Chemical reactions are the processes that transform one set of chemical substances into another by rearranging atoms and forming new bonds. To organize and predict the outcomes of countless possible chemical changes, chemists classify them into distinct types. This classification system allows for a deeper understanding of how atoms interact, bond, and exchange partners to create the substances that make up the physical world.
Understanding the Fundamentals of Chemical Reactions
Every chemical reaction involves starting materials, known as reactants, which are consumed to create the resulting substances, called products. A reaction’s equation represents this process, showing the reactants on the left side and the products on the right, separated by an arrow. A fundamental law governing these transformations is the conservation of mass, which dictates that atoms are neither created nor destroyed during the reaction.
The Four Core Rearrangement Types
The first four classifications are defined by the specific pattern in which the atoms or ions in the reactants rearrange themselves.
Synthesis reactions, also known as combination reactions, follow the pattern of two or more reactants combining to form a single, more complex product, represented generally as \(\text{A} + \text{B} \rightarrow \text{AB}\). A common instance is the reaction of iron metal with oxygen gas, which produces iron oxide, commonly known as rust. These reactions release energy as new chemical bonds are formed, making them exothermic.
Decomposition reactions are the opposite, involving a single complex reactant breaking down into two or more simpler products, represented by the formula \(\text{AB} \rightarrow \text{A} + \text{B}\). Since energy is required to break the bonds holding the complex molecule together, these reactions require an input of heat, electricity, or light to occur. For example, passing an electric current through water breaks it down into its component elements, hydrogen gas and oxygen gas.
A single displacement reaction occurs when one element takes the place of another element within a compound, following the general form \(\text{A} + \text{BC} \rightarrow \text{AC} + \text{B}\). This reaction only proceeds if the single, uncombined element is chemically more reactive than the element it is attempting to replace. For instance, if a strip of zinc metal is placed in a solution of copper sulfate, the zinc will replace the copper, which then precipitates out as a solid.
Double displacement reactions, or metathesis reactions, involve an exchange of ions between two different compounds, generally represented as \(\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}\). These reactions occur in an aqueous solution, meaning the compounds are dissolved in water. The driving force for this exchange is the formation of a precipitate (an insoluble solid), or the creation of a gas or water.
Specialized Reaction Types
The remaining two classifications are grouped separately because they are defined less by the number of reactants and products and more by the specific types of energy or ion behavior involved.
Combustion is a rapid chemical reaction between a substance, a hydrocarbon fuel, and an oxidizing agent, oxygen gas, which produces heat and light. When a hydrocarbon burns completely, the products are always carbon dioxide and water vapor, following a general pattern. The reaction requires a specific activation energy to begin, which is why a spark is needed to ignite the fuel in a car engine.
Acid-base reactions, also known as neutralization reactions, are defined by the transfer of ions between an acid and a base. An acid releases hydrogen ions (\(\text{H}^+\)), and a base releases hydroxide ions (\(\text{OH}^-\)), which then combine to form water. The remaining ions from the acid and the base join to form an ionic compound, called a salt.
Everyday Examples of Chemical Reactions
These six types of reactions are constantly occurring around us, powering technology and driving biological processes.
- Combustion of gasoline provides the energy to move vehicles, releasing carbon dioxide and water vapor.
- Synthesis reactions, fueled by sunlight, are the basis of photosynthesis, creating glucose and releasing oxygen.
- Decomposition reactions are often at work when food spoils, as complex organic molecules break down into simpler compounds.
- Single displacement reactions are responsible for the corrosion of certain metals, such as the degradation of iron exposed to moisture and air.
- Acid-base reactions occur when taking an antacid tablet, where the basic compounds neutralize excess stomach acid.
- Double displacement reactions are used in water treatment plants to precipitate out heavy metal contaminants.