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HIGH SCHOOL SCIENCE
Physical
Science 12:
Chemical
Reactions
WILLMAR PUBLIC SCHOOL
2013-2014 EDITION
C HAPTER 12
Chemical
Reactions
In this chapter you will:
1. Interpret and balance chemical equations.
2. Describe how to convert between moles and mass.
3. Classify chemical reactions as synthesis, decomposition, single-replacement, double-replacement,
or combustion / endothermic and exothermic.
4. Describe oxidation-reduction reactions.
5. Describe the energy changes that take place and
that energy is conserved during chemical reactions.
6. Describe & classify nuclear decay and radiation.
7. Identify sources of nuclear radiation and describe
how nuclear radiation affects matter.
8.Describe methods of detecting nuclear radiation.
9. Describe half-life and how radioisotopes are used
to estimate the age of materials.
10.Compare and contrast nuclear forces.
11.Describe nuclear fission and nuclear fusion.
S ECTION 12.1
What are Chemical
Reactions?
A chemical reaction is a process in which some substances
change into different substances. A useful description of a
chemical reaction tells you the substances present before and
after the reaction. Substances that start a chemical reaction
are called reactants. Substances that are produced in the
reaction are called products. Reactants and products can be
elements or compounds.
O BJECTIVES :
1. Interpret and balance chemical equations.
2. Describe how to convert between moles and
mass.
Vocabulary:
chemical reaction
reactants
products
chemical equation
law of conservation of mass
mole
Does the term chemical reaction bring to mind a chemist
mixing chemicals in a lab. Many chemical reactions take place
in labs. However, most chemical reactions do not. Where do
they occur? They happen in the world all around you. They
even happen inside your own body. In fact, you are alive only
because of the many chemical reactions that constantly take
place inside your cells.
The reactants and products in a chemical reaction contain the
same atoms, but they are rearranged during the reaction. As a
result, the atoms are in different combinations in the products
than they were in the reactants. This happens because
chemical bonds break in the reactants and new chemical
bonds form in the products. Chemical reactions may occur
quickly or slowly.
Chemical reactions are represented by chemical equations,
like the one below, in which reactants (on the left) are
connected by an arrow to products (on the right).
Reactants
Products
molar mass
2
The arrow (
) shows the direction in which the reaction
occurs. In many reactions, the reaction also occurs in the
opposite direction. This is represented with another arrow
pointing in the opposite direction (
). A chemical
equation is a representation of a chemical reaction in which
the reactants and products are expressed as formulas.
2H2 + O2
2H2O
This chemical equation represents a chemical reaction. In the
reaction, two molecules of hydrogen gas combine one
molecule of oxygen gas to produce two molecules of water.
In order to show that mass is conserved during a reaction, a
chemical equation must be balanced. A coefficient is a
number placed in front of a chemical symbol or formula that
shows how many atoms or molecules of the substance are
involved in the reaction.
Balancing a chemical equation involves a certain amount of
trial and error. In general, however, you should follow these
steps:
1. Count each type of atom in reactants and products. Does
the same number of each atom appear on both sides of the
arrow? If not, the equation is not balanced, and you need to
go to step 2.
2. Place coefficients, as needed, in front of the symbols or
formulas to increase the number of atoms or molecules of
the substances. Use the smallest coefficients possible.
Warning! Never change the subscripts in chemical
formulas. Changing subscripts changes the substances
involved in the reaction. Change only the coefficients.
All chemical equations, like equations in math, must balance.
This means that there must be the same number of each type
of atom on both sides of the arrow. That’s because matter is
always conserved in a chemical reaction. This is the law of
conservation of mass.
The law of conservation of mass states that mass is
neither created nor destroyed in a chemical reaction.
3. Repeat steps 1 and 2 until the equation is balanced.
Chemists need to practical units for counting things.
Although you can describe a reaction in terms of atoms and
molecules, these units are too small to be practical. Chemists
use a counting unit called the mole to measure amounts of a
substance. A mole (mol) is an amount of a substance that
contains approximately 6.02 x 1023 particles of that
substance.
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A dozen eggs has a different mass than a dozen oranges.
Similarly, a mole of carbon has a different mass than a mole of
sulfur. The mass of one mole of a substance is called molar
mass. For an element, the molar mass is the same as its
atomic mass expressed in grams. For a compound, you can
calculate the molar mass by adding up the atomic masses of
its component atoms, and then express this sum in grams.
