Chapter 19

Redox Reactions

Oxidation Numbers

Rules Uncombined element = 0 Monatomic ion = charge on ion (from location of periodic

table) 2 element compound – right gets what it wants (from

periodic table), calculate left 3 element compound – right and left get what they want,

calculate left

Examples

Oxidation-Reduction (Redox)

Oxidation Loss of e- (becomes more positive) Ex: Zn -> Zn+2 +2e-

Reduction Gain of e- (becomes more negative) Ex: F2 +2e- -> 2F-

LEO the lion says GER Loss of e- is oxidation Gain of e- is reduction

Example 2H2 + O2 -> 2H2O

Recognizing Redox Reactions

THERE HAS TO BE A CHANGE IN OXIDATION NUMBER FOR A REACTION TO BE REDOX!!

Examples

Half-Reactions

Separate redox reaction into the individual oxidation and reduction reactions

ExamplesMg + O2 -> MgO

Redox Reactions

Oxidizing Agent Substance that causes oxidation It is Reduced!

Reducing Agent Substance that causes reduction It is Oxidized!

Balancing Half-Reactions

Steps: Write half-reactions separately Balance everything except H and O Balance O by adding water Balance H by adding H+

Balance overall charge by adding e-’s Add oxidation and reduction reactions with

equal e-’s Cancel if possible

Balancing Half-Reactions

Examples

Electrochemistry

Using half-reactions and calculating voltages Anode

Site of oxidation Cathode

Site of reduction Use the REDUCTION POTENTIAL CHARTS and

ONE will FLIP to make the MOST POSITIVE TOTAL MULTIPLYING BY COEFFICIENT DOES NOT

CHANGE VOLTAGE

Electrochemistry

Examples

Calculating Voltage for a Cell

Calculating Voltage for a Cell

What two half reactions will occur? What is the oxidation half reaction? What is the reduction half reaction? What would the voltage reading be for the

entire cell? Which electrode would get bigger? Which electrode would be consumed?