REDOX REACTIONSChapter 20

SECTION 20.1

REDOX REACTIONS A chemical rxn in which electrons are

transferred from one atom to another is called an oxidation-reduction reaction, or redox rxn.

In a redox reaction, the loss of electrons from atoms of a substance is called oxidation.

The gain of electrons is called reduction.

CHANGES IN OXIDATION NUMBER You already learned that the oxidation

number of an atom is equal to the number of electrons gained or lost in an ionic compound.

Oxidation increases an atom’s oxidation number; reduction decreases the oxidation number.

COMPLEMENTARY PROCESSES Oxidation and reduction are complementary

processes that always occur together. The substance that is reduced in a redox

reaction is the oxidizing agent. The substance that is oxidized in a redox rxn

is the reducing agent.

EXAMPLE Redox rxn are not limited to reactions in

which atoms change to ions or vice versa. For example in the rxn, identify the oxidizing

and reducing agentH2 (g) + Cl2 (g) 2HCl (g)

PRACTICE PROBLEMS1. Identify what is oxidized and what is

reduced. Also identify the oxidizing agent and the reducing agent.

a. Zn + Ni2+ Ni + Zn2+

b. 2I- + Br2 I2 + 2Br-

c. 2NO N2 + O2

d. 2H2 + S2 2H2S

DETERMINING OXIDATION NUMBERS Use these rules to determine the oxidation

numbers in redox reactions.1. The oxidation number of an uncombined atom is

zero.2. The oxidation number of a monatomic ion is

equal to the charge on the ion.3. The oxidation number of the more

electronegative atom in a molecule or complex ion is the same as the charge it would have if it were an ion.

4. The most oxidation number of oxygen in compounds is always -2, except in peroxides, where it is -1. When bonded to fluorine the oxidation number of oxygen is 2+.

EXAMPLE Determine the oxidation number of each

element in the following compound and ion.

a. SrCO3 b. Cr2O72-

PRACTICE PROBLEMS2. Determine the

oxidation number of the boldface element in each of these compounds.

a. Li2SiO3

b. Al4C3

c. CaH2

d. BeSeO4

e. K2GeF6

f. Al(ClO3)3

3. Determine the oxidation number of the boldface element in each of these ions.

a. PO43-

b. Hg22+

c. HSO4-

d. PtCl62-

e. PuO2+

f. TeO32-

REACTIONS AS REDOX RXN Not all chemical rxns are redox rxns. Most double-displacement rxns are not redox

rxns, because there is not transfer of electrons between atoms.

Combustion and single-replacement rxns are always redox rxns.

Many synthesis and decomposition rxns are redox rxns as well.

SECTION 20.3

BALANCING REDOX REACTION When balancing redox rxns in addition to

balancing the # of each atom, the number of electrons gained or lost must also be balanced.

The method used is balancing half-reactions. A half-reaction shows the oxidation half of

the rxn, and the reduction half of the rxn in each half-rxn.

One half-rxn shows the oxidation and the other shows the reduction.

BALANCING HALF-REACTIONS The steps to balance a redox half-reaction

are as follows:1. Write the net ionic equation for the rxn,

omitting spectator ions.2. Write the oxidation and reduction half-

reactions for the net ionic equation.3. Balance the atoms and charges in each half-

reaction.4. Adjust the coefficients so that the # of

electrons lost in the oxidation equals the # of electrons gained in reduction.

5. Add the balanced half-reactions and return spectator ions.

EXAMPLE Use the half-reaction method to balance this

equation.K2Cr2O7 (aq) + HCl (aq) CrCl3 (aq) + KCl (aq) +

Cl2 (g)

PRACTICE Use the half-reaction method to balance.

a. I2 (s) + H2SO3 (aq) I- (aq) + HSO4- (aq)

b. Fe2+ (aq) + MnO4- (aq) Fe3+ (aq) +

Mn2+ (aq)c. Zn (s) + Cr2O7

2- (aq) Zn2+ (aq) + Cr3+ (aq)

d. IO3- (aq) + I- (aq) I2 (s)