 # Redox Reactions

```Oxidation                Reduction

combination with O2      loss of O2

loss of electrons        gain of electrons

gain in oxidation        loss in oxidation
number                   number

```

An oxidation agent is a donor of electrons. A reduction agent is the absorber of electrons.

## Oxidation Number Rules

1. Lone elements are zero.
2. Neutral compounds must add to zero.
3. Polyatomic ions must add to the charge of the ion.
4. Oxygen is -2 except for H2O2.
5. Hydrogen is +1.
6. Label the alkali earth metals and the halogens +1 and -1, then procede toward the center of the periodic table, making sure to observe all of the rules above.

## Oxidation Number Reaction Calculation

```Fe2O3 + CO  Fe + CO2

+3 -2  +2 -2  0  +4 -2                  Write the oxidation numbers down
Fe2O3 + CO  Fe + CO2

+3 -2  +2 -2  0  +4 -2                  Notice how the carbon's number changes
Fe2O3 + CO  Fe + CO2                    and how the iron's number changes.
Carbon gains two oxidation numbers,
thus oxidizing. Iron is reducing as it
loses three oxidation numbers.

+3 -2  +2 -2  0  +4 -2                  Write in coefficients so that the
Fe2O3 + 3CO  2Fe + CO2                  oxidation number changes cancel out.

Fe2O3 + 3CO  2Fe + 3CO2                 Now balence the atoms

```

## Half Reaction

```S + HNO3  SO2 + NO + H2O

S + H+ + NO3-  SO2 + NO + H2O                          Write the reaction in
ionic form

S  SO2                     NO3-  NO                     Separate the oxidation
Oxidation                  Reduction                    and reduction processes

2H2O + S  SO2              NO3-  NO + 2H2O              Assuming this reaction
is happening in an acid
solution, use H2O to
balence the oxygen

2H2O + S  SO2 + 4H+        4H+ + NO3-  NO + 2H2O        Balence the hydrogens

2H2O + S  SO2 + 4H+ + 4e-  3e- + 4H+ + NO3-  NO + 2H2O  Add electrons to balence
the charges

6H2O + 3S  3SO2 + 12H+ +   12e- + 16H+ + 4NO3-  4NO +   Multiply the equations
12e-                       8H2O                         to make the electrons
match

6H2O + 3S + 12e- + 16 4H+ + 4NO3-  3SO2 + 12H+ + 12e-  Add the equations and
+ 4NO + 8 2H2O                                         cancel uninvolved
molecules

3S + 4HNO3  3SO2 + 4NO + 2H2O                          If necessary, add
spectator ions like K+
and Cl-

```

## Problems

• Is this a redox reaction?
• 20.h. H2O(l) + SO3(g) H2SO4(aq)
• What is the element being reduced and oxidized?
• What are the reducing agents and oxidizing agents?
• 27.a. MnO2 + HCl MnCl2 + Cl2 + H2O
• Balence the equation using the half reaction method.
• 23.a. MnO4-(aq) + I-(aq) MnO2(s) + I2(aq)
• Balence the equation using the oxidation number method.
• 18.a. SbCl5 + KI SbCl3 + KCl + I2
• Vocabulary terms copied from Kang, Kenneth. Chemistry Notes Semester 1. [http://members.aol.com/ksykang/chemnot2.htm].

Information and problems from Wilbraham, et al. Chemistry. 3rd ed. Addition Wesley, 1993.