Learning Goals: I will be able to 1.Identify the givers and
takers of electrons in a chemical reaction: oxidized vs. reduced
elements; 2.Assign an oxidation state to any element in a chemical
equation. 2
Slide 3
3 Oxidation-Reduction Reactions Involve a transfer of electrons
between metals, nonmetals, or metal and nonmetal Redox reactions
include/are used in: automobile and other engines natural gas
burning to heat buildings coal burning to make electricity
battery-powered calculators, digital watches, radios, MP3 players,
etc. Provide most of the energy for work and play! Oxidation is a
loss of electrons. Ex.: Na Na + Reduction is a gain of electrons.
Ex.: Cl Cl -
Slide 4
The Lion Method: Leo Ger 4 Loss of Electrons is Oxidation Gain
of Electrons is Reduction
Slide 5
Oxidation-Reduction Reactions Which element is oxidized? Which
element is reduced? Na (loses 1 e-) Cl (gains 1 e-)
Slide 6
Copyright by Houghton Mifflin Company. All rights reserved. 6
Oxidation-Reduction Reactions When metal + nonmetal ionic compound,
the metal is oxidized and the nonmetal is reduced. Ex.: 2Na(s) + Cl
2 (g) 2NaCl(s) Na is oxidized to Na +, Cl is reduced to Cl - Na is
the reducing agent it donates electrons Cl 2 is the oxidizing agent
it accepts electrons
Slide 7
Copyright by Houghton Mifflin Company. All rights reserved. 7
Oxidation States Numbers/charges used to keep track of electrons in
a redox reaction: 2Mg(s) + O 2 (g) 2MgO(s) 00 +2 -2 Mg is oxidized,
O is reduced Reducing agent: Mg, Oxidizing agent: O 2 CH 4 (g) + 2O
2 CO 2 (g) + 2H 2 O(g) + energy -4 +1 0 +4 -2 +1 -2 C oxidized, O
reduced Reducing agent: CH 4, Oxidizing agent: O 2
Slide 8
Copyright by Houghton Mifflin Company. All rights reserved. 8 F
> O > N > Cl
Slide 9
B. Oxidation States For elements in compounds, Start by asking
if the compound is ionic or covalent: Ionic compound use p.
table/ion charges Covalent compound (including polyatomic ions) use
the list of rules
Slide 10
Learning Goal: I will be able to Design a voltaic cell
(battery) and describe the oxidation and reduction reactions
separately using the half-reaction method. 10
Slide 11
Copyright by Houghton Mifflin Company. All rights reserved. 11
Balancing Oxidation-Reduction Reactions by the Half-Reaction Method
Write and balance a reduction half-reaction electrons shown on
reactant side: 2e - + Cl 2 2Cl - Write and balance an oxidation
half-reaction electrons shown on product side: 2Na 2Na + + 2e -
Check: no. electrons gained = no. electrons lost Add the
half-reactions and cancel electrons to give the overall balanced
equation: 2e - + Cl 2 2Cl - 2Na 2Na + + 2e - 2e - + Cl 2 + 2Na 2Cl
- + 2Na + + 2e - Cl 2 + 2Na 2Cl - + 2Na +
Slide 12
Copyright by Houghton Mifflin Company. All rights reserved. 12
Balanced half-reactions more examples: The reduction of Br 2 (l) to
Br - (aq) Br 2 (l) + 2e - 2Br - (aq) The oxidation of Zn(s) to Zn
2+ (aq) Zn(s) Zn 2+ (aq) + 2e -
Slide 13
Copyright by Houghton Mifflin Company. All rights reserved. 13
Electrochemistry Study of the interchange of chemical and
electrical energy Involves two types of processes: Production of an
electric current from a chemical reaction Use of an electric
current to produce a chemical reaction When a redox reaction occurs
in solution, no useful work is obtained from the chemical energy To
harness the chemical energy from the reaction, the oxidizing agent
(e - acceptor) and reducing agent (e - donor) must be separated so
electrons are transferred through a wire The current flowing
through the wire can be directed to a device, such as motor, to do
useful work.
Slide 14
Schematic for Separating the Oxidizing and Reducing Agents in a
Redox Reaction * check your activity series to find a pair of
metals that could be used here; subtracting the potentials gives
the potential for the battery/voltaic cell. * Copyright by Houghton
Mifflin Company. All rights reserved. 14 Less active metal in ionic
solution More active metal in ionic solution ZnCu Zn 2+ SO 4 2- Cu
2+
Slide 15
Electron flow Copyright by Houghton Mifflin Company. All rights
reserved. 15
Slide 16
Ion flow keeps the charge neutral. Copyright by Houghton
Mifflin Company. All rights reserved. 16
Slide 17
The salt bridge contains a strong electrolyte and allows for
ion flow. Ion flow is needed to keep the charges neutral on both
sides. Copyright by Houghton Mifflin Company. All rights reserved.
17
Slide 18
The porous disk also allows ion flow. Copyright by Houghton
Mifflin Company. All rights reserved. 18
Slide 19
Schematic of a Battery or Galvanic Cell Electrons from one
electrode to another: anode cathode Copyright by Houghton Mifflin
Company. All rights reserved. 19
Slide 20
Schematic of one cell of the lead battery used in automobiles
Copyright by Houghton Mifflin Company. All rights reserved. 20
Slide 21
A common dry cell battery Copyright by Houghton Mifflin
Company. All rights reserved. 21
Slide 22
A mercury battery Alkaline batteries (basic medium) do not
corrode the steel as quickly and last longer. Copyright by Houghton
Mifflin Company. All rights reserved. 22
Slide 23
Corrosion Most metals are found in nature in compounds with
nonmetals (O, S, etc.) Corrosion - oxidation of a metal; returns
metal to its natural state Most metals react with O 2 and develop a
thin oxide coating this protects the metal from further
oxidation/corrosion: Aluminum Al 2 O 3 (very effective) Iron FeO
(not very effective) Gold shows no significant corrosion in air
Copyright by Houghton Mifflin Company. All rights reserved. 23
Slide 24
Mg donates the electrons and the iron pipe is preserved.
Copyright by Houghton Mifflin Company. All rights reserved. 24
Corrosion Protection Methods Coating paint or metal plating;
chromium and tin are often used Alloy use - metal mixture instead
of pure metal; stainless steel contains chromium and nickel, which
form a protective coating Cathodic protection a metal that donates
electrons more easily is connected by a wire (picture)
Slide 25
Copyright by Houghton Mifflin Company. All rights reserved. 25
Electrolysis Forcing a current through a cell (eg., battery) to
produce a chemical change that would not occur otherwise Used to
reverse redox reactions; allows recharging of lead and other
batteries Electrolysis of water: 2H 2 O(l) 2H 2 (g) + O 2 (g)
Occurs whenever a current is forced through an aqueous solutions a
nonreacting electrolyle is needed. Used in the production of metals
from oxides aluminum from aluminum ore (bauxite) Price of 1 lb.
aluminum: 1855, $100,000; 1890, $2; 1990, $0.74
Slide 26
Electrolysis of Aluminum Oxide 26 Electric current provides
electrons to reduce Al ions: Al 3+ + 3e - Al (pure metal) Purified
Al 2 O 3 dissolved in molten cryolite (NaF, AlF 3 ) at about 950 o
C
