Learning Outcomes
Classify a reaction as combination, decomposition, single-replacement, double-replacement, or combustion.
Predict the products and balance a combustion reaction.
Many chemical reactions can be classified as one of five basic types. Having a thorough understanding of these types of reactions will be useful for predicting the products of an unknown reaction. The five basic types of chemical reactions are combination, decomposition, single-replacement, double-replacement, and combustion. Analyzing the reactants and products of a given reaction will allow you to place it into one of these categories. Some reactions will fit into more than one category.
Combination Reactions
A combination reaction, also known as a synthesis 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 - (5.3.1)
One combination reaction is two elements combining to form a compound. Solid sodium metal reacts with chlorine gas to product solid sodium chloride.
2Na(s)+Cl2(g)→2NaCl(s)(5.3.2)
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 sort 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)(5.3.3)
Decomposition Reactions
A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances. The general form of a decomposition reaction is:
AB→A+B(5.3.4)
Most decomposition reactions require an input of energy in the form of heat, light, or electricity.
Binary compounds are compounds composed of just two elements. The simplest kind of decomposition reaction is when a binary compound decomposes into its elements. Mercury (II) oxide, a red solid, decomposes when heated to produce mercury and oxygen gas.
2HgO(s)→2Hg(l)+O2(g)(5.3.5)
A reaction is also considered to be a decomposition reaction even when one or more of the products is still a compound. A metal carbonate decomposes into a metal oxide and carbon dioxide gas. For example, calcium carbonate decomposes into calcium oxide and carbon dioxide.
CaCO3(s)→CaO(s)+CO2(g)(5.3.6)
Metal hydroxides decompose on heating to yield metal oxides and water. Sodium hydroxide decomposes to produce sodium oxide and water.
Explanation:
