An exothermic example

Several general types of chemical reactions can occur based on what happens when going from reactants to products. The more common reactions are 

Combination Decomposition Single displacement Double displacement Combustion Redox 

In combination reactions, two or more reactants form one product. The reaction of sodium and chlorine to form sodium chloride, 

Note that, depending on conditions or the relative amounts of the reactants, more than one product can be formed in a combination reaction. Take the burning of coal, for example. If an excess of oxygen is present, the product is carbon dioxide. But if a limited amount of oxygen is available, the product is carbon monoxide  

Decomposition reactions are really the opposite of combination reactions. In decomposition reactions, a single compound breaks down into two or more simpler substances (elements and/or compounds). The decomposition of water into hydrogen and oxygen gases, 

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