Chemical Reaction Definition and Examples

A chemical reaction is a chemical change which forms new substances. A chemical reaction may be represented by a chemical equation, which indicates the number and type of each atom, as well as their organization into molecules or ions. A chemical equation uses the element symbols as shorthand notation for the elements, with arrows to indicate the direction of the reaction. A conventional reaction is written with reactants on the left side of the equation and products on the right side. The state of matter of the substances may be indicated in parenthesis (s for solid, l for liquid, g for gas, aq for aqueous solution). The reaction arrow may go from left to right or there may be a double arrow, indicating reactants turn to products and some product undergoes the reverse reaction to reform reactants.

While chemical reactions involves atoms, typically only the electrons are involved in the breaking and formation of chemical bonds. Processes involving the atomic nucleus are called nuclear reactions.

The substances that participate in a chemical reaction are called reactants. The substances that are formed are called products. The products have different properties from the reactants.

Also Known As: reaction, chemical change

Chemical Reaction Examples

The chemical reaction H₂(g) + ½ O₂(g) → H₂O(l) describes the formation of water from its elements.

The reaction between iron and sulfur to form iron(II) sulfide is another chemical reaction, represented by the chemical equation:

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Reaction Definition in Chemistry

By Anne Marie Helmenstine, Ph.D.

8 Fe + S₈ → 8 FeS

Types of Chemical Reactions

There are countless reactions, but they can be grouped into four basic categories:

Synthesis Reaction

In a synthesis or combination reaction, two or more reactants combine to form a more complex product. The general form of the reaction is: A + B → AB

Decomposition Reaction

A decomposition reaction is the reverse of a synthesis reaction. In a decomposition, a complex reactant breaks into simpler products. The general form of a decomposition reaction is: AB → A + B

Single Replacement Reaction

In a single replacement or single displacement reaction, one uncombined element replaces another in a compound or trades places with it. The general form of a single replacement reaction is: A + BC → AC + B

Double Replacement Reaction

In a double replacement or double displacement reaction, the anions and cations of the reactants trade places with each other two form new compounds. The general form of a double replacement reaction is: AB + CD → AD + CB

Because there are so many reactions, there are additional ways to categorize them, but these other classes will still fall into one of the four main groups. Examples of other classes of reactions include oxidation-reduction (redox) reactions, acid-base reactions, complexation reactions, and precipitation reactions.

Factors That Affect Reaction Rate

The rate or speed at which a chemical reaction occurs is affected by several factors, including:

• reactant concentration
• surface area
• temperature
• pressure
• presence or absence of catalysts
• presence of light, especially ultraviolet light
• activation energy