Electrochemistry #1

Redox Review A reaction in which electrons are transferred from one atom to another is called an oxidation–reduction reaction.

Example Determine what is oxidized, reduced, the oxidizing agent, & reducing agent HCl + HNO3  HOCl + NO + H2O

Half Reaction Half reaction – one of the two parts of a redox reaction One half will be oxidation One half will be reduction

Step to Balancing Half Reaction Method Separate the reactions into 2 half reactions (one for oxidation & one for reduction) Balance each ½ reaction un the following order: Balance elements other than H & O Balance O by adding H2O Balance H by adding H+ Balance the charge to make each side equal

Step to Balancing Half Reaction Method Multiply each ½ reaction by a number to make the number of electrons gained = the number of electrons lost Add the ½ reactions Simplify Check your work Tip – Break things into ions or binary compounds & keep weak acids & bases together!

Example Fe + CuSO4  Cu + Fe2(SO4)3 ( ) 3 Fe  2Fe +3 2Fe  2Fe +3 0  +6 0  +6 (+6e-) 2Fe  2Fe +3 +6e- Cu +2  Cu +2  0 +2 + 2e-  0 Cu +2 + 2e-  Cu 3Cu +2 + 6e-  3Cu ( ) 3 2Fe + 3Cu +2 + 6e-  2Fe +3 +6e- + 3Cu x x 2Fe + 3Cu +2  2Fe +3 + 3Cu

Another Example AsO4 -3 + Zn  H3As + Zn +2 +11 -3 0 0 +8 0 (+8 e-) AsO4 -3  H3As AsO4 -3  H3As + 4H2O 11 H+ +AsO4 -3  H3As + 4H2O +11 -3 0 0 +8 0 (+8 e-) 11 H+ +AsO4 -3 + 8e-  H3As + 4H2O Zn  Zn +2 0 +2 (+2 e-) Zn  Zn +2 +2 e- 4Zn  4Zn +2 +8 e- ( )4 11H+ +AsO4 -3+ 8e- + 4Zn H3As + 4H2O + 4Zn +2 + 8 e- 11H+ +AsO4 -3+ 4Zn H3As + 4H2O + 4Zn +2

Try This one… MnO4 -1 + C2O4 -2  Mn +2 + CO2 2MnO4 -1+16 H++ 5C2O4-2  2Mn+2 +8H2O + 10CO2

Last one… KMnO4 + HCl  KCl + MnCl2 + H2O + Cl2 6 H+ + 2MnO4- + 10 HCl  2Mn +2 +8H2O + 5Cl2

Balancing Basic Balancing in basic solution is just like acidic with one additional step When balancing in acidic you will have H+ at the end In basic you don’t want H+, you want OH-

To change to basic To change to basic, simply add an equivalent number of OH- to each side and simplify

Example Balancing the following by using the ½ reaction method in a basic solution AsO4-3 + Zn  H3As + Zn +2 Acidic: 11H+ + AsO4-3 + 4Zn  H3As + 4 H2O + 4Zn+2 Basic: +11 OH- + 11OH- 11 H2O + AsO4-3 + 4Zn  H3As + 4 H2O + 4Zn+2+ 11OH- 7 H2O + AsO4-3 + 4Zn  H3As + 4Zn+2+ 11OH-

Another Example Ag + NO3-  Ag + + N+2 Acidic: 3Ag +6 H+ + NO3-  3Ag + + N+2+ 3H2O Basic: + 6OH- + 6OH- 3Ag +6H2O + NO3-  3Ag + + N+2+ 3H2O + 6OH- 3Ag +3H2O + NO3-  3Ag + + N+2 + 6OH-

One More Mn(NO3)2 + NaBiO3 + HNO3  NaMnO4 + Bi(NO3)3 + H2O + NaNO3 Acidic: 2Mn+2 + 14H+ + 5 BiO3-  2MnO4- + 5Bi +3 + 7H2O Basic: + 14 OH- + 14 OH- 2Mn+2 + 14H2O + 5 BiO3-  2MnO4- + 5Bi +3 + 7H2O+ 14 OH- 2Mn+2 + 7H2O + 5 BiO3-  2MnO4- + 5Bi +3 + 14 OH-

MEMORIZE!

Redox Reaction Types Watchout for: Keywords "acidified solution" or an acid included in the reactants. Anytime you see a neutral element, Cu°, O2, H2, etc. it must be redox. When you recognize great oxidizers like Cr2O72-, MnO4-, and MnO2

Reaction Example 1 Solid copper reacts with dilute nitric acid solution

Reaction Example 2 A solution of potassium permanganate is mixed with an alkaline solution of sodium sulfite

Reaction Example 3 Hydrogen peroxide is added to a solution of iron (II) sulfate