Chemical Reactions

What Are the Reactants and Products of a Chemical Reaction?

Categorization of Chemical Reactions

Chemical reactions involve a process that changes one or more substances, or reactants, into one or more new substances, or products. An example of a chemical reactionglossary term (opens in a new window) is the combination of one oxygen atom with two hydrogen atoms to form a water molecule.

There are six distinct types of chemical reactions:

• Synthesis reactionglossary term (opens in a new window): Two or more reactants combine, forming a single productglossary term (opens in a new window) that is more complex than either of the reactants. The general formula is A + B→AB$\text{A + B}\to \text{AB}$.
• Decomposition reactionglossary term (opens in a new window): A single compound breaks down into two or more simpler substances. This process is the opposite of a synthesis reaction. The general formula is AB→A + B$\text{AB}\to \text{A + B}$.
• Combustion reactionglossary term (opens in a new window): Heat is produced when oxygen and one or more other compounds combine. This process often forms water and carbon dioxide.
• Single displacement reactionglossary term (opens in a new window): One element replaces another element in a compound. The general formula is A + BX→AX + B$\text{A + BX}\to \text{AX + B}$, also called single replacement.
• Double displacement reactionglossary term (opens in a new window): Two compounds interchange elements to form two new compounds. The general formula is AX + BY→AY +BX$\text{AX + BY}\to \text{AY +BX}$, also called double replacement.
• Acidglossary term (opens in a new window)–baseglossary term (opens in a new window) reaction: An acid and a baseglossary term (opens in a new window) combine in a special kind of double displacement reaction. During an acid–base reaction, an acid typically donates a proton to a base, forming water. The remaining ions form salt.

Types of Chemical Reactions

Synthesis, decomposition, single displacement, double displacement, acid–base, and combustion are all different types of chemical reactions. How are each of these reactions distinguished according to their reactants and products?

Conservation of Matter

The law of conservation of matter governs chemical reactions. This law is sometimes referred to as the law of conservation of mass. The law states that matter, including all of the atoms that make up matter, cannot be gained or lost through chemical reactions. Thus, the atoms that combine and interact when chemical reactions occur are not created or destroyed.

Reactants may change into products to form new substances. However, the atoms that are the basis of these reactions become rearranged. They do not disappear. The law of conservation of matter is the reason why great care must be taken to ensure the products and reactants of a reaction are carefully balanced. All elements and numbers of atoms must be the same on both the left and right hand sides of a chemical equation. For example, take the overall reaction for the combustion of glucose:

Notice how the equation is balanced to ensure equal numbers of carbon, hydrogen, and oxygen atoms in total on either side of the arrow.

The Law of Conservation of Matter?

The law of conservation of matter states that matter cannot be created or destroyed in chemical reactions. How does this law apply to the rules that govern the writing of an equation for a chemical reaction?

Chemical Equations

Chemical equations are a type of mathematical expression that can be used to describe what occurs in a chemical reaction.

In chemical equations, the symbols that represent elements and molecules in the reactantglossary term (opens in a new window)(s) are shown on the left side of the equation. Next, an arrow points to the right side of the equation, where the final product or products are shown.

Chemical equations must be balanced, meaning that an equal number of atoms must appear on each side of the equation. This corresponds to the law of conservation of matter that governs chemical reactions.
The chemical equation for the chemical reaction that occurs when carbon and oxygen react to create carbon dioxide is C + O2→CO2${\text{C + O}}_2\to {\text{CO}}_2$. It is important to note that this equation is balanced; each side of the equation has one carbon atom and two oxygen atoms.

Chemical Equations: Sample Problem

Combining hydrogen and oxygen creates water. How is this chemical reaction written as a chemical equation?

Solution:

First, find the correct element symbols for the reactants, hydrogen and oxygen. Write these symbols on the left side of the equation. Be sure to include the correct number of atoms for any diatomic elements, which contain two atoms. In nature, hydrogen and oxygen are both found as diatomic elements:

Next, draw a right-pointing arrow to show that the two reactants will react:

Last, determine which symbols will need to be written on the right side of the equation. In order to conserve matter, we know that the right side of the equation must have the same number of atoms as the left side of the equation. We also know that the chemical formula for water is H2O${\text{H}}_2\text{O}$.