Ox Zn ---> Zn2+ Red VO2+ ---> VO2+ Step 2: Balance each for mass. VO2+ ---> VO2+ + H2O 2 H+ + Add H2O on O-deficient side and add H+ on other side for H-balance.

Balancing Equations e- + 2 H+ + VO2+ ---> VO2+ + H2O Step 3: Add electrons to half reaction. Zn ---> Zn2+ + 2e- e- + 2 H+ + VO2+ ---> VO2+ + H2O Step 4: Multiply by an appropriate factor. 2e- + 4 H+ + 2 VO2+ ---> 2 VO2+ + 2 H2O

Balancing Equations Step 5: Add balanced half-reactions Zn + 4 H+ + 2 VO2+ ---> Zn2+ + 2 VO2+ + 2 H2O Check by adding charges on both sides and by counting atoms