Download Unit 11: Redox A) Assigning Oxidation Numbers (States

Unit 11: Redox
A) Assigning Oxidation Numbers (States)
 Uncombined element (only 1 type of element) = 0
 Ion = charge
 When combined: Group 1 = +1, Group 2 = +2, Halogens = -1, Aluminum = +3
 Hydrogen = +1 with nonmetals and -1 with metals only
 Oxygen = -2, in peroxides (H2O2, BaO2, Na2O2) -1, and +2 with fluorine
 Sum of all numbers in a compound is zero
 Sum of all numbers in a polyatomic ion is equal to its charge
 Use subscripts to find total charge of element (answer is before multiplying)
B) Oxidation and Reduction (always occur together)
 Redox reaction – if oxidation numbers change during a reaction
 Oxidation: loss of electrons, oxidation number goes up
 Reduction: gain of electrons, oxidation number goes down
 “LEO says GER” and LEO Jumps Up
 Oxidizing Agent – gets reduced
 Reducing Agent – gets oxidized
C) Half-Reactions
 Oxidation: electrons on right (lost)
Reduction: electrons on left (gained)
 Energy, mass, and charge must be conserved
 1) assign oxidation numbers
2) determine elements oxidized (ox. # goes up) and reduced (ox. # goes down)
3) write half-reactions with number of electrons on correct side
4) make sure reactions are balanced (charges total to the same # on both sides)
by multiplying half-reactions to get electrons equal and balance the original
reaction if necessary
D) Table J
 More reactive elements replace less reactive elements (high replaces low)
 Metals higher = easier to oxidize, metals lower = easier to reduce
 Nonmetals higher = easier to reduce, nonmetals lower = easier to oxidize
 Spontaneous – if metal oxidized is higher or nonmetal reduced is higher
E) Voltaic Cells
 Spontaneous redox reaction: chemical energy  electrical energy (batteries)
 Anode – site of oxidation, negative, “An Ox” – LEO
 Cathode – site of reduction, positive, “Big Red Cat” – GER
 Electrodes in different half cells so electrons can flow through a wire
 Metal oxidized at anode is higher on Table J
 Electrons flow from anode  cathode through wire
 Salt bridge completes circuit and keeps charges equal (migration of ions)
F) Electrolytic Cells
 Needs (requires) electricity for a nonspontaneous redox reaction
 Electrical energy  chemical energy
 Similarities to voltaic cells:
1) anode – oxidation and cathode – reduction
2) electrons flow from anode to cathode
 Differences from voltaic cells:
1) anode and cathode in same container
2) anode is positive and cathode is negative
 Electrolysis – 1) molten salts (makes highly reactive elements), 2) breaks apart
water, and 3) electroplating
 Electroplating – uses electricity to deposit a coating of metal on object
 Anode – metal used for plating, Cathode – object to be coated