The six types of chemical reactions are synthesis, decomposition, single-replacement, double-replacement, acid-base, and combustion. Chemical reactions can be generalized by chemical groups. These groups are labeled A, B, C, and D. Synthesis and decomposition reactions occur when chemical groups combine or separate. Single and double-replacement reactions are “shuffles” between either three (single replacement) or four (double replacement) distinct chemical groups. Acid-base and combustion are identified by distinct reactants and products.
Synthesis reaction: Note if the reaction has only one (complicated) product. If there is a single production with notation “AB” (or ABC, etc...), then you can be sure this is a synthesis reaction. Synthesis reactions are a union of two (or more) reactants (A and B) into one new product (AB). The reaction has form A + B -> AB. Even though entropy decreases—going from two free chemical groups to one—the energy release is enough driving force for many synthesis processes.
Decomposition reaction: Look for a “breakup” to identify decomposition reactions. Decompositions are synthesis-in-reverse. A complicated molecule of form “AB” separates into its constituents. If you see one “complex” molecule breaking up into several simpler ones in form AB -> A + B, you’ve found a decomposition reaction.
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Single-replacement: Remember that single-replacement reactions switch the identity of the simpler, un-bonded group. The general formula for single replacement reactions is: A + BC -> AB + C (or AC + B). Before the reaction, the “A” is by itself, while chemical groups B and C are combined. Single-replacement processes shuffle this order, so that group A bonds to either B or C.
Double-replacement: Keep in mind that double-replacement reactions have products as complicated—in terms of bonded chemical groups—as starting reactants. The process is: AB + CD -> AC + BD. Each chemical group (A, B, C and D) essentially switches partners.
Acid-base reaction: Observe that acid-base processes are a special case of double replacement. They can be identified by crystalline salt and “H2O” presence among the products. For example, sodium hydroxide (NaOH, a base) and hydrochloric acid (HCl) give sodium chloride—common salt—and water through the reaction NaOH + HCl -> NaCl + HOH (H2O). Here the chemical group formula is: A = Na, B = OH, C = Cl, D = H.
Combustion reaction: Identify combustion through unique reactant/product features. First, it has molecular oxygen (O2) as a reactant, but never as product. The other reactant is a hydrocarbon such as “C6H6” or “C8H10”. Water (H2O) and carbon dioxide (CO2) are products of a combustion reaction.
Combustion processes are rarely perfect. Realistically, you would see secondary combustion reactions. Secondary reactions often give out products such as carbon monoxide (CO). Carbon monoxide produced by this process indicates incomplete combustion. Though less prominent than the primary CO2-generating reaction, incomplete combustion matters. Running a car engine in a closed garage can be fatal—the small percentage of gas burned “incompletely” into CO adds up to toxic levels.