The chemical reactions we have described are only a tiny sampling of the infinite number of chemical reactions possible. How do chemists cope with this overwhelming diversity? How do they predict which compounds will react with one another and what products will be formed? The key to success is to find useful ways to categorize reactions. Familiarity with a few basic types of reactions will help you to predict the products that form when certain kinds of compounds or elements come in contact.
Most chemical reactions can be classified into one or more of five basic types: acid–base reactions, exchange reactions, condensation reactions (and the reverse, cleavage reactions), and oxidation–reduction reactions. The general forms of these five kinds of reactions are summarized in Table 7.10.1 , along with examples of each. It is important to note, however, that many reactions can be assigned to more than one classification, as you will see in our discussion.
Table 7.10.1 : Basic Types of Chemical Reactions
Name of ReactionGeneral FormExamples
Oxidation–Reduction (redox)oxidant + reductant → reduced oxidant + oxidized reductantC7H16(l) + 11O2(g) → 7CO2(g) + 8H2O(g)
Acid–Baseacid + base → saltNaOH(aq) + HNO3(aq) → NaNO3(aq) +H2O(l)
Exchange: Single Replacement
AB + C → AC + BZnCl2(aq)+ Mg(s) → MgCl2(aq)+ Zn(s)
Exchange: Double ReplacementAB + CD → AD + CBBaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
Combination (Synthesis)A + B → AB
CO2(g) + H2O(l) → H2CO3(aq)
N2(g) + 2O2(g)→ 2NO2(g)
DecompositionAB → A + BCaCO3(s) → CaO(s) + CO2(g)
The classification scheme is only for convenience; the same reaction can be classified in different ways, depending on which of its characteristics is most important. Oxidation–reduction reactions, in which there is a net transfer of electrons from one atom to another, and condensation reactions are discussed in this section. Acid–base reactions are one kind of exchange reaction—the formation of an insoluble salt, such as barium sulfate, when solutions of two soluble salts are mixed together.
Combination Reactions
A combination reaction is a reaction in which two or more substances combine to form a single new substance. Combination reactions can also be called synthesis reactions. The general form of a combination reaction is:
A+B→AB(7.10.1)
One combination reaction is two elements combining to form a compound. Solid sodium metal reacts with chlorine gas to produce solid sodium chloride.
2Na(s)+Cl2(g)→2NaCl(s)
Notice that in order to write and balance the equation correctly, it is important to remember the seven elements that exist in nature as diatomic molecules ( H2 , N2 , O2 , F2 , Cl2 , Br2 , and I2 ).
One type of combination reaction that occurs frequently is the reaction of an element with oxygen to form an oxide. Metals and nonmetals both react readily with oxygen under most conditions. Magnesium reacts rapidly and dramatically when ignited, combining with oxygen from the air to produce a fine powder of magnesium oxide:
2Mg(s)+O2(g)→2MgO(s)
Sulfur reacts with oxygen to form sulfur dioxide:
S(s)+O2(g)→SO2(g)
When nonmetals react with one another, the product is a molecular compound. Often, the nonmetal reactants can combine in different ratios and produce different products. Sulfur can also combine with oxygen to form sulfur trioxide:
2S(s)+3O2(g)→2SO3(g)
Transition metals are capable of adopting multiple positive charges within their ionic compounds. Therefore, most transition metals are capable of forming different products in a combination reaction. Iron reacts with oxygen to form both iron (II) oxide and iron (III) oxide:
2Fe(s)+O2(g)→2FeO(s)
4Fe(s)+3O2(g)→2Fe2O3(s)
Example 7.10.1 : Combustion of Solid Potassium
Potassium is a very reactive alkali metal that must be stored under oil in order to prevent it from reacting with air. Write the balanced chemical equation for the combination reaction of potassium with oxygen.
Solution
StepsExample Solution
Plan the problem.
Make sure formulas of all reactants and products are correct before balancing the equation. Oxygen gas is a diatomic molecule. Potassium oxide is an ionic compound and so its formula is constructed by the crisscross method. Potassium as an ion becomes K+ , while the oxide ion is O2− .
Solve.
The skeleton (unbalanced) equation:
K(s)+O2(g)→K2O(s)
The equation is then easily balanced with coefficients.
4K(s)+O2(g)→2K2O(s)
Think about your result.Formulas are correct and the resulting combination reaction is balanced.
Combination reactions can also take place when an element reacts with a compound to form a new compound composed of a larger number of atoms. Carbon monoxide reacts with oxygen to form carbon dioxide according to the equation:
2CO(g)+O2(g)→2CO2(g)(7.10.2)
Two compounds may also react to form a more complex compound. A very common example is the reactions of oxides with water. Calcium oxide reacts readily with water to produce an aqueous solution of calcium hydroxide:
