Chapter 6
The Nature of Chemical Reactions
Table of Contents
Section 1 The Nature of Chemical Reactions
Section 2 Reaction Types
Section 3 Balancing Chemical Equations
Section 4 Rates of Change
Chapter 6
Section 1 The Nature of Chemical
Reactions
Objectives
• Recognize some signs that a chemical reaction
may be taking place.
• Explain chemical changes in terms of the
structure and motion of atoms and molecules.
• Describe the differences between endothermic
and exothermic reactions.
• Identify situations involving chemical energy.
Chapter 6
Section 1 The Nature of Chemical
Reactions
Chemical Reactions Change
Substances
• Chemical reactions occur when substances
undergo chemical changes to form new
substances.
• Production of gas and change of color are signs
of chemical reactions.
• Chemical reactions rearrange atoms.
• A reactant is a substance or molecule that
participates in a chemical reaction.
• A product is a substance that forms in a
chemical reaction.
Chapter 6
Section 1 The Nature of Chemical
Reactions
Energy and Reactions
• Energy must be added to break bonds.
• Many forms of energy can be used to break bonds:
•
•
•
•
heat
electricity
sound
light
• Forming bonds releases energy.
• Example: When gasoline burns, energy in the form
of heat and light is released as the products of the
isooctane-oxygen reaction and other gasoline
reactions form.
Chapter 6
Section 1 The Nature of Chemical
Reactions
Reaction Model
Chapter 6
Section 1 The Nature of Chemical
Reactions
Energy and Reactions, continued
• Energy is conserved in chemical reactions.
• Chemical energy is the energy released when a chemical
compound reacts to produce new compounds.
• The total energy that exists before the reaction is equal to the
total energy of the products and their surroundings.
• An exothermic reaction is a chemical
reaction in which heat is released to the
surroundings.
• An endothermic reaction is a chemical
reaction that absorbs heat.
Chapter 6
Section 1 The Nature of Chemical
Reactions
Energy and Reactions, continued
• The graphs below represent the changes in
chemical energy for an exothermic reaction
and an endothermic reaction.
Chapter 6
Section 2 Reaction Types
Objectives
• Distinguish among five general types of
chemical reactions.
• Predict the products of some reactions based
on the reaction type.
• Describe reactions that transfer or share
electrons between molecules, atoms, or ions.
Chapter 6
Section 2 Reaction Types
Classifying Reactions
• A synthesis reaction is a reaction in which two or
more substances combine to form a new
compound.
• Synthesis reactions have the following general form: A
+ B → AB
• Example: In the following synthesis reaction, the metal
sodium reacts with chlorine gas to form sodium
chloride, or table salt.
• 2Na + Cl2 → 2NaCl
Chapter 6
Section 2 Reaction Types
Classifying Reactions, continued
• A decomposition reaction is a reaction in a
single compound breaks down to form two or
more simpler substances.
• Decomposition reactions have the following
general form: AB → A + B
• Example: The following shows the decomposition
of water.
• 2H2O → 2H2 + O2
• Electrolysis is the process in which an electric
current is used to produce a chemical reaction,
such as the decomposition of water.
Chapter 6
Section 2 Reaction Types
Classifying Reactions, continued
• A combustion reaction is the oxidation
reaction of an organic compound, in which
heat is released.
• Combustion reactions use oxygen as a reactant.
• Water is a common product of combustion
reactions.
• In combustion the products depend on the
amount of oxygen available for the reaction.
Chapter 6
Section 2 Reaction Types
Classifying Reactions, continued
• A single-displacement reaction is a reaction in
which one element or radical takes the place
of another element or radical in the
compound.
• Single-displacement reactions have the following
general form: AX + B → BX + A
• Example: The single-displacement reaction
between copper(II) chloride and aluminum is
shown as follows.
3CuCl2 + 2Al → 2AlCl3 + 3Cu
Chapter 6
Section 2 Reaction Types
Classifying Reactions, continued
• A double-displacement reaction is a reaction
in which a gas, a solid precipitate, or a
molecular compound forms from the apparent
exchange of atoms or ions between two
compounds.
• Double-displacement reactions have the following
general form: AX + BY → AY + BX
• Example: The double-displacement reaction that
forms lead chromate is as follows.
Pb(NO3)2 + K2CrO4 → PbCrO4 + 2KNO3
Chapter 6
Section 2 Reaction Types
Double Displacement Reaction
Chapter 6
Section 2 Reaction Types
Electrons and Chemical Reactions
• An oxidation-reduction reaction is any chemical
change in which one species gains electrons and
another species loses electrons.
• Oxidation-reduction reactions are often called redox reactions for short.
• Substances that accept electrons in a redox reaction are said to be
reduced.
• Substances that give up electrons in a redox reaction are said to be
oxidized.
• A radical is an organic group that has one or
more electrons available for bonding.
• Polymerization reactions can occur when radicals
are formed.
Chapter 6
Section 3 Balancing Chemical
Equations
Objectives
• Demonstrate how to balance chemical equations.
• Interpret chemical equations to determine the
relative number of moles of reactants needed
and moles of products formed.
• Explain how the law of definite proportions
allows for predictions about reaction amounts.
• Identify mole ratios in a balanced chemical
equation.
• Calculate the relative masses of reactants and
products from a chemical equation.
Chapter 6
Section 3 Balancing Chemical
Equations
Describing Reactions
• One way to record the products and reactants of a reaction is to write a
word equation.
• Example: methane + oxygen → carbon dioxide + water
• A chemical equation is a representation of a chemical reaction that
uses symbols to show the relationship between the reactants and the
products.
• In a chemical equation, such as the one above, the reactants, which are
on the left-hand side of the arrow, form the products, which are on the
right-hand side.