Section Review:
1.What does a useful description of a chemical reaction tell
you?
2.Do chemical reactants and products contain the same
atoms in a chemical reaction?
Once you know the molar mass of a substance, you can
convert moles into mass, or mass into moles. For either
calculation, you need to express the molar mass as a
conversion factor. The multiple either the moles or mass with
the conversion factor.
3.How do the atoms form different combinations in the
products from the reactants?
For example, the molar mass of CO2 is 44.0 grams.
6.Why must a chemical equation be balanced?
(44.0 g CO2)/(1 mol CO2)
7.Write a balanced equation for the reaction between
copper and oxygen to produce copper (II) oxide.
or
(1 mol CO2)/( 44.0 gCO2)
Suppose you have 55 g of CO2. To calculate how many moles
of CO2 you have, multiply the mass by the second conversion
factor.
(55g CO2)*(1 mol CO2)/(44.0 g CO2)= 1.25 mol CO2
4.How are chemical reactions represented?
5.What do the arrows show?
Cu+
O2
CuO
8.Which of the following chemical equations is balanced?
a) Zn+HCl
b) 2Zn+2HCl
ZnCl2 +H2
ZnCl2 +H2
c) 2Zn+HCl
ZnCl2 +H2
d) Zn+2HCl
ZnCl2 +H2
9.How do you determine molar mass for an element?
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Section Review Continued:
10. How do you determine molar mass for a compound?
11. How do you convert moles into mass?
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S ECTION 12.2
The type of reactant or the number of reactants and products
often classifies reactions. Some general types of chemical
reactions are synthesis, decomposition, single replacement,
double replacement, and combustion.
Types of Reactions
O BJECTIVES :
Types of Chemical Reactions
1. Classify chemical reactions as synthesis,
decomposition, single-replacement, doublereplacement, or combustion / endothermic
and exothermic.
Type of
Reaction
General Equation
Example
Synthesis
A+BèC
2Na + Cl2 è 2NaCl
Decomposition
ABèA+B
2H2O è2H2 + O2
2. Describe oxidation-reduction reactions.
Single
Replacement
A+BCèB+AC
2K+2H2Oè2KOH+H2
3. Describe the energy changes that take place
and that energy is conserved during chemical
reactions.
Double
AB+CDèAD+CB
NaCl+AgF èNaF+AgCl
fuel + oxygen è
CH4 +2O2 èCO2
carbon dioxide +
+2H2O
Replacement
Combustion
Vocabulary:
synthesis reaction
water
decomposition reaction
single replacement reaction
combustion
double replacement reaction
oxidation
oxidation-reduction reaction
reduction
chemical energy
exothermic reaction
endothermic reaction
activation energy
law of conservation of energy
A synthesis reaction occurs when two or more reactants
combine to form a single product. An example of a synthesis
reaction is the combination of sodium (Na) and chlorine (Cl)
to produce sodium chloride (NaCl). This reaction is
represented by the chemical equation:
2Na + Cl2
2NaCl
Sodium is a highly reactive metal, and chlorine is a poisonous
gas. The compound they synthesize has very different
properties. Sodium chloride is commonly called table salt,
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which is neither reactive nor poisonous. In fact, salt is a
necessary component of the human diet.
A decomposition reaction occurs when one reactant
breaks down into two or more products. Carbonic acid
(H2CO3) is an ingredient in soft drinks. A decomposition
reaction takes place when carbonic acid breaks down to
produce water (H2O) and carbon dioxide (CO2). This occurs
when you open a can of soft drink and some of the carbon
dioxide fizzes out. The equation for this reaction is:
H2CO3
H2O + CO2
Another decomposition reaction occurs when water (H2O)
breaks down to produce hydrogen (H2) and oxygen (O2)
gases. The equation for this reaction is:
2H2O
2H2 + O2
This happens when an electric current passes through the
water.
A replacement reaction occurs when elements switch places in
compounds. A single replacement reaction occurs when
one element replaces another in a single compound. An
example of a single replacement reaction occurs when
potassium (K) reacts with water (H2O). A colorless solid
compound named potassium hydroxide (KOH) forms, and
hydrogen gas (H2) is set free. The equation for the reaction is:
2K + H2O
In this reaction, a potassium ion replaces one of the hydrogen
atoms in each molecule of water. Potassium is a highly
reactive group 1 alkali metal, so its reaction with water is
explosive.
A double replacement reaction occurs when two ionic
compounds exchange ions.An example of a double
replacement reaction is sodium chloride (NaCl) reacting with
silver fluoride (AgF). This
reaction is represented by the equation:
NaCl+AgF
NaF+AgCl
During the reaction, chloride and fluoride ions change places,
so two new compounds are formed in the products: sodium
fluoride (NaF) and silver chloride (AgCl).
A combustion reaction occurs when a substance reacts
quickly with oxygen (O2). Combustion is commonly called
burning, and the substance that burns is usually referred to as
fuel. The fuel that burns in a combustion reaction contains
compounds called hydrocarbons. Hydrocarbons are
compounds that contain only carbon (C) and hydrogen (H).
The charcoal pictured above consists of hydrocarbons. So do
fossil fuels such as natural gas. The main component of
natural gas is the hydrocarbon called methane (CH4). The
combustion of methane is represented by the equation:
CH4 +2O2
CO2 +2H2O
2KOH + H2
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Natural gas is a fuel that is commonly used in home furnaces
and gas stoves.
As scientists learned more about the structure of the atom,
they found different ways to describe how reactions take
place. The discovery of subatomic particles enabled scientists
to classify certain chemical reactions as transfers of electrons
between atoms. A reaction in which electrons are transferred
from one reactant to another is called oxidation-reduction
reaction, or redox reaction.
A type of synthesis reaction in which a metal combines with
oxygen, traditionally have been classified as oxidations.
However, today we say that any process in which an element
loses electrons during a chemical reaction is called
oxidation. The process in which an element gains electrons
during a chemical reaction is called reduction. Oxidation
and reduction always occur together. When one element loses
electrons, another must gain electrons.
All chemical reactions involve energy. Chemical energy is
the energy stored in the chemical bonds of a substance.
Energy is used to break bonds in reactants, and energy is
released when new bonds form in products. In terms of
energy, there are two types of chemical reactions:
endothermic reactions and exothermic reactions.
In exothermic reactions, more energy is released when
bonds form in products than is used to break bonds in
reactants. These reactions release energy to the environment,
often in the form of heat or light. All combustion reactions
are exothermic reactions. During combustion, a substance
burns as it combines with oxygen, releasing energy in the
form of heat and light.
An endothermic reaction is a chemical reaction in which
more energy is needed to break bonds in the reactants than is
released when new bonds form in the products. A constant
input of energy, often in the form of heat, is needed to keep an
endothermic reaction going. One of the most important series
of endothermic reactions is photosynthesis. The energy
needed for photosynthesis comes from light.
Chemical reactions also need energy to be activated. They
require a certain amount of energy just to get started. This
energy is called activation energy. Turning the key causes a
spark that activates the burning of gasoline in the engine. The
combustion of gas won’t occur without the spark of energy to
begin the reaction. You have probably used activation energy
to start a chemical reaction. For example, if you’ve ever struck
a match to light it, then you provided the activation energy
needed to start a combustion reaction. When you struck the
match on the box, the friction started the match head burning.
Combustion is exothermic. Once a match starts to burn, it
releases enough energy to activate the next reaction, and the
next, and so on. However, the match won’t burst into flames
on its own.
Whether a chemical reaction absorbs or releases energy, there
is no overall change in the amount of energy during the
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reaction. That’s because energy cannot be created or
destroyed. This is the law of conservation of energy.
Section Review:
1.Identify these reactions as synthesis, decomposition,
single replacement, double replacement, or combustion.
a)Pb(NO3)2 + 2HCl
b)2C2H6 + 7O2
c)Ca + 2HCl
d)2SO2 + O2
a)CaCO3
PbCl2 + 2HNO3
4CO2 + 6H2O
CaCl2 + H2
2 SO3
CaO + CO2
2. Why are oxidation and reduction always together?
3. Contrast exothermic and endothermic chemical
reactions.
4. Give an example when you have used activation energy
to start a chemical reaction.
5. Why is the amount of energy the same before and after a
chemical reaction?
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S ECTION 12.3
Nuclear Chemistry
O BJECTIVES :
1.
