Chemical reactions involve a rearrangement of the ways atoms are grouped together. A chemical equation represents a chemical reaction. Reactants are shown to the left of an arrow. Products are shown to the right of the arrow.

In a chemical reaction atoms are not created or destroyed. All atoms present in the reactants must be accounted for in the products. Same number of each type atom on both sides of arrow Coefficients = number of atoms, molecules, or moles. You cannot change the formula of any reactant or product when you balance an equation. You can only place coefficients in front of the formulas.

Balancing a Chemical Equation Unbalanced equation Balancing the equation The balanced equation CH4 + 2O2  CO2 + 2H2O

Physical states of compounds are often given in a chemical equation.

A chemical reaction is balanced by using a systematic approach: Write the formulas of the reactants and products to give the unbalanced chemical equation. Balance by trial and error starting with the most complicated molecule(s). At the end check to be sure the equation is balanced (same numbers of all types of atoms on the reactant and product sides).

Example: Hydrogen gas and oxygen gas combine to form liquid water. Write the formulas of the reactants and products to give the unbalanced chemical equation. H2(g) + O2(g)  H2O(l)

Example:  Hydrogen gas and oxygen gas combine to form liquid water. Balance by trial and error starting with the most complicated molecule(s).

Example: Hydrogen gas and oxygen gas combine to form liquid water. At the end check to be sure the equation is balanced (same numbers of all types of atoms on the reactant and product sides). 2H2(g) + O2(g)  2H2O(l)