Balanced Equation Definition and Examples

Chemistry Glossary Definition of Balanced Equation

A balanced chemical equation describes the reactants, products, and quantities of chemicals in a reaction.
A balanced chemical equation describes the reactants, products, and quantities of chemicals in a reaction. Jeffrey Coolidge, Getty Images

Balanced Equation Definition

A balanced equation is an equation for a chemical reaction in which the number of atoms for each element in the reaction and the total charge are the same for both the reactants and the products. In other words, the mass and the charge are balanced on both sides of the reaction.

Also Known As: Balancing the equation, balancing the reaction, conservation of charge and mass.

Examples of Unbalanced and Balanced Equations

An unbalanced chemical equation lists the reactants and products in a chemical reaction, but doesn't state the amounts required to satisfy conservation of mass. For example, this equation for the reaction between iron oxide and carbon to form iron and carbon dioxide is unbalanced with respect to mass:

Fe2O3 + C → Fe + CO2

The equation is balanced for charge, because both sides of the equation have no ions (net neutral charge).

The equation has 2 iron atoms on the reactants side of the equation (left of the arrow), but 1 iron atom on the products side (right of the arrow). Even without counting up the quantities of other atoms, you can tell the equation is not balanced. The goal of balancing the equation is to have the same number of each type of atom on both the left and right sides of the arrow.

This is achieved by changing the coefficients of the compounds (numbers placed in front of compound formulas).

The subscripts are never changed (small numbers to the right of some atoms, as for iron and oxygen in this example). Changing the subscripts would alter the chemical identity of the compound!

The balanced equation is:

2 Fe2O3 + 3 C → 4 Fe + 3 CO2

Both the left and right sides of the equation have 4 Fe, 6 O, and 3 C atoms.

When you balance equations, it's a good idea to check your work by multiplying the subscript of each atom by the coefficient. When no subscript is cited, consider it to be 1.

It's also good practice to cite the state of matter of each reactant. This is listed in parentheses immediately following the compound. For example, the earlier reaction could be written:

2 Fe2O3(s) + 3 C(s) → 4 Fe(s) + 3 CO2(g)

where s indicates a solid and g is a gas

Balanced Ionic Equation Example

In aqueous solutions, it's common to balance chemical equations for both mass and charge. Balancing for mass produces the same numbers and kinds of atoms on both sides of the equation. Balancing for charge means the net charge is zero on both sides of the equation. The state of matter (aq) stands for aqueous, meaning only the ions are shown in the equation and that they are in water. For example:

Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) → AgCl(s) + Na+(aq) + NO3-(aq)

Check that an ionic equation is balanced for charge by seeing if all of the positive and negative charges cancel each other out on each side of the equation. For example, on the left side of the equation, there are 2 positive charges and 2 negative charges, which means the net charge on the left side is neutral.

On the right side, there is a neutral compound, one positive, and one negative charge, again yielding a net charge of 0.