oxidation-reduction reactions redox reactions. oxidation state oxidation numbers are very similar to...
TRANSCRIPT
Oxidation-Reduction Reactions
REDOX Reactions
Oxidation State• Oxidation numbers are very
similar to charge. • There are some different rules
for assigning them.
Rules for Oxidation State• 1. The oxidation number for
any uncombined element is zero
• Ex. The oxidation state for Na is zero.
Rule 2• The oxidation number for a
monatomic ion equals the charge on the ion.
• Ex. Cl-1 has an oxidation of -1.
Rule 3• The more electronegative
element in a binary compound is assigned the number equal to the charge it would have if it were an ion.
• Ex. The oxidation number of O in NO is -2 because oxygen is more electronegative than nitrogen.
Rule 4• The oxidation number of
fluorine in a compound is always -1.
• Ex. The oxidation number of F in LiF is -1.
Rule 5• Oxygen has an oxidation
number of -2 unless it is combined with fluorine, when it is +2, or it is in a peroxide such as H2O2, when it is -1.
• Ex. The oxidation of O in NO2 is -2.
Rule 6• The oxidation state of
hydrogen in most of its compounds is +1 unless it is combined with a metal, in which case it is -1.
• Ex. The oxidation of H in LiH is -1. The oxidation of H in HCl is +1.
Rule 7• In compounds, Group 1 and 2
elements and aluminum have oxidation numbers of +1, +2, and +3, respectively.
• Ex. The oxidation number of Ca in CaCO3 is +2.
Rule 8• The sum of the oxidation
numbers of all atoms in a neutral compound is 0.
• Ex. The oxidation number of each atom in CaCO3 can be found by knowing the rules above and knowing the compound is neutral
• Ca = +2 (rule 7)• O = -2 x 3 atoms = -6 (rule 5)• C = +4 (rule 8—balances out
the charge)
Rule 9• The sum of the oxidation
numbers of all atoms in a polyatomic ion equals the charge of the ion.
• Ex. The oxidation of each element in H2PO4
2- can be determined by knowing the overall charge is -2
• H = +1 x 2 atoms = +2 (rule 6)• O = -2 x 4 atoms = -8 (rule 5)• P = +4 (rule 9)
Types of Reactions5 types plus a new one
Type of Reactions
Chemical reactions are classified into five general types
Combination
Decomposition
Single Replacement
Double Replacement
Combustion
Combination (Synthesis)
Two or more elements or simple compounds combine to form (synthesize) one product
• A + B AB
• 2Mg + O2 2MgO
• 2Na + Cl2 2NaCl
• SO3 + H2O H2SO4
Decomposition
One substance is broken down (split) into two or more simpler substances.
• AB A + B
• 2HgO 2Hg +
O2
• 2KClO32KCl + 3
O2
Learning Check R1
Classify the following reactions as 1) combination or
2) decomposition:
___A. H2 + Br2 2HBr
___B. Al2(CO3)3 Al2O3 +
3CO2
___C. 4 Al + 3C Al4C3
Solution R1
Classify the following reactions as 1) combination or 2) decomposition:
_1_A. H2 + Br2 2HBr
_2_B. Al2(CO3)3 Al2O3 +
3CO2
_1_C. 4 Al + 3C Al4C3
Single Replacement
One element takes the place of an element in a reacting compound.
