Predicting REDOX Reactions

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Predicting REDOX Reactions
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Redox Rxn

• occur due to the transfer of e- from one
  substance to another.
• is a competition of e- between substances.
• if one substance pulls e- from another, a
  spontaneous rxn occurs.

eg. Cu 2 Ag ? Cu2 2 Ag
?
e- deficient, pulls e- from Cu harder than Cu
does for itself.
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• Reducing agents (RA)
• a substance that loses e- to another substance
  and undergoes oxidation.
• Oxidizing agents (OA)
• a substance that gains e- to another substance
  and undergoes reduction.
• eg. Cu 2Ag ? Cu2 2Ag
• RA OA OA RA
•  
• ox Cu ? Cu2 2e-
• red Ag e- ? Ag

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REDOX TABLEp.805
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• rxns are listed as reductions, S Strong, W
  Weak.
• SOA OA1 ne- ? RA1 WRA
• WOA OA2 ne- ? RA2 SRA
• spontaneous rxns occur only if the OA is above
  and
• to the left of the RA.
• For spontaneous rxn
• Ag e- ? Ag
• Cu2 2e- ? Cu

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• eg. Will a mixture of aqueous chromium (II)
  nitrate and tin (II) nitrate react and if so what
  is the overall equation?
• ? List all species THEN determine the SOA,
  highest on the left and SRA, lowest on the right
  on the REDOX table
• Cr2 NO3- Sn2 H2O

SOA
H2 (-0.83)
Sn (-0.14)
OA
Cr (-0.91)
(not in H)
SRA
Cr3 (0.41)
RA
Sn4 (-0.15)
O2 (-1.23)
ox Cr2 ? Cr3
e-
2
2

2
2x
2e-
red Sn2 ? Sn

2 Cr2 Sn2 ? 2 Cr3 Sn
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eg. Create ½ rxns from the following reaction
data and place in order of strongest to weakest
OA
Ion/Metal Zn Pb Ag I1-
Zn2 x x x x
Pb2 ? x x x
Ag ? ? x ?
I2 ? ? x x
1. Pb2 Zn ? Pb Zn2

1. 2Ag Zn ? 2Ag Zn2

3. 2Ag Pb ? 2Ag Pb2
4. 2Ag 2 I- ? 2Ag I2
5. I2 Zn ? 2 I- Zn2
6. I2 Pb ? 2 I- Pb2
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• Now create ½ reactions for each of the 4
  ions/metals

Zn2 2e- ? Zn
Pb2 2e- ? Pb
Ag e- ? Ag
I2 2e- ? 2 I-

• Using the experimental reactions, create a REDOX
  table with strongest to weakest OA

1. Pb2 Zn ? Pb Zn2

• this means Pb2 is a stronger OA than Zn2 table

Pb2 2e- ? Pb
Correct order
Zn2 2e- ? Zn
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• Ag

2. Ag Zn ? Ag Zn2
Ag is a SOA than Zn2
So the Ag rxn is above Zn, but is it above Pb?
3. Ag Pb ? Ag Pb2
Ag is a SOA than Pb2
? Ag rxn is above Pb
The table now
Ag e- ? Ag
Pb2 2e- ? Pb
Zn2 2e- ? Zn
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I2
4. Ag 2 I- ? Ag I2
Ag is a SOA than I2
So the Ag rxn is above I2, but is it above Pb
and/or Zn?
5. I2 Zn ? 2 I- Zn2
6. I2 Pb ? 2 I- Pb2
? I2 rxn is above Zn and Pb
The table now
Ag e- ? Ag
I2 2e- ? 2 I-
Pb2 2e- ? Pb
Finished!
Zn2 2e- ? Zn
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• eg. Create ½ rxns from the following reactions
  and place in order of strongest to weakest OA

? Cu2 is SOA than Pb2
Cu2 Pb ? Cu Pb2
? Br2 is a SOA than Ag
Ag Br2 ? AgBr
Ag Cu ? Ag Cu2
? Ag is a SOA than Cu2
Au Br2 ? no rxn
? Au3 is a SOA than Br-
Au3
½ rxns
Finally
Br2
Ag
Cu2 2e- ? Cu
Au3 3e- ? Au
Cu2
?
x
Pb2
Pb2 2e- ? Pb
Br2 2e- ? 2 Br-
Ag e- ? Ag
Ag e- ? Ag
x
Br2
Br2 2e- ? 2 Br-
Cu2 2e- ? Cu
x
Ag
Au3 3e- ? Au
Au3
Pb2 2e- ? Pb
Br2
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• eg. Create ½ rxns from the following reactions
  and place in order of strongest to weakest OA

W M2 ? W2 M-
X is the only 1 above M and is never the lowest
in the pair
M2
X2
W2
Y2
M2
Y2 Z ? Y Z2
M is above W which is never the lowest in the pair
Z2
W and X are above Y
X M2 ? no rxn
W2
X2
M2
Y is above Z
Y2
W2
W Y2 ? no rxn
Y2
done
Z2
X2
X2 Z ? X Z2
Y2
Table
X2 2e- ? X
M2 2e- ? 2 M-
W2 2e- ? W
Y2 2e- ? Y
Z2 2e- ? Z