ENERGY AND REACTIONS
Chapter Eighteen: Energy and
Reactions
18.1 Energy and Chemical
Reactions
18.2 Chemical Reaction Systems
18.3 Nuclear Reactions
Chapter 18.1 Learning Goals
Contrast endothermic and exothermic
reactions.
Explain why activation energy is
needed to begin chemical reactions.
Describe what happens when ionic
compounds are dissolved in water.
Investigation 18A
Energy and Chemical Changes
Key Question:
How do chemical changes involve energy?
Investigation 18B
Thermodynamics of Hot Packs/
Cold Packs
Key Question:
 Can we measure the heat released/energy
absorbed by instant hot and cold packs?
18.1 Energy and Reactions
 Energy is involved in chemical
reactions in two ways:
1. to break some (or all) bonds between
atoms in the reactants so the atoms can
form new bonds or
2. when the atoms or products form new
bonds to make new products.
18.1 Two Types of Reactions
We classify chemical reactions based
on how the energy of the reactants
compares to the energy of the
products.
18.1 Exothermic reactions
If forming new
bonds releases more
energy than it takes
to break the old
bonds, the reaction
is exothermic.
18.1 Exothermic reactions
A good example is the reaction of
hydrogen with oxygen.
Once started, exothermic reactions tend to
keep going as each reaction releases more
energy to fuel neighboring molecules.
18.1 Endothermic reactions
If forming new
bonds in the
products releases
less energy than it
took to break the
original bonds, the
reaction is
endothermic.
18.1 Endothermic reactions
An important endothermic reaction is
photosynthesis.
Plants need energy from sunlight to
make glucose and oxygen from carbon
dioxide and water.
18.1 Activation Energy
Activation energy is the energy
needed to begin a reaction and break
chemical bonds in the reactants.
18.1 Activation Energy
This is why a
flammable material
like gasoline does
not burn without a
spark or flame.
18.1 Carbon Reactions
Carbon dioxide and other atmospheric
gases are called “greenhouse gases.”
Scientists believe the rise in amount of
greenhouse gases will result in rises in sea
level and changes in weather.
18.1 Examples of
Endothermic Reactions
Most of the reactions used in industry to
produce useful materials require more
energy than they produce.
One process that uses endothermic
reactions is the refining of ores to produce
useful metals.
18.1 Examples of
Endothermic Reactions
Most of the reactions used
in industry to produce
useful materials require
more energy than they
produce.
The reaction taking place
inside an instant cold pack
is endothermic.
18.1 Examples of
Endothermic Reactions
When you squeeze the plastic bag the water
reacts with the ammonium nitrate crystals,
and the reaction dissolves the ionic bonds
in the ammonium nitrate.
18.1 Examples of
Endothermic Reactions
The reaction is also a dissolution reaction.
Dissolution occurs when an ionic compound
(like ammonium nitrate) dissolves in water
to make an ionic solution.
Chapter Eighteen: Energy and
Reactions
18.1 Energy and Chemical
Reactions
18.2 Chemical Reaction Systems
18.3 Nuclear Reactions
Chapter 18.2 Learning Goals
Discuss how chemical equations
are similar to recipes.
Identify limiting and excess
reactants in chemical reactions.
Describe factors that may
influence the rate at which a
chemical reaction occurs.
18.2 Chemical Reaction Systems
A balanced chemical equation is like a
recipe.
If you write the equation for making
chocolate cake, you will see it is similar to a
real recipe for making water.
18.2 Chemical Reaction Systems
18.2 Information from Balanced
Chemical Equations
If the recipe for chocolate cake gives
you ratios among the ingredients
needed to make eight servings, how
many servings are possible if you
only have half a cup of flour?
18.2 Information from Balanced
Chemical Equations
To make a good-tasting
chocolate cake with
half as much flour, you
would have to use half
as much of the other
ingredients, too.
By halving the recipe,
you can make four
servings of chocolate
cake.
18.2 Information from Balanced
Chemical Equations
A balanced chemical
equation shows the
ratios of the number
of molecules of
reactants needed to
make a certain
number of molecules
of products using
coefficients.
18.2 What a balanced equation
doesn’t tell you
A balanced equation does not describe
the exact conditions under which a
reaction will occur.
The right conditions for most of the
reactions that are used in science and
industry are the result of careful
research and experimentation.
