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LEO SAYS GERLEO SAYS GERLEO SAYS GERLEO SAYS GER
Textbook ch 4.4
Unit 1: Chemistry Basics
1.42 Oxidation-Reduction Oxidation-Reduction ReactionsReactions“Redox”“Redox”
Oxidation and Reduction (Redox)
Electrons are transferred
Spontaneous redox rxns can transfer energy
Electrons (electricity) Heat
Non-spontaneous redox rxns can be made to happen with electricity
Oxidation Reduction Reactions(Redox)
Each sodium atom loses one electron:
Each chlorine atom gains one electron:
11
2
00
22
ClNaClNa
eNaNa10
10 CleCl
LEO says GER :Lose Electrons = Oxidation
Sodium is oxidized
Gain Electrons = Reduction
Chlorine is reduced
eNaNa10
10 CleCl
One cannot occur without the other
Rules for Assigning Oxidation Numbers
Rules 1 & 21. The oxidation number elements in their
elemental form is zero.
2. The oxidation number of a monatomic ion equals its charge
11
2
00
22
ClNaClNa
Rules for Assigning Oxidation Numbers
Rules 3 & 4
3. The oxidation number of oxygen in compounds is -2
4. The oxidation number of hydrogen in compounds is +1
2
2
1
OH
Rules for Assigning Oxidation Number Rule 5
5. The sum of the oxidation numbers in the formula of a compound is 0
2(+1) + (-2) = 0 H O
(+2) + 2(-2) + 2(+1) = 0 Ca O H
2
2
1
OH 2
122
)(
HOCa
Rules for Assigning Oxidation Numbers
Rule 66. The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its charge
3
2?
ON
24
2?
OS
Not All Reactions are Redox Reactions
Reactions in which there has been no change in oxidation number are not redox rxns.
Examples:
1 5 2 1 1 1 1 1 5 2
3 3( ) ( ) ( ) ( )Ag N O aq NaCl aq AgCl s Na N O aq
1 2 1 1 6 2 1 6 2 1 2
2 24 2 42 ( ) ( ) ( ) ( )NaOH aq H S O aq Na S O aq H O l
Trends in Oxidation and Reduction
Active metals: Lose electrons easily Are easily oxidized
Active nonmetals: Gain electrons easily Are easily reduced
Calculate the oxidation #s
Weirdo Weirdo exceptionsexceptions: : Rules 3 & 43. The oxidation number of oxygen in compounds is -2 (Note: Except in the peroxide ion in which it has an oxidation number of −1.)
4. The oxidation number of hydrogen in compounds is +1 (Note: Except when bonded to a metal, in which it has an oxidation number of -1)
2
1
2
1
OH
2
12
HaC
Oxidation of a metal in Single Replacement reactions
• Hydrochloric acid reacts with Zinc metal.
• Write oxidation #s 2HCl(aq) + Zn(s) ZnCl2 + H2(g)
Note: As written Zinc replaces the hydrogen ion in the reaction
WAIT! - How do you know the reaction WILL occur???
• Will hydrochloric acid oxidize zinc metal?HCl(aq) + Zn(s) ??????
Sometimes chemicals don’t react.
For single replacement reactions we consult activity series to predict if reaction will occur.
We want to “see” if single element can oxidized and “replace” a like ion in the compound.
Look at single element…is it higher in the chart than a like ion
(metal replace metals)• I will give you a copy of
the activity series to use on test
Activity series
•metals–greater activity = easier to lose electrons– i.e. easier to become a cation
•nonmetals–greater activity = easier to gain
electrons– i.e. easier to become an anion
• Will an aqueous solution hydrochloric acid oxidize zinc metal?
• If so, write the net ionic equation
• Because Zn is higher on activity series than H-
the reaction will occur. Zn will replace H in the compound.
HCl(aq) + Zn(s)
• calcium and lead (II) nitrate
• copper and lithium sulfate