Untitled - Kelly Walsh High School

Aqueous Solutions 57
Quick Tip
Quick Tip
Oxidizing and reducing agents are reactants, not products.
Another way to determine what is undergoing oxidation and what is undergoing
reduction is by looking at the change in oxidation numbers of the reactant
species. Oxidation occurs when there is an increase in oxidation number. In the
example above, the Zn metal went from an oxidation state of 0 to 2.
Reduction occurs when there is a decrease in oxidation number. Cu 2 went
from an oxidation state of 2 to 0. In order to determine if a particular reaction
is a redox reaction, determine the oxidation numbers of each element in the
reaction. If at least one element changes oxidation number, it is a redox reaction.
Refer to your textbook for rules in assigning oxidation numbers.
We may predict many redox reactions of metals by using an activity series. An
activity series lists reactions showing how various metals and hydrogen oxidize
in aqueous solution. Elements at the top of the series are more reactive (active)
than elements below. A reaction occurs when an element interacts with a cation
of an element lower in the series. The more active elements have a stronger tendency
to oxidize than the less active elements. The less active elements tend to
reduce instead of oxidize. The reduction reactions are the reverse of the oxidation
reactions given in the activity series table, Table 4-1. This is an abbreviated
table. Refer to your textbook for a more complete table.
These types of reactions are sometimes called displacement reactions in which
one atom is displacing another atom from a compound.
We may use the activity series to predict certain types of redox reactions. For
example, suppose you wanted to write the equation between magnesium metal
and hydrochloric acid:
Mg(s) HCl(aq) l