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REDOX REACTI
ONS
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OXIDATION NUMBER
The oxidation state of an element in an elemental state is zero.
(O2, Fe, He)
The oxidation state of an element in a monoatomic ion is the charge of that ion.
(oxidation number of iron in Fe+3 is +3) The oxidation number of Group 1 elements in all their
compounds is +1. The oxidation number of Group 2 elements in all their
compounds is +2.
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OXIDATION NUMBER
The oxidation number for fluorine in a compound is -1. The oxidation number of H is almost always +1. The oxidation of O is ordinarily -2. The sum of oxidation numbers in a molecule is zero.
H2SO4
In a polyatomic ion, the sum of the oxidation numbers is the charge of the ion.
Cr2O7-2
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DEFINITIONS
Oxidation: An increase in oxidation number
Reduction: A decrease in oxidation number
Reducing agent: Whatever is oxidized
Oxidizing agent: Whatever is reduced
Zn(s)+ 2H+(aq) Zn+2
(aq) + H2(g)
Zn is oxidized (ox # 0+2) Reducing agentH+ is reduced (ox # +10) Oxidizing agent
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BALANCING HALF-EQUATIONS
1) Balance the atoms of the atoms of the element being oxidized or reduced.
2) Balance the oxidation number by adding electrons.
3) Balance charge by adding H+ in acidic solution, OH- in basic solution.
4) Balance hydrogen by adding H2O molecules.
5) Check to make sure oxygen is balanced.
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BALANCIN
G EQUAT
IONS IN
ACIDIC
SOLU
TION
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MnO4-(aq) Mn+2
(aq)
MnO4-Mn+2
MnO4- + 5e-Mn+2
MnO4- + 5e- + 8H+Mn+2
MnO4- + 5e- + 8H+ Mn+2 + 4H2O
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BALANCIN
G EQUAT
IONS IN
BASIC S
OLUTI
ON
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Cr(OH)3(s) CrO4-2
(aq)
Cr(OH)3 CrO4-2
Cr(OH)3 CrO4-2 + 3 e-
Cr(OH)3 + 5OH-CrO4-2 + 3e-
Cr(OH)3 + 5OH-CrO4-2 + 3e- + 4H2O
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BALANCIN
G REDOX
EQUATIO
NS
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1) Split the equation into two half-equations.
2) Balance one of the half equations with respect to atoms and charge.
3) Balance the other half equation.
4) Combine the two half equations in such a way as to eliminate electrons.
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Balance the following redox equation in acidic solution.Fe+3
(aq) + MnO4-(aq)Fe+2
(aq) + Mn+2(aq)
Oxidation: Fe+3Fe+2 Reduction: MnO4-Mn+2
Balanced half equationsFe+2 -Fe+3+ e-
MnO4- + 5e-+ 8H+ Mn+2 + 4H2O
To eliminate e-, multiply the oxidation half-equation by 5 and add to the reduction half equation.5Fe+2 + MnO4
- + 5e- + 8H+ 5Fe+3 + Mn+2 + 4H2O + 5e-
5Fe+2 + MnO4- + 8H+ 5Fe+3 + Mn+2 + 4H2O
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Balance the following redox equation in basic solution.Cl2(g) + Cr(OH)3(s) Cl-
(aq)+ CrO4-2
(aq)
Cl2+ 2e- 2Cl-
Cr(OH)3 CrO4-2 + 3e- + 4H2O
Multiply reduction equation by 3 and the oxidation equation by 2.3Cl2+ 6e- + 2Cr(OH)3 6Cl- + 2CrO4
-2 + 6e- + 8H2O
3Cl2+ 2Cr(OH)3 6Cl- + 2CrO4-2 + 8H2O
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STOIC
HIOMETR
Y
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What volume of of 0.684 M KMnO4 solution is required to react completely with 27.50 mL of 0.250 M Fe(NO3)2?
5Fe+2 + MnO4- + 8H+ 5Fe+3 + Mn+2 + 4H2O
Moles Fe+2 = 0.02750 L x 0.250 moles Fe(NO3)2 x 1 mole Fe+2 =
1 L 1 mole Fe(NO3)2
6.88 x 10-3moles Fe+2
Moles MnO4- = 6.88 x 10-3moles Fe+2 x 1 mole MnO4
- = 0.00138 moles MnO4-
5 moles Fe+2
Moles KMnO4 = moles MnO4- = 0.00138 moles KMnO4
V = moles/M 0.00138 moles KMnO4/0.684 M KMnO4 = 2.02 x 10-2L = 2.02 mL
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REDOX R
EACTION
PROBLE
MSPage 9749-5664, 66, 6870, 72