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:

• the release or absorption of heat
• emission of light
• formation of a solid (observation of a "precipitate")
• formation of a gas (observation of bubbles)
• change of color

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

LEARNING OBJECTIVES:

• Chemical Reactions
• Recognizing reactions
• Writing reactions and chemical equations
• Balancing chemical equations
• Types of Chemical Reactions
• Decomposition
• Single-Replacement
• Double-Replacement
• Precipitation
• Gas Formation
• Neutralization
• Combustion Reaction

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.

• reactants
• are chemical substances written on the left of the reaction arrow
• products
• are chemical substances written on the right of the reaction arrow
• symbols indicating conditions
• reaction arrow indicates that a chemical reaction has occurred to the reactants which has resulted in the products and is read "reacts to form"
• small, subscript numbers are part of the chemical formula, they indicate the relative ratio of atoms within a chemical formula
• large coefficients to the left of the formula indicate the number of molecules ( or "moles") of the substance that undergo reaction or are formed relative to all other substances (we will discuss "moles" in chapter 7)
• small subscript letter in parentheses indicates the state of the substance reacting or formed
• symbols over or under the arrow indicate other conditions of the reaction (often whether the reaction is heated and/or if a catalyst is used)
• examples:
  CH_4(g) + 2O_2(g) ® CO_2(g) + 2H₂O_(l)
  methane gas and oxygen gas react to form carbon dioxide gas and water liquid

          C₄H_8(g) + 6O_2(g) ® 4CO_2(g) + 4H₂O_(g)
          butane gas and oxygen gas react to form carbon dioxide gas and water gas
                            _D

          2KClO_3(s) ® 2KCl_(s) + 3O_2(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) + O_2(g) ® NO_2(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:

• you cannot change the subscripts once the formula is correctly written
• remember, the numerical subscripts indicate the relative ratio of atoms within a molecule or formula--if you change that ratio, the molecule is changed and is no longer what was originally intended
• you can change the coefficient to the left of the chemical formula (in other words, you can only change the relative number of molecules or formula units that participate in the reaction)
• the coefficients must be
• whole numbers
• show the simplest ratio of reactants and products
• if no coefficient is shown, it is assumed to be 1
• examples:
  
• 2NO_(g)     +       O_2(g)      ®       2NO_2(g)
  2 molecules of nitrogen monoxide gas and 1 molecule of oxygen gas react to form 2 molecules of nitrogen dioxide gas
           reactants                         products
          2 N atoms                        2 N atoms
          4 O atoms                        4 O atoms
                reaction is balanced!
  
• C_(s)        +          2H_2(g)      ®            CH_4(g)
  1 atom of carbon solid and 2 molecules of hydrogen gas react to form 1 molecule of methane gas
             reactants                               products
             1 C atom                               1 C atom
             4H atoms                              4H atoms
                             balanced!
  
• PCl_3(l)         +        3H₂O_(l)         ®          H₃PO_3(aq)        +        3HCl_(aq)
  1 molecule of phosphorus trichloride and 3 molecules of water liquid react to form 1 molecule of phosphorus acid and 3 molecules of hydrochloric acid
              reactants                                                          products
             1 P atom                                                           1 P atom
             6 H atoms                                                         6 H atoms
             3 O atoms                                                         3 O atoms
                                         balanced!
  

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:

• Balance:
  2 C₄H_10(g) + 13 O_2(g) ® 8 CO_2(g) + 10 H₂O_(g)
  

• note that the coefficients 2 : 13 : 8 : 10 in reaction 1 and 2 : 11 : 8 : 10 in reaction 2 are the smallest combination (the simplest ratio) of numbers, in other words, they cannot be divided by a common denominator other than 1.

• note that in the last reaction involving the ionic compounds, the cations and anions are treated as units--if there is no coefficient (as is the case for BaCl_2(aq) and Ba(NO₃)_2(aq)), it means that a coefficient of 1 is implied.
  

