Classification of Chemical Reactions

There are many kinds of chemical reactions and also many ways of classifying them.   We will look at a few the ways to classify chemical reactions. Five traditional types of chemical reactions are

1. Decomposition reactions
2. Combination reactions (Synthesis reactions)
3. Single-replacement reactions (Displacement reactions)
4. Double-replacement reactions (Metathesis reactions)
5. Combustion reactions
6. Oxidation-reduction reactions (Redox reactions)

Decomposition Reactions

A decomposition reaction is a reaction in which a single compound decomposes to two or more other substances.  A general equation that describes a decomposition reaction is AB --> A + B  where A and B can be elements or compounds.  Most compounds can be broken down into simpler substances or decomposed.   Often this can be done by heating the compound.  For example the industrial preparation of lime (calcium oxide) involves the decomposition of calcium carbonate by heating it.   

  CaCO₃(s) ---> CaO(s) + CO₂(g)

Combination Reactions

A combination reaction is a reaction in which two substances combine to form a third.  A general equation that describes a combination reaction is   A + B  --> AB.  Again A and B can be either elements or compounds.   Decomposition and combination reactions can be considered to be the reverse of each other.  Under some conditions it is possible to change conditions and cause a decomposition reaction to become a combination reaction or vice versa.  The reaction of calcium oxide with sulfur dioxide to form calcium sulfite is an example of a combination reaction. 

CaO(s) + SO₂(g) ---> CaSO₃(s)

Single-Replacement Reactions

A single-replacement reaction is a reaction in which an element reacts with a compound and replaces another element in the compound.  A general equation that describes a single-replacement reaction is A + BC --> AB + C, where A and C are elements and BC and AB are compounds.  The reaction in which copper displaces silver from an aqueous solution of silver nitrate is an example of a single-replacement reaction.

Cu(s) + 2 AgNO₃(aq) ---> Cu(NO₃)₂(aq)  + 2 Ag(s)

Double-Replacement Reactions

A double-replacement reaction (also called a metathesis reaction) is a reaction in which there is an exchange of positive ions between two compounds.  These reactions generally take place between two ionic compounds in aqueous solution.  A general equation that describes a double-replacement reaction is AB + CD --> AD + CB, where A and C are cations and B and D are anions. For a double-replacement reaction to occur, at least one of the products must be a gas or water, or a precipitate.

• Precipitation reactions are one type of double-replacement reaction.  An example is

AgNO₃(aq) + NaCl(aq) ---> AgCl(s) + NaNO₃(aq)

This is what is called a molecular equation, which is a chemical equation in which the compounds are written as if they were molecular substances,even if they exist in solution as ions.  Another type chemical equation is an ionic equation.  The ionic equation shows soluble ionic compounds as individual ions in solution.  Lets rewrite the equation above.

Ag⁺(aq) + NO₃^-(aq) + Na⁺(aq) + Cl^-(aq) ---> AgCl(s) + Na⁺(aq) + NO₃^-(aq)

Next we cancel any ions that appear on both sides of the equation.

Ag⁺(aq) + NO₃^-(aq) + Na⁺(aq) + Cl^-(aq) ---> AgCl(s) + Na⁺(aq) + NO₃^-(aq)

Remove the cancelled ions (called spectator ions) from the equation and we have

Ag⁺(aq) + Cl^-(aq) ---> AgCl(s)

This is called a net ionic equation.

• A neutralization reaction is a reaction that occurs between an acid and a base with the formation of an ionic compound and water, if the reaction is in an aqueous solution.  This is another type of double-replacement reaction.  An example is

HCl(aq) + NaOH(aq) ---> H₂O(l) + NaCl(aq)

The net ionic equation would be

H⁺(aq) + OH^-(aq) ---> H₂O(l)

Combustion Reactions

A combustion reaction is a reaction in which a substance reacts with oxygen, usually with the rapid release of heat and the production of a flame.   Organic compounds usually burn in the oxygen in air to produce carbon dioxide and if the compound contains hydrogen, another product will be water.  For example butane burns in air as follows.

2 C₄H₁₀(g) + 13 O₂(g) ---> 8 CO₂(g) + 10 H₂O(l)

Summary of the Types of Chemical Reactions

Many chemical reactions can be classified into one of five general types. In a combination reaction, two or more elements or compounds combine chemically to form a new compound.  By contrast, a single reactant is the identifying characteristic of a decomposition reaction.  Usually in a decomposition reaction, a compound is decomposed into two or more elements or compounds. 

When oxygen reacts with a compound composed of the elements carbon and hydrogen, a combustion reaction takes place.  If all the carbon in the product is present as carbon dioxide, the reaction is complete combustion.  If carbon monoxide is formed, the reaction is incomplete combustion.

The other two types of chemical reactions are single-replacement and double-replacement reactions.  In a single-replacement reaction an element replaces another element in a compound.  A new compound and a new element are formed.  The element that is being displaced must be less active than the element replacing it. This can be determined from the activity series of metals.  In a double-replacement reaction two ionic compounds react by exchanging cations to form two new compounds.  A double-replacement reaction usually takes place in aqueous solution.  Double-replacement reactions are driven by the formation of a precipitate, a gaseous product, or water.