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REDOX
electrochemistry
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• Spontaneous redox reactions can transfer energy– Electrons (electricity)– Heat
• Non-spontaneous redox reactions can be made to happen with electricity.
Redox reactions involve the transfer of the electron.
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Trends in Oxidation and Reduction
Active metals: Lose electrons easily Are easily oxidized Are strong reducing
agents
Active nonmetals: Gain electrons easily Are easily reduced Are strong oxidizing agents
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Oxidation
- is defined as the loss of an electron
• (LEO - loss of electron; oxidation)
- when oxidation happens, charge becomes more positive
• Fe → Fe2+ + 2e-
4 Fe (s) + 3 O2 (aq) → 2 Fe2O3 (s)
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Reduction
• is defined as the gain of an electron• (GER- gain of electrons – reduction• - when reduction happens, charge becomes
more negative• Fe2+ + 2e- → Fe• - term is derived from the observation that metal
oxides lose mass when refined to produce the pure metal:
• 2 Fe2 O3 (s) → 4 Fe (s) + 3 O2 (aq)
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• N2O → NO• Has the nitrogen gained or lost
electrons? • N2O → NO +1 +2Nitrogen has lost one electron and is
therefore being oxidized.
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11
2
00
22
ClNaClNa
Each sodium atom loses one electron:
Each chlorine atom gains one electron:
eNaNa10
10 CleCl
Lose Electrons = Oxidation
Gain Electrons = Reduction
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• These pairs of reactions are called half-reactions. For example, in the reaction
• Cu2+ (aq) + Fe (s) → Cu (s) + Fe2+ (aq)• two half-reactions occur; iron is oxidized
while the copper (II) ion is reduced. Seen another way, the copper (II) ion causes the oxidation of the iron; it is the oxidizing agent. Likewise, the iron is the reducing agent for the copper (II) ion.
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There are five basic rules for the determination of oxidation number:
• Rule 1: The oxidation number for any atom in its elementary state is 0.
• Rule 2: The oxidation number for any simple ion is the change on the ion.a. The oxidation number of alkali metals in compounds is 1+
(Li1+ , Na1+ , K1+ , Rb1+ , Cs1+ , Fr1+ ).b. The oxidation number of alkaline-earth metals in compounds is 2+
(Mg2+ , Ca2+ , and Ba2+ ).
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Find the oxidation number for magnesium and chlorine
12
2
00
ClMgClMg
22 ClMgClMg
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• Rule 3: The oxidation number for oxygen usually is 2-. In peroxides, it is 1-.
2
2
1
OH 2
1
2
1
OHwater peroxide
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• Rule 4: The oxidation number for hydrogen is 1+ in all its compounds except in metallic hydrides like NaH or BaH2 , where it is 1-.
• Rule 5: All other oxidation numbers are assigned so that the sum of oxidation numbers equals the net charge on the molecule or polyatomic ion.
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Find the oxidation numbers
• H2S Ca(OH)2
2
2
1
SH2(+1) + (-2) = 0 H O
2
122
)(
HOCa(+2) + 2(-2) + 2(+1) = 0 Ca O H
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3
2?
ONX + 3(-2) = -1N O
24
2?
OS
X = +5 X = +6
X + 4(-2) = -2S O
Find the oxidation numbers
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Find the oxidation number of Cr:
• Ex: Cr2O72-
2X + 7(-2) = -2Cr O
X = +6
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Write the oxidation numbers for each atom:
• S8
• SO2
• S2O32-
• SO42-
• MgSO4
• H2SO4
0
S = +4 and O = -2
S = +2 and O = -2
S = +6 and O = -2
Mg = +2, S = +6 and O = -2
H = +1, S = +6 and O = -2
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Find the oxidation numbers
• P4
• P2O5
• PCl5• H3PO4
• PO43-
• Na3PO4
0
P = +5 and O = -2
P = +5 and Cl = -1
H = +1, P = +5 and O = -2
P = +5 and O = -2
Na = +1, P = +5 and O = -2
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A summary of terminology for oxidation-reduction (redox) reactions
X Y
e-
transfer or shift of electrons
X loses electron(s) Y gains electron(s)
X is oxidized Y is reduced
X is the reducing agent Y is the oxidizing agent
X increases its oxidation number
Y decreases its oxidation number
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Not all reactions are redox
)()()()( 3
2511111
3
251
aqONNasClAgaqClNaaqONAg
)()()()(22
2
1
4
26
2
1
4
26
2
1121
lOHaqOSNaaqOSHaqHONa
Reactions in which there has been no change in oxidation number are not redox reactions.
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• Just like the Bronsted-Lowry theory of acids, oxidation and reduction also happens in pairs; a species cannot donate an electron unless another species gains the electron. Since oxidation and reduction always happen together, these are most often called redox reactions.
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Reducing and oxidizing agents
The substance reduced is the oxidizing agent The substance oxidized is the reducing agent
eNaNa10
10 CleCl
Sodium is oxidized – it is the reducing agent
Chlorine is reduced – it is the oxidizing agent
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• 2H2(g) + O2(g) → 2H2O(g)
• 2H2(g) + O2(g) → 2H2O(g)
0 0 +1 -2
The O.N. of H increases; it is oxidized; it is the reducing agent.
The O.N. of O decreases; it is reduced; it is the oxidizing agent.
FIND THE OXIDATION NUMBERS:
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Find the oxidation numbers:
2Al(s)+ 3H2SO4(aq) Al2(SO4)3(aq) + 3H2(g)
2Al(s) + 3H2SO4(aq) Al2(SO4)3(aq) +3H2(g)
0 0+6+1 -2 +3 +6 -2
The O.N. of Al increases; it is oxidized; it is the reducing agent.
The O.N. of H decreases; it is reduced; it is the oxidizing agent.
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Find the oxidation numbers
• PbO(s) + CO(g) → Pb(s) + CO2(g)
• PbO(s) + CO(g) → Pb(s) + CO2(g)
+2 -2 +2 -2-2 0 +4
The O.N. of C increases; it is oxidized; it is the reducing agent.
The O.N. of Pb decreases; it is reduced; it is the oxidizing agent.