Chemical Equations

Chemical Equations

• Lavoisier observed that mass is conserved in a chemical reaction.
 This observation is known as the law of conservation of mass.
• The quantitative nature of chemical formulas and reactions is called stoichiometry.
• Chemical equations give a description of a chemical reaction.
• There are two parts to any equation:
 Reactants (written to the left of the arrow) and
 Products (written to the right of the arrow):

2 H2 + O2 --> 2 H2O
• There are two sets of numbers in a chemical equation:
 Numbers in front of the chemical formulas (called stoichiometric coefficients) and
 Numbers in the formulas (they appear as subscripts).
• Stoichiometric coefficients give the ratio in which the reactants and products exist.
• The subscripts give the ratio in which the atoms are found in the molecule.
 Example:
• H2O means there are two H atoms for each one molecule of water.
• 2 H2O means that there are two water molecules present.
• Note: In 2 H2O there are four hydrogen atoms present (two for each water molecule).
• Matter cannot be lost in chemical reactions.
 Therefore, the products of a chemical reaction have to account for all the atoms present in the reactants.
• Consider the reaction of methane with oxygen.

CH4 + O2 --> CO2 + H2O

• Counting atoms in the reactants:
 1 C;
 4 H; and
 2 O
• In the products:
 1 C;
 2H; and
 3O
• It appears as though H has been lost and C has been created.
• To balance the equation, we adjust the stoichiometric coefficients:

CH4 + 2 O2 --> CO2 + 2 H2O