Chapter 20 – Chapter 20 – Oxidation and Oxidation and

ReductionReduction

Oxidation-Reduction reactions Oxidation-Reduction reactions called redox reactionscalled redox reactions

OO22 + Fe → + Fe → FeFe22OO33

►Oxidation – is the loss of electrons through Oxidation – is the loss of electrons through transfer or unequal sharingtransfer or unequal sharing

► Reduction – the gain of electrons through Reduction – the gain of electrons through transfer or unequal sharingtransfer or unequal sharing

► Iron was oxidized. Oxygen is the oxidizing Iron was oxidized. Oxygen is the oxidizing agent (it causes the iron to be oxidized)agent (it causes the iron to be oxidized)

►Oxygen was reduced. Iron is the reducing Oxygen was reduced. Iron is the reducing agent (it causes oxygen to be reduced)agent (it causes oxygen to be reduced)

►We cannot have oxidization without We cannot have oxidization without reduction If something gains electrons reduction If something gains electrons something must lose electronssomething must lose electrons

Assigning Oxidation NumbersAssigning Oxidation Numbers► The oxidation number of any uncombined element The oxidation number of any uncombined element

is zero. Examples: Fe His zero. Examples: Fe H22 OO33 He He ► The oxidation # of any ion is the charge on that The oxidation # of any ion is the charge on that

ion. Examples Clion. Examples Cl-- CaCa+2+2 OH OH-- SO SO44-2-2

► We assign elements oxidation numbers based on We assign elements oxidation numbers based on their oxidation number given on the periodic table. their oxidation number given on the periodic table. Many elements have only one oxidation number. Many elements have only one oxidation number. Examples: HFExamples: HF NaCl NaCl AlBrAlBr33 Fe Fe22OO33

► Hydrogen is assigned a +1 and oxygen is assigned Hydrogen is assigned a +1 and oxygen is assigned a -2 when combined to form compounds. a -2 when combined to form compounds.

► The algebraic sum of the oxidation numbers of The algebraic sum of the oxidation numbers of each atom in a neutral compound is zero or the each atom in a neutral compound is zero or the charge on the polyatomic ion. charge on the polyatomic ion.

► Assign those oxidation numbers you can be Assign those oxidation numbers you can be sure of, and then solve for the oxidation sure of, and then solve for the oxidation number you are not sure of. number you are not sure of.

► Do several examplesDo several examples

► Demonstrate sample problem page 661Demonstrate sample problem page 661► Assignment: page 661 practice problems 1 Assignment: page 661 practice problems 1

and 2and 2► Demonstrate sample problem page 664Demonstrate sample problem page 664► Assignment: page 665 practice problems 3 Assignment: page 665 practice problems 3

and 4and 4

►Where on the periodic table do you Where on the periodic table do you think strong oxidizing agents are think strong oxidizing agents are found?found?

►Where are strong reducing agents? Where are strong reducing agents? Why?Why?

►Synthesis, decomposition, and single Synthesis, decomposition, and single replacement are usually redox replacement are usually redox reactions.reactions.

►Double replacement reactions are Double replacement reactions are usually not redox reactions. usually not redox reactions.

See metal activity series on page See metal activity series on page 668668

►K is a reactive metal (is easily oxidized)K is a reactive metal (is easily oxidized)►Au is not a reactive metal (not easily Au is not a reactive metal (not easily

oxidized)oxidized)►AuAu3+3+ ion is easily reduced (very reactive) ion is easily reduced (very reactive)►KK++ ion is not easily reduced. (not reactive) ion is not easily reduced. (not reactive)►Active metals have stable ions.Active metals have stable ions.►Stable metals have reactive ions. Stable metals have reactive ions.

How does Zn prevent the How does Zn prevent the rusting of iron?rusting of iron?

►The OThe O22 takes electrons from Zn in takes electrons from Zn in preference to Fe because Zn gives up preference to Fe because Zn gives up electrons easier. electrons easier.

►Other applications of redox reactions Other applications of redox reactions are bleaching, fuels, explosives and are bleaching, fuels, explosives and photography. photography.

Balancing Redox ReactionsBalancing Redox Reactions

►Difficult equations can be balanced by Difficult equations can be balanced by using the change in oxidation using the change in oxidation numbers as your starting point. numbers as your starting point.

►You then balance the change in charge You then balance the change in charge (which is really balancing electrons). (which is really balancing electrons).

►See example on page 676See example on page 676►Assignment: practice problems 5 and 6 Assignment: practice problems 5 and 6

page 677page 677

►Complete chapter questions on page Complete chapter questions on page 680 and 681 (1-16, 18, 19, 20, 23, 26, 680 and 681 (1-16, 18, 19, 20, 23, 26, 28)28)