Balancing Reactions

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Balancing Reactions
Several methods exist for balancing chemical
equations 1- inspection (experienced trial and
error) 2- oxidation number method (REDOX) 3-
half-cell method (REDOX) 4- simultaneous
equations (computer application)
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Balancing redox reactions

• Identify half reactions
• Balance all atoms but O, H
• Balance the charge
• Multiply so both half reactions share the same
  of electrons
• Balance O and H by adding OH-, H or H2O
• Add the half reactions together
• Cancel like terms on both sides

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Balancing Redox Equations

• gt Half-reaction technique
• Examples
• Sn2 Fe3 ? Sn4 Fe2
• Fe2 MnO4- ? Fe3 Mn2 (in acid)

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Sn2 Fe3 ? Sn4 Fe2

• Sn2 ? Sn4 2 e-
• e- Fe3 ? Fe2
• 2(e- Fe3 ? Fe2)
• --------------------------------------------------
  --
• Sn2 2 Fe3 ? Sn4 2 Fe2

Check Atoms Sn 1
1 Fe 2
2 Charge
8 8
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Fe2 MnO4- ?Fe3 Mn2

• MnO4- ? e- ? Mn2
• oxidation numbers -1 1(x)Mn 4(-2)O 2
• x 8 - 1 7
• MnO4- ? e- ? Mn2
• 7 2
• MnO4- 5 e- ? Mn2
• MnO4- 5 e- ? Mn2 4H2O
• MnO4- 8 H 5 e- ? Mn2 4H2O

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Fe2 MnO4- ?Fe3 Mn2

• MnO4- 8 H 5 e- ? Mn2 4H2O
• Fe2 ? Fe3 e-
• 5(Fe2 ? Fe3 e-)
• ----------------------------------------
• 5Fe2 MnO4- 8H ? 5Fe3 Mn2 4H2O

Check Atoms Fe 5
5 Mn 1
1 O
4 4 H
8 8 Charge
5(2) -1 8(1) 5(3) 2(1)
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Your turn

• Cu (s) NO3- (aq) ? Cu2 (aq) NO (g) in acid

Oxidation half-cell Cu ? Cu2
2 e-

5 -2 2 -2 Reduction
half-cell NO3- 3e- ? NO
4 H NO3- 3 e- ? NO 2 H2O
Balance electron exchange 3(Cu ? Cu2 2 e-)
2(4 H NO3- 3 e- ? NO
2 H2O)
--------------------------------------------------
-----------------------------
3 Cu 8 H 2 NO3- ? 3 Cu2 2 NO 4 H2O
Check Atoms Cu 3
3 H
8 8
N 2
2 O 6
6 Charge
6 6
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How to determine if electrons have transferred

• Oxidation Numbers
• Assign electrons to the most electronegative
  element in a bond. The oxidation number is the
  charge an atom would have if the compound were
  completely composed of ions.
• RECORD KEEPING.

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Oxidation Numbers Useful Method

• Oxidation numbers are an electron count
  bookkeeping scheme.
• When a compound is formed, the electrons
  distribute themselves, according to
  electronegativity differences
• Metals tend to give up electrons (oxidation)
• gt becoming positive
• Non-metals tend to receive electrons (reduction)
• gt becoming negative

It is useful to have a method for keeping track
of the electrons
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Oxidation Numbers

• Rules for assigning oxidation numbers
• 1- treat ions separately
• 2- sum of oxidation numbers on species (molecule,
  ion) is equal to the charge
• 3- Hydrogen usually has ox no 1
• exception hydride, -1
• 4- Oxygen usually has ox no -2
• exceptions peroxide, -1 superoxide,
  -1/2
• 5- halogens usually are -1 (F, Cl, Br, I)
• 6- most electronegative atom often has ox no
  equal to number of electrons needed to complete
  its octet

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Rules for Determining Oxidation States

• Oxygen has a -2 oxidation state in its covalent
  compounds
• EXO in CO, CO2, SO2, SO3
• Exception oxygen has a -1 oxidation state in
  peroxides such as H2O2

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Rules for Assigning Oxidation State

• Any element in its free state has an oxidation
  state of zero
• EXmetals Cu, Ag, Hgnon-metals H2, O2, He,
  Ne, etc.

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Rules for Assigning Oxidation States

• In binary compounds (AaBb), the element with the
  greater electronegativity is assigned the
  negative oxidation state
• EXNH3 (N is -3)H2S (S is -2)

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Rules for Assigning Oxidation States

• The oxidation state on a monatomic ion is its
  charge which can be determined by the position of
  the element in the Periodic Table
• EXNa, Mg2, F-, Cl-

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Rules for Assigning Oxidation States

• The sum of the oxidation states of all atoms in a
  compound is zero
• EXKMnO4NaCl

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Rules for Assigning Oxidation States

• The sum of the oxidation states of all atoms in a
  polyatomic ion is the charge on the ion
• EXPO43-ClO4-

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Oxidation Numbers

• Examples with a strategy
• NH4NO3 NH4 NO3- treat
  separately
• Ox no for each element x 1
  -3 1
• N H4
  ? N H4
• Sum of ox numbers x 4(1) 1
• x
  4 1 x -3
• Ox no for each element x -3
  5 -2
• N O3-
  ? N O3-
• Sum of ox numbers x 3(-2) -1
• x -
  6 -1 x 5

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Oxidation Numbers

• Examples with a strategy
• FeO and
  Fe2O3
• Ox no for each element x -2
  2 -2
• FeO
  ? FeO
• Sum of ox numbers x (-2) 0
• x -
  2 0 x 2
• Ox no for each element x -2
  3 -2
• Fe2O3
  ? Fe2O3
• Sum of ox numbers 2(x) 3(-2) 0
• 2x
  - 6 0 x 3
• Same element exists in three different oxidation
  states! Fe 0, 2, 3
• Others Sn 0, 2, 4 N 0, -3, 1, 2, 3,
  4, 5 Cl 0, -1, 1, 3, 5, 7

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Oxidation Numbers

• Examples with a strategy
• FeO (s) CO (g) ? Fe (s) CO2 (g)

Fe gains two electrons
C gives up two electrons
In a balanced REDOX equation Number of
electrons received Number of electrons given
up Number electron in reduction Number
electrons in oxidation
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check the oxygen balance
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Problem 1 and 2

• Determine the oxidation number of each atom in
  the following species
• BaCl2
• LiH
• Zn
• O2
• H2CO3
• Cr2O72-
• Determine if the following changes represents an
  oxidation or a reduction
• MnO2 to Mn2O3
• Pb(OH)42- to PbO2

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Anode Pb grid
Cathode Pb grid filled w/ PbO2
H2SO4 electrolyte
-0.3588
E Ecat Ean 1.69 - (-0.3588) 2.0488V
Spontaneous?
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-0.7618
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