5 Steps to a 5 AP Chemistry

with nonmetals to form ionic compounds, and the reaction of nonmetals with other

nonmetals to form covalent compounds.

In the first reaction, we have the combination of an active metal with an active

nonmetal to form a stable ionic compound. The very active oxygen reacts with hydrogen

to form the stable compound water. The hydrogen and potassium are undergoing

oxidation, while the oxygen and chlorine are undergoing reduction.

Decomposition Reactions

Decomposition reactionsare reactions in which a compound breaks down into two or more

simpler substances. Although not all decomposition reactions are redox reactions, many are.

For example, the thermal decomposition reactions, such as the common laboratory experi-

ment of generating oxygen by heating potassium chlorate, are decomposition reactions:

In this reaction the chlorine is going from the less stable +5 oxidation state to the more

stable −1 oxidation state. While this is occurring, oxygen is being oxidized from −2 to 0.

Another example is electrolysis, in which an electrical current is used to decompose

a compound into its elements:

The spontaneous reaction would be the opposite one; therefore, we must supply energy

(in the form of electricity) in order to force the nonspontaneous reaction to occur.

Single Displacement Reactions

Single displacement (replacement) reactionsare reactions in which atoms of an element

replace the atoms of another element in a compound. All of these single replacement reac-

tions are redox reactions, since the element (in a zero oxidation state) becomes an ion. Most

single displacement reactions can be categorized into one of three types of reaction:

 A metal displacing a metal ion from solution

 A metal displacing hydrogen gas (H 2 ) from an acid or from water

 One halogen replacing another halogen in a compound

Remember: It is an elementdisplacing another atom from a compound. The displaced

atom appears as an element on the product side of the equation.

Reactions will always occur in the free-response section of the AP Chemistry exam. This

may not be true in the multiple-choice part.

For the first two types, a table of metals relating their ease of oxidation to each other is

useful in being able to predict what displaces what. Table 6.1 shows the activity series for

metals, which lists the metal and its oxidation in order of decreasing ease of oxidation. An

alternative to the activity series is a table of half-cell potentials, as discussed in Chapter 16.

In general, the more active the metal, the lower its potential.

22 HOl 22 ()⎯→⎯⎯electricity H g O g() 2 () +

223 KClO s 32 ()⎯→⎯+Δ KCl s() O g( )

22

22

2

22 2

K s Cl g KCl s

Hg Og HOl

() ( ) ()

() () ()

+→

+→

Reactions and Periodicity  73

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