Recognizing Reaction Patterns
Chemical equations can be grouped by how reactant particles are reorganized into products. Pattern recognition helps describe a reaction and predict the general form of its products.
1. Combination reactions
Two or more reactants form one product. The product contains the elements or groups that were present in the reactants. Example: (2Mg+O_2\rightarrow2MgO). There are two reactant substances and one product substance.
2. Decomposition reactions
One reactant forms two or more products. Energy such as heating, light, or electricity is often required, although the equation pattern defines the type. Example: (CaCO_3\rightarrow CaO+CO_2). One compound produces two compounds.
3. Single-displacement reactions
An element and a compound react to form a new compound and a different free element. One element replaces another in the compound. Example: (Zn+2HCl\rightarrow ZnCl_2+H_2). Whether a particular displacement occurs depends on relative reactivity; recognizing the pattern does not by itself prove that any chosen substances will react.
4. Double-displacement reactions
Two compounds exchange ions or groups to form two new compounds. In aqueous mixtures, such reactions commonly proceed when a precipitate, gas, or weakly ionized product such as water forms. Example: (AgNO_3+NaCl\rightarrow AgCl+NaNO_3), where solid AgCl is a precipitate.