Lecture 24 3/14/06

Balancing Redox reactions

H2C2O4 (aq) + MnO4- Mn2+ + CO2 (g)

oxalic acid permanganate

1. Break into half-reactions2. Balance the atoms3. Combine the half-reactions4. Check for mass and charge balance

Redox example: respiration/combustion

Balance redox reaction for glucose (C6H12O6) respiration (same as combustion reaction)

How do the oxidation numbers of carbon and oxygen change?

What is being oxidized? What is being reduced?

What is the oxidizing agent? The reducing agent?

Acid Mine Drainage

FeS2 (pyrite)

www.state.sd.us/denr/DES/mining/P000106.jpg

http://www.nelpi.org/tarcreek/slides/5s.html

Redox under basic conditions:

Balance the following redox reaction in a basic solution

CrO42- + SO3

2- Cr(OH)3 + SO42-

Displacement reactions

Some metals react with acids to produce salts and H2 gas

Balance the following displacement reaction:

Zn (s) + HBr (aq) ZnBr2 (aq) + H2 (g)

Apparatus with oxidation and reduction Apparatus with oxidation and reduction reactions in different compartmentsreactions in different compartments redox reaction occurs by transferring electrons redox reaction occurs by transferring electrons

through an external connectorthrough an external connector

voltaic or galvanic cell product favored reactionproduct favored reaction produces electric current produces electric current

electrolytic cell reactant favored reactionreactant favored reaction external electric current used to drive reactionexternal electric current used to drive reaction

Electrochemical cells

Voltaic (Galvanic) Cells

Redox reaction must favor products

Electrodes Anode: where oxidation occurs Cathode: where reduction occurs

Salt bridge Need salt bridge or porous barrier in order to

maintain charge balance in each compartment

External circuit Wire to allow electrons to flow between electrodes