AP Chemistry - Balancing Chemical Equations
There are a great number of chemical reactions.  In order to be a chemical reaction at least one new substance must be produced.  Recall that there are definite signs that a new substance gets produced
a)  a change in colour
b)  the formation of a gas
c)  the formation of a precipitate
d)  the release or absorption of energy (heat).

You may get only one of these, or a combination of them.

Chemical reactions are typically written one of three ways.  There are word equations, skeleton equations and balanced equations.

Word Equations
Word equations are used to describe the reaction in sentence form or in a literally form.  As an example we will use the results from a lab that you have already done.  Magnesium burns in air to produce a white powder.  The white powder has been experimentally determined to be a compound of magnesium and oxygen.  The product is magnesium oxide.  The word equation would be:

                 magnesium   +   oxygen   ------->   magnesium oxide

The "+" means "reacts with", and the arrow means "to produce".  The word equation can therefore be read as follows:  "Magnesium reacts with oxygen to produce magnesium oxide."   Magnesium and oxygen are reactants and the magnesium oxide is the lone product.

A word equation gives limited information.  It identifies only the reactant and products. It does not give their formulae, nor does it tell you the masses of reactants needed or the mass of product produced.

However at this point in time, you should be able to make up chemical formula based upon the name.

Elements in Skeleton Equations
There are 109 elements.  A few need to be treated in a special way because of how they bond with each other.  You never find elemental oxygen by itself.  Elemental oxygen is always O2.   Oxygen is one of the diatomic elements. 

The other diatomic elements are: H2, F2, Cl2, Br2, I2, O2, and N2.

Please note that these are all gases.   When substituing these into skeleton equations make sure that you use the correct formulas.   All other elements can be treated as if they were monoatomic. (Act like lone atoms.)

Types of Chemical Reactions
The vast number of chemical reactions can be classified in any number of ways. Under one scheme they can be categorized either as oxidation-reduction (electron transfer) reactions or non-oxidation-reduction reactions. Another completely different but common classification scheme recognizes four major reaction types:
(1)  combination or synthesis reactions
(2)  decomposition reactions
(3)  substitution or single replacement reactions
(4)  metathesis or double displacement reactions

The Four Major Types of  Reactions
      Name                                           General Reaction Pattern
Combination or synthesis                                 A  +  B ----> AB

Decomposition                                                      AB ---->  A  +  B

Substitution or Single Replacement            A  +  BC ---->  B  +  AC

Metathesis or Double Displacement        AB  +  CD ---->  AD  +  CB

Combination or Synthesis Reactions   Two or more reactants unite to form a single product.

                                     S   +   O2 --------->  SO2
                               sulphur    oxygen            sulphur dioxide

                                  2 S      +    3 O2 --------->  2 SO3
                                sulphur            oxygen                 sulphur trioxide

                                2 Fe   +   O --------->  2 FeO
                                iron        oxygen                    iron (II)  oxide
 
 


Decomposition Reactions   A single reactant is decomposed or broken down into two or more
                                              products.

                                       CaCO3   ---------->    CaO     +      CO2
                                calcium carbonate                calcium oxide     carbon dioxide

                                                     2 H2O ----------->  2 H2    +     O2
                                                         water                           hydrogen        oxygen

                                                 2 KClO3 ----------->  2 KCl   +     3 O2
                                           potassium chlorate          potassium chloride   oxygen


Substitution or Single Replacement Reactions  A single free element replaces or is substituted for one of the elements in a compound.  The free element is more reactive than the one its replaces.

                          Zn    +     2 HCl   ---------->  H2    +  ZnCl2
                                  zinc      hydrochloric acid             hydrogen    zinc chloride

                       Cu   +   2 AgNO3  ----------->  2 Ag    +    Cu(NO3)2
                   copper        silver nitrate                             silver          copper (II) nitrate

                               H2    +   2 AgNO3 ----------->  2 Ag   +   2 HNO3
                           hydrogen     silver nitrate                            silver           nitric acid

                                  2 Na   +  2 H2O  ----------->  2 NaOH    +     H2
                                 sodium          water                          sodium hydroxide     hydrogen


Metathesis or Double Displacement Reactions  This reaction type can be viewed as an "exchange of partners."  For ionic compounds, the positive ion in the first compound combines with the negative ion in the second compound, and the positive ion in the second compound combines with the negative ion in the first compound.

                       HCl    +     NaOH  ----------->  NaCl   +  HOH
                  hydrochloric        sodium                              sodium         water
                              acid               hydroxide                           chloride

                          BaCl2   +   2 AgNO3 ---------->  2 AgCl    +   Ba(NO3)2
                            barium                  silver                                    silver                barium
                            chloride                nitrate                                 chloride              nitrate
                                                                                                    (precipitate)

                      CaCO3   +   2 HCl   ----------->  CaCl2   +  H2CO3
                              calcium         hydrochloric                        calcium          carbonic
                            carbonate             acid                                 chloride            acid


Skeleton Equations
Skeleton equations are simply the bare bones of a chemical equation.  The chemical formulae are substituted into the word equation.  The skeleton equation for the reaction above is:

                  Mg      +       O2   --------->      MgO


The formulas are written first.  Each formula should be checked at this time to make sure that they are correct.  If they are not correct then the equation probably will not balance later.

Balanced Equations
A skeleton equation again provides limited information.  It tells you what the chemical formulas are for the reactants and products, but again it tells you nothing about how much.  If you observe the skeleton equation above carefully,  you'll see that there is 1 Mg atom on the left and 1 Mg atom on the right.  At least that part is balanced.  There is however, 2 O atoms on the left and 1 O atom on the right.  If even one atom does not match up evenly on both sides then the equation is unbalanced.

A chemical equation must be balanced.  The number of atoms of each type must be the same on both sides of the equation.  You must never change the subscripts inside a formula.  After all, these have been discovered experimentally and cannot be changed at the whim of a chemistry student or teacher.   The only way to balance an equation is to place numbers, called coefficients, in front of whole formulas.  A coefficient applies to the entire molecule that follows it.

Start with a skeleton equation:         Mg   +     O2    ------->   MgO

The oxygen atoms did not match so place a 2 in front of the MgO so that there are now two O on the right.

                                                       Mg   +    O2   -------->    2 MgO


Now the oxygen atoms balance but the magnesium atoms no longer match.  We have placed a 2 in front of the MgO which means we have 2 Mg atoms and 2 O atoms in total.  If we place a 2 in front of the Mg on the left side then we get a balanced equation.

                                                    2 Mg   +   O2   -------->    2 MgO


If you check the number of atoms on each side of the equation you'll see that it is balanced.  The method used above is called balancing by inspection.  It is used on the simpler types of equations.
Go to the Sight Balancing Worksheet

Balancing More Complex Equations
Many chemical reactions involve more than a total of three reactants and products.  Balancing these more complex equations involves the same principles as used above but in a set pattern.

Guidelines for balancing complex chemical equations:
1.  
Balance the element that has the greatest number of atoms in any reactant or product, other than oxygen or hydrogen.
 
2.    Balance any other elements, other than hydrogen or oxygen.
 
3.    Balance oxygen or hydrogen, whichever one is present in the combined state.  Leave until the last whichever one is present in the uncombined state.
 
4.    Check that the equation is balanced by counting the number of atoms of each element on each side of the equation.
 
5.    The coefficients should be in whole numbers and in their lowest terms.  It is customary to clear any fractions.
Go to the Balancing More Complex Reactions Worksheet
Go to the Extra Balancing Practice Questions
Go to the Translating English Into Chemistry Worksheet

Go to the Next Stoichiometry Section on the Common Reactions