WEEK TWO -- CHAPTER 6: Chemical Reactions and Equations

This chapter will discuss the basic chemical reactions that you must know. We will try to classify them (categorize them) in order to make it easier to see the patterns involved in those reactions. First, let's review what a chemical reaction is:

A chemical reaction occurs when a substance is changed (chemically) into another substance

In other words, substances change their chemical composition and energy changes occur at the same time. We typically look for some physical signs that a reaction has occurred, such as:

However, some reactions are subtle enough that it is difficult to determine whether a reaction has occurred.

LEARNING OBJECTIVES:

CHEMICAL REACTIONS:

Before we look at specific chemical reactions we need to understand the language of writing and displaying chemical reactions. Chemical reactions consist of reactants and products. Reactants undergo chemical change to form products. In addition to reactants and products there are numerous symbols which describe the conditions under which the reactions occu.

          C4H8(g) + 6O2(g) ® 4CO2(g) + 4H2O(g)
          butane gas and oxygen gas react to form carbon dioxide gas and water gas
                            D

          2KClO3(s) ® 2KCl(s) + 3O2(g)

                                  MnO2

          potassium chlorate solid reacts with heat (D ) and a catalyst (MnO2) to form potassium chloride
          solid and oxygen gas

 

But is the following reaction correct?

NO(g) + O2(g) ® NO2(g)

nitrogen monoxide gas reacts with oxygen gas to form nitrogen dioxide gas

According to the Law of Conservation of Matter atoms are neither created nor destroyed by a chemical reaction but only rearranged to form new substances. The equation above cannot be correct since we show a total of three atoms of O on the reactant side and only two atoms of O on the product side it would appear that we "lost" an atom of O in the process of the equation, and that cannot happen. It turns out that the equation which represents the reaction is not balanced. All chemical equations which represent a chemical reaction must be balanced so that the reactions, and the equations which represent them, obey the Law of Conservation of Matter.

Balancing Chemical Equations:

To balance a chemical equations:

 

Remember: the total number of each type of atom must be the same on both sides of the equation but you cannot change the formula to make the equation balance! 

Problems:

2 C4H10O2(l) + 11 O2(g) ® 8 CO2(g) + 10 H2O(g)

2 AgNO3(aq)  +   BaCl2(aq)   ®   2 AgCl(s)   +   Ba(NO3)2(aq)
                 reactants                                   products
             2 Ag+ cations                            2 Ag+ cations
             1 Ba2+ cation                             1 Ba2+ cation
             2 Cl- anions                                2 Cl- anions
             2 NO3- anion units                    2 NO3- anion units
                          reaction is balanced!

Hints for Balancing Chemical Equations:

TYPES OF CHEMICAL REACTIONS:

Summary of Types:

Decomposition:     a compound decomposes into two or more different substances
                                   
                                   General Form:       A     ®     B    +    C

                                   Examples:        2HgO(s)   ®    2Hg(l)   +   O2(g)
                                                             CaCO3(s)   ®    CaO(s)   +    CO2(g)

Combination: two or more substances combine to form a third, different, substance

                        General Form:           A     +      B      ®        C

                        Examples:                 2K(s)   +    Cl2(g) ®    2KCl(s)
                                                           4Fe(s)   +    3O2(g)  ®   2Fe2O3(s)

Single Replacement: occurs between an element and a compound such that the element replaces another element in the compound

                        General Form:               A       +       BX        ®       AX     +        B

                        Examples:                     C(s)     +      PbO(s)   ®      CO(g)  +     Pb(s)
                                                            element         compound         compound       element

                                                                2Al(s)  +      Cr2O3(s)  ®    Al2O3(s)  +    2Cr(s)
                                                             element         compound           compound         element


Double Replacement: these reactions occur between two ionic compounds in which the compounds exchange ion partners

                                     General Form:            AX      +       BY     ®      AY    +       BX

                                                                    where A & B are cations and X & Y are anions

Combustion: the reaction of anything (element or compound) with oxygen (O2)

                                  General Form:            A        +       O2       ®       AO2      or

                                                                      AX4     +       2O2     ®        AO2   +      2X2O

                                  Examples:                2Hg(l)   +        O2(g)   ®         2HgO(s)

         CH4(g) +         2O2(g) ®           CO2(g)   +     2H2O(g)

          2H2(g)   +         O2(g)  ®            2H2O(g)

 

In order to be able to predict double replacement precipitation reactions, you must know which substances are soluble and which substances are insoluble. The term "soluble" means that the substance dissolves in water. In other words, if the substance is ionic, such as salt (NaCl), then the ions (Na+ and Cl-) are separated by the water molecules and the white solid dissolves (disappears) and a solution is formed, as when you put a teaspoon of salt in a glass of water. The term "insoluble" meas that the substance does not dissolve in water, but remains in solid form--much like coffee grounds in water, they remain solid. Below are given a few general rules for predicting whether a substance is soluble or insoluble--know these rules!

SOLUBILITY RULES

soluble ionic compounds

insoluble ionic compounds

  1. 1. All common compounds of Grp. IA ions and NH4+ ions are soluble

 

  1. 1. All common metal hydroxides are insoluble, except Grp. IA and the larger Grp. IIA starting with Ca+2 ion
  1. 2. All common nitrates (NO3-), acetates (CH3COO-), and most perchlorates (ClO4-) are soluble

 

  1. 2. All common carbonates (CO3-2) and phosphates (PO4-3) are insoluble, except those of Grp. IA and NH4+
  1. 3. All common chlorides (Cl-), bromides (Br-), and iodides (I-) are soluble, except those of Ag+, Pb+2, Cu+ and Hg2+2
  1. 3. All common sulfides are insoluble, except those of Grp. IA, NH4+, and Ca2+ and Ba2+
  1. 4. All common sulfates (SO4-2) are soluble, except those of Ca+2, Sr+2, Ba+2 and Pb+2
 

 

 

 

 

 

 

Click here to go to Chapter 7 Chemical Composition