Elements and compounds react with each other in numerous ways. Memorizing every type of reaction would be challenging and also unnecessary since nearly every inorganic chemical reaction falls into one or more of four broad categories.
Two or more reactants form one product in a combination reaction. An example of a combination reaction is the formation of sulfur dioxide when sulfur is burned in air:
- S (s) + O2 (g) → SO2 (g)
In a decomposition reaction, a compound breaks down into two or more substances. Decomposition usually results from electrolysis or heating. An example of a decomposition reaction is the breakdown of mercury (II) oxide into its component elements.
- 2HgO (s) + heat → 2Hg (l) + O2 (g)
Single Displacement Reactions
A single displacement reaction is characterized by an atom or ion of a single compound replacing an atom of another element. An example of a single displacement reaction is the displacement of copper ions in a copper sulfate solution by zinc metal, forming zinc sulfate:
- Zn (s) + CuSO4 (aq) → Cu (s) + ZnSO4 (aq)
- Single displacement reactions are often subdivided into more specific categories (e.g., redox reactions).
Double Displacement Reactions
Double displacement reactions also may be called metathesis reactions. In this type of reaction, elements from two compounds displace each other to form new compounds. Double displacement reactions may occur when one product is removed from the solution as a gas or precipitate or when two species combine to form a weak electrolyte that remains undissociated in solution. An example of a double displacement reaction occurs when solutions of calcium chloride and silver nitrate are reacted to form insoluble silver chloride in a solution of calcium nitrate.
- CaCl2 (aq) + 2 AgNO3 (aq) → Ca(NO3)2 (aq) + 2 AgCl (s)
- A neutralization reaction is a specific type of double displacement reaction that occurs when an acid reacts with a base, producing a solution of salt and water. An example of a neutralization reaction is the reaction of hydrochloric acid and sodium hydroxide to form sodium chloride and water:
- HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
Remember that reactions can belong to more than one category. Also, it would be possible to present more specific categories, such as combustion reactions or precipitation reactions. Learning the main categories will help you balance equations and predict the types of compounds formed from a chemical reaction.