Zumdahl chemistry Chapter 3: Chemical equation

Chemical Reaction: chemical change involves reorganization of the atoms in one or more substances.
For example: CH4 burn with O2 to form CO2 and H2O
The chemical reaction represented by chemical equation.

Reactants: the starting materials on the left side.
For example: CH₄ and O₂
Products: on the right side.
For example: CO₂ and H₂O

CH₄ + O₂ → CO₂ + H₂O 

The equation (shown above) for the reaction between CH₄ and O₂ is not balanced, we can see this from the following representation of the reaction: 

and to balance the equation there must be the same number of each type of atom on the products side and on the reactants side of the arrow. In the previous notice that the number of oxygen atoms (in O₂) on the left of the arrow is two, while on the right there are three O atoms (in CO₂ and H₂O). Also, there are four hydrogen atoms (in CH₄) on the left and only two (in H₂O) on the right. To balance the equation We can represent the preceding situation in a shorthand manner by the following chemical equation:

The meaning of a chemical equation: The chemical equation for a reaction gives two important types of information: the nature of the reactants and products and the relative numbers of each. Also the equation often gives the physical states of the reactants and products:

For example, the balanced equation
CH₄(g) + 2O₄(g) → CO₂(g) + 2H₂O(g) 

We can get a great deal of information