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Chapter 10: Completing Chemical Equations

A chemical equation is written as an expression similar to a mathematic equation that can be compared to a recipe that a chemist follows in order to produce desired results.


Chemical Equations 

All chemical equations have reactants and products. 

We express a chemical equation as follows: 

Reactants Products 

The arrow is equivalent to an “=“ math.  When we describe the equation we use the word “yields” or “produces” instead of equals


Conservation of matter  

The law of conservation of matter tells us that matter can neither be created nor destroyed.  It is transformed from one form to another.  

We follow the conservation of matter law when we balance equations.


Balancing a Chemical Equation  

A chemical equation is balanced when the ions or atoms found on the reactant side of the equation equals that found on the product side. 

The arrow can be considered the balance point.



We use coefficients in front of elements or compounds on either side of the reaction to balance the equation.  (The coefficients go in front of the chemical!) 

We cannot change subscripts because it would create new materials that are not part of reaction process.


Balance this equation! 

   Na  +   Cl2       NaCl

          Na- 1  Na-1

           Cl- 2  Cl-1 

**note that the number of sodiums balance but the chlorine does not.  We will have to use coefficients in order to balance this equation.


Inserting subscripts 

         Na  +   Cl2         2 NaCl

          Na- 1  Na-1 2

           Cl- 2  Cl-1 2 

** Now the chlorine balances but the sodium does not!  So we go back and balance the sodium.


Finally balanced! 

         2Na  +   Cl2      2 NaCl

          Na- 1 2  Na-1 2

           Cl- 2  Cl-1 2 

**Since the number of each element on the reactant side and the product side of the equation are equal, the equation is balanced.


Balancing Equation Practice 

CuCl3 + Li2S    Cu2S3 +  LiCl

NiNO3 +  KCl NiCl + KNO3

FeCl3 + Na2O Fe2O3 + NaCl



1. 2CuCl3 + 3Li2S  Cu2S3 +  6LiCl

2. NiNO3 +  KCl NiCl + KNO3

         (already balanced)

3. 2FeCl3 + 3Na2O Fe2O3 + 6NaCl


Identifying Reaction Types 

Reactions are classified by their products.   

There are several different types of chemical reactions.  The 5 basic types of chemical reactions we will study are:  synthesis or combination reactions, decomposition reactions, single replacement reactions, double replacement reactions, and combustion reactions.


Synthesis or Combination reactions 

Synthesis (meaning to make) or combination reactions are typified by their single product. 

If you have a reaction in which at least 2 elements or compounds are reacted and produce a single product, the reaction is a synthesis reaction.


Synthesis Reactions 

A + B AB 

Note:  Single Product!  This is your clue that this is a synthesis or combination reaction.


Decomposition Reactions 

Decomposition reactions are really just the opposite of a synthesis reaction.  Remember, if you can make a substance, you should be able to break it back apart into its components. 

A good way to remember decomposition reactions to to remember what happens when something decomposes.  It falls apart!


Decomposition Reactions 

AB A + B 

Note:  Single Reactant!  The single reactant is your clue that this is a decomposition reaction.   


Single Replacement Reactions 

Single replacement reactions occur when one chemical takes the place of another in a reaction. 

In the typical single replacement reaction, an element trades places with one of the ions in a compound.


Single Replacement Reactions 

A + CD   AD + C 

Note:  Element A replaces ion C in the reaction.  The clue in this reaction is:

Element+Compound Element + Compound


Double Replacement Reactions 

Double replacement reactions are identified by two ions trading places and forming new compounds. 


Double Replacement Reactions 

AB + CD AD + CB 

Note:  Notice that one ion from compound AB replaces one ion from compound CD.


Compound+Compound Compound+Compound


Combustion Reactions 

Combustion reactions are the ones that burn (or explode!).  There are two types of combustion reactions—complete or incomplete reactions. 

These reactions are identified by their products.  They either produce carbon monoxide and water or carbon dioxide and water.


Complete Combustion Reactions 

These reactions burn “efficiently” which means they produce carbon dioxide and water.  These reactions typically burn cleanly and leave very little residue behind. 


Complete Combustion Reactions 

CH4 + O2 CO2 + H2O 

They may also be written:

CH4 CO2 + H2O 

(O2 is usually written above the arrow.) 

Clue: CO2 (carbon dioxide) in the product along with water


Incomplete Combustion Reactions 

Incomplete combustion reactions occur when something does not burn efficiently.  This can cause a lot of harm if the gases produced cannot escape.  Carbon monoxide,an oderless and colorless gas, is dangerous.  People poisoned by this gas usually become sleepy and can die due to exposure.


Incomplete Combustion Reactions 

CH4 + O2 CO + H2O 

These reactions may also be written by:

CH4 CO + H2O

(the O2 is usually written over the arrow.) 

Clue:  CO (Carbon monoxide as a product.)


Completing Reactions 

Completing reactions requires knowledge of the different reaction types (sometimes called mechanisms). 

You must first identify the reaction type by the reactants.  The only type of reaction that cannot be predicted this way is the combustion reaction since the products are very similar.

 First Step: 

Identify reaction type 


Al + O2  

Clue:  2 elements – Synthesis or combination reaction 

 Second Step: 

Write the net ionic equation for the reactants 

Al + O2  

Al3+  +  O2-  

 Step 3 

Using clues, complete reaction taking care to write each formula correctly by checking charges and “criss-crossing” if necessary. 

Al + O2   Al3+O2- 

Al + O2 Al2O3