Chapter 3: Stoichiometry: Calculations with Chemical Formulas and Equations

I love chemical equations! (Don't you?)

In Chapter 3 we are going dwell on the application of the law of conservation of mass by studying the quantitative relationship which exists between the reactants and products in chemical reactions. To communicate such quantitative information about chemical reactions, we will employ chemical equations as a symbolic way of representing a chemical reaction. The study of the quantitative relationships in chemical reactions is called stoichiometry.

We will look at six kinds of chemical reactions. You will need to able to recognize the different types of reactions and to write the correct products give the reactants. We will see examples of the different types of reactions in class. You must know these examples for the examination. You will also be expected to extend the information you receive in class to other examples.

The six different reaction types are;

  1. Formation
    • A reaction between elements in their standard state to form a single product in its standard state. Such reactions may require heat or other form of energy to occur.
    • Fe(s) + S(s) -(heat)--> FeS(s)
    • Al(s) + Br2(s) ---> Al2Br6(s)
  2. Combustion
    • A reaction between a substance and oxygen. The common example is the combustion of a hydrocarbon. A hydrocarbon is a covalent (organic) compound which only contains carbon and hydrogen. The combustion of a hydrocarbon produces carbon dioxide (CO2) and water (H2O)
    • CH4(g) + O2(g) ---> CO2(g) + H2O(g)
  3. Decomposition
    • A reaction of a single substance to produce two or more new substances. Such reactions may require heat or other form of energy to occur.
    • NaN3(s) ---> Na(s) + N2(g)
  4. Single replacement
    • A reaction in which an element in a reactant compound is replaced by a second reacting element producing a new compound and an element.
    • Na(s) + H2O(l) ---> NaOH(aq) + H2(g)
  5. Double replacement
    • A reaction between two ionic compounds producing two new ionic compounds where the cations and anions have exchanged 'partners'.
    • NaCl(aq) + AgNO3(aq) ---> NaNO3(aq) + AgCl(s)
  6. Neutralization
    • A reaction between an acid and a base to produce a salt and water.
    • HCl(aq) + NaOH(aq) ---> NaCl(aq) + H2O(g)

Important Acids

Name of Acid

Formula of Acid

Hydrochloric Acid


Sulfuric Acid


Nitric Acid


Phosphoric Acid


Important Bases

Name of Base

Formula of Base

Sodium hydroxide


Potassium hydroxide


Barium hydroxide




How to balance equations

One very important aspect of any chemical equation is that it be balanced. While the process of balancing chemical equations is straight forward, the real challenge is in the laboratory trying to identify the products of a chemical reaction. In this course our goal will generally be to simply balance a chemical equation. The primary rule that must be remembered when balancing equations is not to change subscripts in the formulas. Balancing equations is accomplished by changing coefficients. Coefficients are whole numbers immediately preceding the chemical formula for the substance in the equation. Never change a subscript or add or remove elements or compounds in an equation.

__Fe(s) + __S8(s) ----> __FeS(s)

To balance this equation, that is, obtain equals numbers of atoms of each element in the products and the reactants, can be accomplished by placing a coefficient of 8 in front of iron(II) sulfide, and than a 8 in front of iron. Now the equation is balanced.

There are some simple rules for balancing equations:

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