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Balancing Chemical Equations
Chemical equations can be viewed as recipes for chemical reactions. They give a description of what
chemicals are combined together and what chemicals are made when a reaction occurs. The law of
conservation of mass says that matter can neither be created nor destroyed, and this requires that all
chemical reactions be balanced.
Consider the following balanced equation:
Cu (s) + 4 HNO3 (aq)  Cu(NO3)2 (aq) + 2 NO2 (g) + 2 H2O (l)
Reactants are the chemicals that are initially combined together. They are written on the left side of the
reaction arrow. In the case above, Cu and HNO3 are reactants.
Products are the chemicals produced by a reaction. They are written on the right side of the reaction
arrow. In the case above, Cu(NO3)2, NO2, and H2O are products.
Note that state symbols (s for solid, l for liquid, g for gas, and aq for aqueous which means a chemical
dissolved in water) indicate the state of matter.
The equation above is balanced because the number of atoms on each side is equal. For instance, there is 1
Cu atom on each side and 4 N atoms on each side. For practice, list the number of each type of atom on
each side of this equation.
Chemical equations can be written in words too. The equation below is called a word equation
Copper + nitric acid  copper (II) nitrate + nitrogen dioxide + water
Word equations can be quickly turned into a skeleton equation, which is an unbalanced equation without
coefficients like the one below:
Cu (s) + HNO3 (aq)  Cu(NO3)2 (aq) + NO2 (g) + H2O (l)
Notice that the way to turn a skeleton equation into a balanced equation is to insert coefficients in front of
the chemical formulas:
_____ Cu (s) + _____ HNO3 (aq)  _____ Cu(NO3)2 (aq) + _____ NO2 (g) + _____ H2O (l)
Balancing equations is usually a trial and error process where one tries different numbers for coefficients
until a given combination balances the equation.
Please note the following rule when balancing chemical equations:
NEVER CHANGE THE SUBSCRIPTS!!!
Don’t forget the seven diatomic elements: H2 N2 O2 F2 Cl2 Br2 and I2. These elements occur as diatomic
molecules when they are uncombined with other elements.
Try to write the following word equations first as skeleton equations and then as balanced equations.
Calcium metal reacts with liquid water to produce aqueous calcium hydroxide and hydrogen gas
Upon heating, solid ammonium nitrite produces nitrogen gas and water vapor