5 Steps to a 5 AP Chemistry 2019

Reactions and Periodicity ‹ 75

There are several types of redox reaction that are given specific names. In the next few

pages, we will examine some of these types of redox reaction.

Combination Reactions

Combination reactions are reactions in which two or more reactants (elements or com-

pounds) combine to form one product. Although these reactions may be of a number of

different types, some types are definitely redox reactions. These include reactions of metals

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

nonmetals to form covalent compounds.

+→

+→

2K(s)Cl(g) 2KCl(s)

2H(g)O(g)2HO(l)

2

22 2

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 hydro-

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

oxidation, while the oxygen and chlorine are undergoing reduction.

Decomposition Reactions

Decomposition reactions are 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:

2KClO( 32 s)→∆+2KCl(s)3O(g)

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:

2H 22 O(l)→electricity+2H(g)O 2 (g)

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) reactions are 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 element displacing another atom from a compound. The displaced

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

STRATEGY

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