Chemical Reactions

Chemical reactions occur when chemical bonds between atoms are formed or broken. The substances that go into a chemical reaction are called the reactants, and the substances produced at the end of the reaction are known as the products. An arrow is drawn between the reactants and products to indicate the direction of the chemical reaction, though a chemical reaction is not always a "one-way street".

Reversibility and equilibrium

Some chemical reactions simply run in one direction until the reactants are used up. These reactions are said to be irreversible. Other reactions, however, are classified as reversible.

Reversible reactions can go in both the forward and backward directions.

In a reversible reaction, reactants turn into products, but products also turn back into reactants. In fact, both the forward reaction and its opposite will take place at the same time. This back and forth continues until a certain relative balance between reactants and products is reached—a state called equilibrium. At equilibrium, the forward and backward reactions are still happening, but the relative concentrations of products and reactants no longer change.

Each reaction has its own characteristic equilibrium point, which we can describe with a number called the equilibrium constant. To learn where the equilibrium constant comes from and how to calculate it for a specific reaction, check out the equilibrium topic.


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

·        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,

and the burning of coal (carbon) to give carbon dioxide,

are examples of combination reactions.


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,

and the decomposition of hydrogen peroxide to form oxygen gas and water,

are examples of decomposition reactions.


In single displacement reactions, a more active element displaces (kicks out) another less active element from a compound. For example, if you put a piece of zinc metal into a copper(II) sulfate solution, the zinc displaces the copper, as shown in this equation:

The notation (aq) indicates that the compound is dissolved in water — in an aqueous solution. Because zinc replaces copper in this case, it’s said to be more active. If you place a piece of copper in a zinc sulfate solution, nothing will happen.


In single displacement reactions, only one chemical species is displaced. In double displacement reactions, or metathesis reactions, two species (normally ions) are displaced. Most of the time, reactions of this type occur in a solution, and either an insoluble solid (precipitation reactions) or water (neutralization reactions) will be formed.


If you mix a solution of potassium chloride and a solution of silver nitrate, a white insoluble solid is formed in the resulting solution. The formation of an insoluble solid in a solution is called precipitation.

Here is the molecular equation for this double-displacement reaction:

The white insoluble solid that’s formed is silver chloride.


The other type of double-displacement reaction is the reaction between an acid and a base. This double-displacement reaction, called a neutralization reaction, forms water. Take a look at the mixing solutions of sulfuric acid (auto battery acid) and sodium hydroxide (lye).

Here is the molecular equation for this reaction:


Combustion reactions occur when a compound, usually one containing carbon, combines with the oxygen gas in the air. This process is commonly called burning. Heat is the most-useful product of most combustion reactions.

Here’s the equation that represents the burning of propane:

Propane belongs to a class of compounds called hydrocarbons, compounds composed only of carbon and hydrogen. The product of this reaction is heat.

Combustion reactions are also a type of redox reaction.


Redox reactions, or reduction-oxidation reactions, are reactions in which electrons are exchanged:

The preceding reactions are examples of other types of reactions (such as combination, combustion, and single-replacement reactions), but they’re all redox reactions. They all involve the transfer of electrons from one chemical species to another. Redox reactions are involved in combustion, rusting, photosynthesis, respiration, batteries, and more.


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