Chapter 4  Quantities of reactants  and products

 

4.1  Chemical equations

 

A chemical equation is a shorthand description of a chemical reaction, using symbols and formulas to represent the elements and compounds involved.

 

Reactants:  the starting substances for a reaction.  These are the materials you put into the test tube/beaker/flask.  Written on the left side of the arrow

 

Products:  the substances formed by the reaction.  Written on the right side of the arrow.

 

Phase labels:  letters written in parenthesis after a reactant or product to indicate whether the substance is a solid (s), liquid (l), gas (g) or dissolved in water (aq).

 

Stoichiometric coefficients:  number placed in front of formulas in a chemical equation to balance the numbers of each type of atom in the entire chemical equation.  We must do this to obey the law of conservation of matter. 

 

Examples:

 

            N2 (g) + O2 (g) → 2NO (g)

 

            Ca (s) + Br2 (l) → CaBr2 (s)

 

4.2  Types of chemical reactions

 

Combination reaction:  a reaction in which two or more reactants become bonded together to form a single product.

 

Decomposition reaction:  a reaction in which bonds in a single reactant break to form two or more products from that reactant.

 

Displacement reaction:  a reaction in which one element reacts with a compound to form a new compound and release a different element.

 

Exchange reaction:  a reaction in which there is an interchange of partners between two compounds.

 

Identify the following reaction types:

 

•         2 Al(s) + 3 Br2 (l) ®  Al2Br6(s)                                                combination

 

•         2K(s) + H2O(l)  ®  2KOH(aq) + H2(g)                                   displacement

 

•         AgNO3(aq) + NaCl(aq) ®  AgCl(s) + NaNO3(aq)                  exchange

 

•         NH4NO3(s)  ®   N2O(g) + 2H2O (g)                                       decomposition

 

4.3  Writing and Balancing Chemical Equations

 

 

The process of adding the stoichiometric coefficients is called balancing a chemical equation.

 

One strategy for balancing a chemical equation:

 

1.  If an element is present in just one compound on each side of the equation, balance that first.

 

2.  In some reactions, groupings of atoms (such as polyatomic ions) remains unchanged.  Balance these groups as a unit.

 

3.  Balance reactants or products that exist as free elements last.

 

4.  Sometimes balancing is simplest if you use fractional coefficients first then clearing the fractions by multiplying the entire equation with a common multiplier.

 

A second strategy for balancing chemical reactions:

 

1. Write an unbalanced equation with correct formulas for all substances.

 

2.  Balance the atoms of one element.

    1. Start with the most complex molecule
    2. Change the coefficients in front of the molecules
    3. Do NOT alter the chemical formulas

 

3.  Balance the remaining elements.

 

4.  Check the atoms are all balanced.

 

Examples:

                                    unbalanced                                         balanced

                  SiCl4 + H2O → SiO2 + HCl                       SiCl4 + 2H2O → SiO2 + 4HCl

 

                 PCl5 + H2O → H3PO4 + HCl                      PCl5 + 4H2O → H3PO4 + 5HCl

 

                 CaO + P4O10 → Ca3(PO4)2                                6CaO + P4O10 → 2Ca3(PO4)2

 

                 C4H10 + O2 → CO2 + H2O                         2C4H10 + 13O2 → 8CO2 + 10H2O

           

Polyatomic ions that appear on both sides of the equation should be balanced as a unit.

 

Examples:

 

            unbalanced                                                                   balanced

FeCl3 + NaOH → Fe(OH)3 + NaCl                             FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl

 

Ba(NO3)2 + Al2(SO4)3 → BaSO4 + Al(NO3)3               3Ba(NO3)2 + Al2(SO4)3 → 3BaSO4 + 2Al(NO3)3