18.2 Limiting and Excess
Reactants
When a chemical reaction occurs, the
reactants are not always present in the
exact ratio indicated by the balanced
equation.
What usually happens is that a
chemical reaction will run until the
reactant that is in short supply is used
up.
18.2 Limiting and Excess
Reactants
The reactant that is used up first in a
chemical reaction is called the limiting
reactant.
The limiting reactant limits the amount
of product that can be formed.
Which reactant
will be used up
first?
18.2 Limiting and Excess
Reactants
A reactant that is not completely used
up is called an excess reactant
because some of it will be left over
when the reaction is complete.
Which reactant
will be left over
(mixed with)
product?
18.2 Percent Yield
 Often the amount of
product you are
able to collect and
measure is less
than the amount
you would expect.
 Experimental error
often affects how
much product is
produced.
18.2 Percent Yield
 The percent yield (%) is the actual
yield divided by the predicted yield
and then multiplied by one hundred.
Solving Problems
Aspirin can be made in the laboratory
through a series of reactions.
If the actual yield for aspirin was 461.5
grams when the reactions were performed,
and the predicted yield was 500 grams,
what was the percent yield?
Solving Problems
1. Looking for:
 …percent yield of reaction
2. Given
 Actual yield = 461.5 g
 Predicted yield = 500.0 g
3. Relationships:
 percent yield = actual yield
predicted yield
× 100%
4. Solution
 percent yield = (461.5 g ÷ 500.0 g) × 100 = 92.3%
18.2 Reaction Rates
In all phases of matter, atoms and
molecules exhibit random motion.
This concept is part of the kinetic
theory of matter.
The speed at which atoms or
molecules move depends on the state
of matter and temperature.
18.2 Reaction Rates
 The reaction rate for a chemical
reaction is the change in
concentration of reactants or
products over time.
 Reaction rates can be increased by:
1. adding heat to increase molecular motion
2. increasing the concentration of the
reactants
3. increasing the chances that two
molecules will collide.
18.2 Catalysts
 A catalyst is a molecule
that can be added to a
reaction to speed it up.
 Catalysts work by
increasing the chances
that two molecules will
be positioned in the
right way for a reaction
to occur.
18.2 Chemical equilibrium
A reaction may reach chemical
equilibrium, the state in which the rate
of the forward reaction equals the rate
of the reverse reaction.
In chemical equilibrium, the reaction
can proceed both left and right
simultaneously.
Chapter Eighteen: Energy and
Reactions
18.1 Energy and Chemical
Reactions
18.2 Chemical Reaction Systems
18.3 Nuclear Reactions
Chapter 18.3 Learning Goals
Compare and contrast chemical
and nuclear reactions.
Explain the significance of the
strong nuclear force.
Explore benefits and negative
effects of nuclear reactions.
18.3 Chemical vs. Nuclear Reactions
The involvement of
energy in chemical
reactions has to do with
the breaking and forming
of chemical bonds.
A nuclear reaction
involves altering the
number of protons and/or
neutrons in an atom.
18.3 Chemical vs. Nuclear Reactions
For complex reasons,
the nucleus of an atom
becomes unstable if it
contains too many or too
few neutrons relative to
the number of protons.
The forces inside the
atom result in it
breaking apart or
releasing particles.
18.3 Radioactive Decay
This process of
radioactive decay
results in an
unstable,
radioactive isotope
like carbon-14
becoming the more
stable isotope
nitrogen-14.
18.3 Radioactive Decay
 There are three types of
radioactive decay:
1. alpha decay,
2. beta decay, and
3. gamma decay.
18.3 Two types of Nuclear
Reactions
There are two kinds of nuclear
reactions: fusion and fission.
 Nuclear fusion is the process of
combining the nuclei of lighter atoms to
make heavier atoms.
18.3 Fusion
Nuclear fusion occurs in the Sun and
the resulting energy released provides
Earth with heat and light.
18.3 Types of Nuclear Reactions
Nuclear fission is the
process of splitting
the nucleus of an
atom.
A fission reaction
can be started when
a neutron bombards
a nucleus.
18.3 Fission
A nuclear reactor is a power plant that
uses fission to produce heat.
Investigation 18C
Nuclear Reactions
Key Question:
How do we model nuclear reactions?
Your Footprint Matters
Every person can help
address the problem
of global warming by
making changes in
personal habits that
reduce his or her
individual carbon
footprint.