Hints for Balancing Chemical Equations:

• begin by balancing the element which has the fewest atoms present in only one of the chemical formulas
• for example:              C₂H₆   +    7/2 O₂      ®       2 CO₂     +      3 H₂O
  balance the C atoms first (there are only 2 C atoms in C₂H₆), then the H atoms, then, finally, the O atoms (since O atoms occur in more than one chemical formula on the product side, we leave them to last)
• the above reaction is balanced, although it is, strictly speaking, incorrectly balanced
                                              2 [C₂H₆    +   7/2 O₂    ®       2 CO₂    +         3 H₂O]
  not only should the coefficients be in the smallest possible ratio, they should also be whole numbers--to get the whole number, simplest ratio of coefficients, we multiply all coefficients by two (the common denominator of any fractions), which, in effect, removes the 7/2 fraction.
                                                 2 C₂H₆   +      7 O₂      ®      4 CO₂    +         6 H₂O
  the ratio 2 : 7/2 : 2 : 3 is the same ratio as 2 : 7 : 4 : 6 , but the latter is the correct value of the simplest ratio of whole number coefficients

TYPES OF CHEMICAL REACTIONS:

Summary of Types:

• decomposition
• one chemical compound changes to two or more different chemical compounds
• combination
• two or more chemical compounds change to one chemical compound of a different type
• single replacement
• an element or an elemental molecule changes to a compound in which it is chemically combined with another element
• double replacement
• the ion partners of two different ionic compounds exchange, there are several physical occurrences which indicate a double replacement reaction:
• formation of a solid (precipitation)
• formation of a gas (bubbling occurs)
• formation of a molecular (non-ionic) compound (neutralization)
• combustion
• reaction with oxygen (O₂)

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

                                   Examples:        2HgO_(s)   ®    2Hg_(l)   +   O_2(g)
                                                             CaCO_3(s)   ®    CaO_(s)   +    CO_2(g)

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

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

• single replacement reactions occur based upon the relative "activity" of the elements that undergo the exchange (between element and compound)--for example, in the reaction above, the reaction is based upon the relative activities of Al and Cr--the more active metal will always end up in the compound form rather than the elemental form
• which is the more active metal in the reaction immediately above--Al or Cr?          Al
• on page 164 of your text, table 6.2 gives an activity series for some common metals; the metals at the top of the series are the most active while they become less active down the series table

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

• precipitation -- formation of a solid
  
                                                         AgNO_3(aq)    +     NaCl_(aq)     ®    AgCl_(s)   +     NaNO_3(aq)
  
• gas formation:
  
                                                         CaCO_3(s)   +       2HCl_(aq)   ®    CaCl_2(aq)   +   H₂O_(l)  +  CO_2(g)
  
                                                         K₂SO_3(aq)   +    2HCl_(aq)    ®    2KCl_(aq)   + H₂O_(l)    +    SO_2(g)
  
                                                         Na₂S_(aq)       +    2HCl_(aq)   ®      2NaCl_(aq)   +    H₂S_(g)
  
• please note: anytime a carbonate (any ionic compound with CO₃^2- anion), or a sulfite (any ionic compound with SO₃^2- anion), or a sufide (any ionic compound with S^2- anion) react with any acid, CO_2(g), SO_2(g) and H₂S_(g) will be formed, respectively
• molecular compound formation (neutralization):
  
                                                           HCl_(aq)    +      NaOH_(aq)     ®      H₂O_(l)     +     NaCl_(aq)
  
• note that H₂O is a molecular compound--not an ionic compound--while NaOH is an ionic compound, NaCl is an ionic compound and HCl_(aq) is an acid which when dissolved produces H⁺ and Cl^- ions

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

                                  General Form:            A        +       O₂       ®       AO₂      or

                                                                      AX₄     +       2O₂     ®        AO₂   +      2X₂O

                                  Examples:                2Hg_(l)   +        O_2(g)   ®         2HgO_(s)

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!