Chemical Reactions
Describing Chemical Reactions
11.1
Introduction to Chemical Equations
 In a chemical reaction, one or more reactants change
into one or more products
 Chemical equations are used to show what happens in
a chemical reaction
Word Equation
 Word equations are shorthand methods used for writing a
description of a chemical reaction
 The reactants are written on the left and the products on
the right, and an arrow separates them
 The arrow means to give, to yield, to produce, and in some
causes to decompose
 Reactants are separated from each other by a plus sign
 The plus sign here means “reacts with”
 Products are separated from each other by a plus sign
 The plus sign here means “and”
Na(s) + H2O (l)  NaOH (aq) + H2 (g)
Solid sodium reacts with water to form sodium hydroxide
and hydrogen.
Chemical Equations
 A chemical equation is a representation of a chemical reaction
 A skeleton equation is a chemical equation that does not indicate the
relative amounts (coefficients) of the reactants and products
 Sometimes skeleton equations are already balanced
Key Question
How do you write a skeleton equation?
Write the chemical formulas for the reactants to the left of the arrow and
the formulas for the products to the right of the arrow.
 You must give the states of matter as (s), (l), (g), or (aq)
 (aq) is used when a substance is dissolved in water
 A catalyst is a substance that speeds up the reaction but is not used up
in the reaction
 A catalyst is neither a reactant nor a product, so its formula is written
above the arrow
See Table 11.1
Balancing Chemical Equations


Coefficients are small whole numbers that are placed in
front of the formulas in an equation in order to balance it
A balanced equation is obtained when each side of the
equation has the same number of atoms of each element
and mass is conserved
Key Question
What are the steps for writing and balancing a chemical
equation?
See Next Slide
Rules for Writing and Balancing
Equations (Graphic Organizer)
Determine the correct formulas for all the reactants and
products
2. Write the skeleton equation
3. Determine the number of atoms of each element in the
reactants and products
1.

Count polyatomic ions as a single unit if it appears on both
sides of the equation
4. Balance the elements one at a time by using coefficients,
which will show the law of conservation on mass
5. Make sure all coefficients are in the lowest possible ratio
Types of Chemical Reactions
11.2
Classifying Reactions
Key Question
What are the five general types of reactions?
Combination, decomposition, single-replacement,
double-replacement, combustion
Combination Reactions
 A combination reaction is a chemical change in
which two or more substances react to form a single
new substance
Mg (s) + O2  MgO (s)
Decomposition Reactions
 A decomposition reaction is a chemical change in
which a single compound breaks down into two or
more simpler products
NaN3 (s)  Na (s) + N2 (g)
Model
Writing and Balancing Combination
Reactions and Decomposition
Reactions
Page 359, numbers 12-14 (Blue Book)
Pages 331 and 332, numbers 13-16 (Red book)
Single-Replacement Reaction
 A single replacement reaction is a chemical change in which one
element replace a second element in a compound
Zn (s) + AgNO3 (aq)  Zn(NO3)2 (aq) + Ag(s)
 Whether one metal will displace another metal from a compound
depends upon the relative reactivities of the two metals
 Activity series list metals in order of decreasing reactivity
*Attention to removing H in acids and bases*
 A reactive metal will replace any metal listed below it in the activity
series
 A halogen can also replace another halogen from a compound
 the activity of a halogen decreases as you go down group 7
Double-Replacement Reactions
 A double-replacement reaction is a chemical
change involving an exchange of positive ions between
two compounds
 They generally take place in aqueous solution and
often produce a precipitate, a gas, or a molecular
compound (H2O)
See examples on page 362
Combustion Reactions
 A combustion reaction is a chemical change in which an element or a
compound reacts with oxygen, often producing energy in the form of
heat and light
 A combustion reaction always involves O2 as a reactant, and most of
the time the other reactant is a hydrocarbon
 The complete combustion of a hydrocarbon produces CO2 and H2O
C8H18 (l) + O2 (g)  CO2 (g) + H2O
An example of combustion of a metal
Mg (s) + O2(g)  MgO (s)
A metal oxide is always produced!
Reactions in Aqueous Solutions
11.3
Net Ionic Equations
 Complete ionic equation is an equation that shows
dissolved ionic compounds as dissociated free ions
 An ion that appears on both sides of an equation and
is not directly involved in the reaction is called a
spectator ion
 The net ionic equation is an equation for a reaction
in solution that shows only those particles that are
directly involved in the chemical change
 In writing net ionic equations you must make sure that
the ionic charge is balanced
Key Question
What does a net ionic equation show?
Only those particles involved in the reaction and is
balanced with respect to both mass and charge.
*Both single and double displacement reactions can be
written as net ionic equations!
Predicting the Formation of a
Precipitate
 Mixing solutions of two ionic compounds can sometimes
result in the formation of an insoluble salt called a
precipitate
 Whether or not a precipitate forms depends upon the
solubility of the new compounds that form
Key Question
How can you predict the formation of a precipitate in a
double-replacement reaction?
By using the general rules for solubility of ionic compounds