Chapter 6
Section 3 Balancing Chemical
Equations
Describing Reactions
• When the number of atoms of reactants matches
the number of atoms of products, then the
chemical equation is said to be balanced.
• Balancing equations follows the law of conservation of
mass.
• You cannot balance chemical equations by changing
chemical formulas themselves, because that would
change the substances involved.
• To balance chemical equations, numbers called
coefficients must be placed in front of the
chemical formulas.
Chapter 6
Section 3 Balancing Chemical
Equations
Describing Reactions, continued
• When the numbers of atoms for each
element are the same on each side, the
equation is balanced, as shown below.
Chapter 6
Section 3 Balancing Chemical
Equations
Math Skills
Balancing Chemical Equations Write the
equation that describes the burning of
magnesium in air to form magnesium oxide.
1. Identify the reactants and products.
Magnesium and oxygen gas are the reactants
that form the product, magnesium oxide.
2. Write a word equation for the reaction.
magnesium + oxygen → magnesium oxide.
Chapter 6
Section 3 Balancing Chemical
Equations
Math Skills, continued
3. Write the equation using formulas for the
elements and compounds in the word
equation.
Remember that some gaseous elements, like oxygen, are molecules,
not atoms. Oxygen in air is O2, not O.
Mg + O2 → MgO
4. Balance the equation one element at a
time.
TheBut
same
number
of oxygen
each kind
of atom
must
on one
bothon
sides. So
there
are two
atoms
on the
leftappear
and only
far, right.
there is one atom of magnesium on each side of the equation.
the
Chapter 6
Section 3 Balancing Chemical
Equations
Math Skills, continued
4. Balance the equation one element at a
time, continued
To balance the number of oxygen atoms,
you need to double the amount of
magnesium oxide:
Mg + O2 → 2MgO
This equation gives you two magnesium atoms on the
right and only one on the left. So you need to double the
amount of magnesium on the left,
as follows.
2Mg + O2 → 2MgO
Chapter 6
Section 3 Balancing Chemical
Equations
Math Skills, continued
4. Balance the equation one element at a
time, continued
2Mg + O2 → 2MgO
Now the equation is balanced. It has an equal number of
each type of atom on both sides.
Chapter 6
Section 3 Balancing Chemical
Equations
Determining Mole Ratios
• The law of definite proportions states that a
compound always contains the same elements
in the same proportions, regardless of how the
compound is made or how much of the
compound is formed.
• Because the law of definite proportions holds
true for all chemical substances in all
reactions, mole ratios can be derived from
balanced equations.
• Mole ratio is the relative number of moles of the
substances required to produce a given amount of
product in a chemical reaction.
Chapter 6
Section 3 Balancing Chemical
Equations
Determining Mole Ratios, continued
• The mole ratio for any reaction comes from the balanced
chemical equation.
• Example: The equation for the electrolysis of water shows
that the mole ratio for H2O:H2:O2 is 2:2:1.
• 2H2O → 2H2 + [1]O2
• If you know the mole ratios of the substances in a reaction, you
can find the relative masses of the substances required to react
completely.
• Relative masses can be found by multiplying the molecular
mass of each substance by the mole ratio from the balanced
equation.
Chapter 6
Section 4 Rates of Change
Objectives
• Describe the factors affecting reaction rates.
• Explain the effect a catalyst has on a chemical
reaction.
• Explain chemical equilibrium in terms of equal
forward and reverse reaction rates.
• Apply Le Châtelier’s principle to predict the effect
of changes in concentration, temperature, and
pressure in an equilibrium process.
Chapter 6
Section 4 Rates of Change
Factors Affecting Reaction Rates
• For any reaction to occur, the particles of the
reactants must collide with one another.
Therefore, whatever will help particles collide
with one another will speed up the reaction
rate.
•
•
•
•
•
Most reactions go faster at higher temperatures.
Greater surface area speeds up reactions.
Concentrated solutions react faster.
Reactions are faster at higher pressure.
Massive, bulky molecules react slower.
Chapter 6
Section 4 Rates of Change
Factors Affecting Reaction Rates,
continued
• A catalyst is a substance that changes the
rate of a chemical reaction without being
consumed or changed significantly.
• Catalysts are not reactants or products,
because they are not used up in the reaction.
• Catalysts are often used in industry to make
reactions go faster.
• Catalysts that slow reactions are called
inhibitors.
Chapter 6
Section 4 Rates of Change
Factors Affecting Reaction Rates,
continued
• Enzymes are proteins that serve as
biological catalysts.
• An enzyme is very specific, controlling one
reaction or set of similar reactions.
• Most enzymes are fragile, and stop working
above certain temperatures.
• The substrate is the reactant in reactions
catalyzed by enzymes.
• Example: hydrogen peroxide is the substrate
for catalase:
Chapter 6
Section 4 Rates of Change
Equilibrium Systems
• Some changes are reversible.
• Example: the physical change represented below can go in either
direction.
increase pressure

 CO2 (gas dissolved in liquid)
CO2 (gas above liquid) 
decrease pressure
• Chemical equilibrium is a state of balance in which the
rate of a forward reaction equals the rate of the reverse
reaction.
• Systems in equilibrium respond to minimize change.
• Example: when the top is removed from a carbonated drink, the
system is no longer at equilibrium, and CO2 leaves as bubbles.
Chapter 6
Section 4 Rates of Change
Equilibrium Systems, continued
• Le Châtelier’s principle predicts changes
in equilibrium.
• Le Châtelier’s principle is a general rule that states that if a
change is made to a system in chemical equilibrium, the
equilibrium shifts to oppose the change until a new
equilibrium is reached.
• Le Châtelier’s principle can be used to
control reactions.
• Example: in a reaction that releases energy, if you raise the
temperature, the equilibrium will shift to the left and make
less products.