A + BC AB + C
Zn + 2HCl ZnCl2 + H2
Fe +CuSO4 FeSO4 +
Cu
Learning Check R2
Classify the following reactions as 1) single replacement 2) double replacement
__A. 2Al + 3H2SO4 Al2(SO4)3 + 3H2
__B. Na2SO4 + 2AgNO Ag2SO4 +
2NaNO3
__C. 3C + Fe2O3 2Fe + 3CO
Solution R2
Classify the following reactions as 1) single replacement 2) double replacement
1_A. 2Al + 3H2SO4 Al2(SO4)3
+ 3H2
2_B. Na2SO4 + 2AgNO3 Ag2SO4
+ 2NaNO3
1_C. 3C + Fe 2Fe + 3CO
Combustion
A reaction in which a compound (often carbon) reacts with oxygen
C + O2 CO2
CH4 + 2O2 CO2 + 2H2O
C3H8 + 5O2 3CO2 + 4H2O
C6H12O6 + 6O2 6CO2 + 6H2O
Learning Check R3
Balance the combustion equation
___C5H12 + ___O2 ___CO2 + ___H2O
Solution R3
Balance the combustion equation
1 C5H12 + 8 O2 5 CO2 + 6 H2O
Oxidation-Reduction Reactions
REDOX Rxns
Oxidation and Reduction
Reactions that involve a loss or gain of electrons
Occurs in many of the 4 types of reactions and combustion
Important in food metabolism, batteries, rusting of metals
Requirements for Oxidization-Reduction
Electrons are transferred
Two processes occur
Oxidation = Loss of electrons (LEO)
Zn Zn2+ + 2e-
Reduction = Gain of electrons (GER)
Cu2+ + 2e- Cu
Balanced Red-Ox Equations
Combine the oxidation and reduction reactions to make
Loss of electrons = Gain of electrons
Zn + Cu2+ + 2e- Zn2+ + 2e- + Cu
Zn + Cu2+ Zn2+ + Cu
Gain/Loss of Hydrogen
In organic and biological reactionsoxidation = Loss of Hreduction = Gain of H
Half-Reaction• The part of a reaction involving
oxidation or reduction alone. • The overall reaction is the sum
of the two half reactions• Ex. • 3Cu 3Cu2++ 6 e-
• 2NO3-1 + 6 e- + 8H+ 2NO + 4
H2O
• 3Cu + 2NO3-1 + 8 H+ 3Cu2+ + 2NO +
4H2O
Learning Check R3
Identify the following as an 1) oxidation or a reduction process:
__A. Sn Sn4+ + 4e-
__B. Fe3+ + 1e- Fe2+
__C. Cl2 + 2e- 2Cl-
Solution R3
Identify the following as an 1) oxidation or a reduction process:
1_ A. Sn Sn4+ + 4e-
2_ B. Fe3+ + 1e- Fe2+
2_ C. Cl2 + 2e- 2Cl-
Learning Check R4
In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction
Ag+ + Cl- Ag + ClA. Which reactant is oxidized
1) Ag+ 2) Cl- 3) AgB. Which reactant is reduced?
1) Ag+ 2) Cl- 3) Cl
Solution R4
In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction
Ag+ + Cl- Ag + ClA. Which reactant is oxidized
2) Cl- Cl- Cl + e-
B. Which reactant is reduced?1) Ag+ Ag+ + e-
Ag
Oxidizing and Reducing Agents
Reducing Agents• A substance that has the
potential to cause another substance to be reduced.
• Lose electrons (gain higher charge)
Oxidizing Agent• Substance that has the
potential to cause another substance to be oxidized
• Gains electrons (become more negative)
Summary Chart of TerminologyTerm Change in
oxidation number
Change in electron population
Oxidation Increases Loss of electrons
Reduction Decreases Gain of electrons
Oxidizing agent
Decreases Gains electrons
Reducing agent
Increases Loses electrons
Autooxidation• A process in which a substance
acts as both an oxidizing agent and a reducing agent
ElectrochemistryElectrodes, half cells, anodes, cathodes,
voltaic and electrolytic cells
Electrochemistry• The branch of chemistry that
deals with electricity-related applications of oxidation-reduction
A bright idea!• As redox rxns occur, electrons
are transferred which also produces heat.
• But if the substances are separated by a porous barrier then they are able to transfer electricity instead of heat
Electrode•Conductor used to establish electrical contact with a nonmetallic part of a circuit such as an electrolyte
Half-cell• A single electrode immersed in
a solution of its ions.
Anode• One half cell is the anode which
is the electrode where oxidation takes place.
Cathode• Other half cell where reduction
takes place.
Electrochemical cell• A system of electrodes and
electrolytes in which either chemical reactions produce electrical energy of an electric current produces chemical change.
Voltaic Cells•If the redox rxn in an electrochemical cell occurs spontaneously and produces electrical energy then it is a voltaic cell.
Types of Dry Cell BatteriesZinc-Carbon Dry Cells
Alkaline BatteriesMercury batteries
Zinc-Carbon Dry Cells•Ex. Flashlight batteries•Have Zn container which is the anode
•Filled with moist paste of MnO2, graphite, and NH4Cl
Alkaline Batteries•Ex. Batteries found in small, portable devices such as CD players•Very similar to the Zn-C but it does not contain the C which allows them to be smaller•Uses paste of Zn and KOH instead of a solid metal anode.
Mercury Batteries•Ex. Calculator batteries•The anode half rxn is the same as the alkaline but the cathode involves HgO
Rechargeable cells•Combination of voltaic and electrolytic cells•Acts as a voltaic cell during use and as an electrolytic cell during charging.•Ex. Car battery