http://www.ck12.org Chapter 22. Oxidation-Reduction Reactions
Step 1: Plan the problem.
Break the reaction down into a net ionic equation and then into half-reactions. The substance that loses electrons is
being oxidized and is the reducing agent. The substance that gains electrons is being reduced and is the oxidizing
agent.
Step 2: Solve.
Cl 2 (g) +2Na+(aq) + 2Br−(aq)→2Na+(aq) + 2Cl−(aq) + Br 2 (l)
Cl 2 (g) + 2Br−(aq)→2Cl−(aq) + Br 2 (l) (net ionic reaction)
Oxidation: Cl 2 (g) +2e−→2Cl−(aq)
Reduction: 2Br−(aq)→Br 2 (l) + 2e−The Cl 2 is being reduced and is the oxidizing agent. The Br−is being oxidized and is the reducing agent.
Practice Problems- Determine what is being oxidized and what is being reduced in each reaction. Identify the oxidizing and
reducing agents.
a. 2Mg(s) + O 2 (g)→2MgO(s)
b. Pb(NO 3 ) 2 (aq) + Zn(s)→Zn(NO 3 ) 2 (aq) +Pb(s)
Redox Reactions and Molecular Compounds
The loss or gain of electrons is easy to see in a reaction in which ions are formed. However, in many reactions, no
such electron transfer occurs. Recall that in a molecular compound, electrons are shared between atoms in a type of
bond called a covalent bond. Even though electrons are not completely transferred between atoms, it is still common
for reactions involving molecular compounds to be classified as redox reactions.
For example, when hydrogen gas is reacted with oxygen gas, water is formed as the product.
2H 2 (g) + O 2 (g)→2H 2 O(l)In the individual hydrogen molecules, a pair of bonding electrons is shared equally between the hydrogen atoms (a
nonpolar covalent bond). Likewise, the bonding electrons in the oxygen molecule are also shared equally between
the two oxygen atoms. However, when the atoms are rearranged to form the water molecule, the electron sharing is
no longer equal. In each hydrogen-oxygen bond in the water molecule, the bonding electrons are more attracted to
the oxygen atom than they are to the hydrogen atom. We know this because oxygen has a higher electronegativity
than hydrogen.
Over the course of this reaction, electrons are shifted away from the hydrogen atoms toward the oxygen atoms. In
the reactants, there are no partial charges on any atoms, because the bonds are completely nonpolar. However, in the
products, the hydrogen atoms carry a partial positive charge, while the oxygen atoms carry a partial negative charge.