Describe & classify nuclear decay and radiation.
2.
Identify sources of nuclear radiation and describe how
nuclear radiation affects matter.
3.
Describe methods of detecting nuclear radiation.
4.
Describe half-life and how radioisotopes are used to
estimate the age of materials.
5.
Compare and contrast nuclear forces.
6.
Describe nuclear fission and nuclear fusion.
Vocabulary:
radioactivity
radiation
radioactive isotope
nuclear radiation
alpha particle
beta particle
gamma ray
background radiation
half-life
radioactive dating
weak nuclear force
strong nuclear force
nuclear fission
uncontrolled nuclear fission
nuclear fusion
For an atom of one element to change into a different
element, the number of protons in its nucleus must change.
That’s because each element has a unique number of protons.
For example, lead atoms always have 82 protons, and gold
atoms always have 79 protons. Alchemists, who lived during
the Middle Ages, were people who strived to turn lead into
gold. They tried all sorts of chemical reactions involving lead,
but they were never able to produce gold. Today, scientists
know that one element cannot be changed into another by
chemical processes.
Radioactivity is the ability of an atom to emit, or give off,
charged particles and energy from its nucleus. The charged
particles and energy are called by the general term radiation.
Only unstable nuclei emit radiation. They are unstable
because they have too much energy, too many protons, or an
unstable ratio of protons to neutrons. For example, all
elements with more than 83 protons—such as uranium,
radium, and polonium—have unstable nuclei. They are called
radioactive isotope or radioisotope. The nuclei of these
elements must lose protons to become more stable. When
they do, they become different elements. Radioactivity was
discovered in 1896 by Antoine Henri Becquerel when he
found that uranium leaves an image like an X-ray on a
photographic plate. Besides uranium, radioactive elements
include radium and polonium, both of which were discovered
by Marie Curie.
Scientists can detect a radioactive substance by measuring the
nuclear radiation it gives off. Nuclear radiation is charged
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particles and energy that are emitted from the nuclei of
radioisotopes. Common types of nuclear radiation include
alpha particles, beta particles, and gamma rays.
An alpha particle is a positively charged particle made up of
two protons and two neutrons. Alpha particles are the least
penetrating type of nuclear radiation. Most alpha particles
travel no more than a few centimeters in air, and can be
stopped by a sheet of paper or clothing.
A beta particle is an electron emitted by an unstable
nucleus. Due to their smaller mass and faster speed, beta
particles are more penetrating than alpha particles. Beta
particles pass through paper, but can be stopped by a thin
sheet of metal.
Not all nuclear radiation consists of charged particles. A
gamma ray is a penetrating ray of energy emitted by an
unstable nucleus. Gamma radiation has no mass or charge.
Gamma rays are much more penetrating than either alpha
particles or beta particles. It can take several centimeters of
lead or several meters of concrete to stop gamma radiation.
You may not know it but you are exposed to nuclear radiation
every day. Most of this is background radiation, or
nuclear radiation that occurs naturally in the environment.
Radioisotopes in air, water, rocks, plants, and animals all
contribute to background radiation. Cosmic rays also
contribute to background radiation.
When nuclear radiation exceeds background levels, it can
damage the cells and tissues of your body. When cells are
exposed to nuclear radiation, the bonds holding together
protein and DNA molecules may break. As these molecules
change, the cell may no longer function properly.
Although you cannot see, hear, or feel the radioactivity around
you, scientific instruments can measure nuclear radiation.
Devices that are used to detect nuclear radiation include
Geiger counters and film badges.
Despite its dangers, radioactivity has several uses. For
example, it can be used to determine the ages of ancient rocks
and fossils. It can also be used as a source of power to
generate electricity. Radioactivity can even be used to
diagnose and treat diseases, including cancer.
A radioisotope decays and changes to a different element at a
constant rate. The rate is measured in a unit called the halflife. Half-life is the length of time it takes for half of a given
amount of the radioisotope to decay. This rate is always the
same for a given radioisotope, regardless of temperature,
pressure, or other conditions outside the nuclei of its atoms.
Different radioisotopes may vary greatly in their rate of decay.
That’s because they vary in how unstable their nuclei are. The
more unstable the nuclei, the faster they break down.
The age of a rock or other specimen can be estimated from the
remaining amount of a radioisotope it contains and the
radioisotope’s known rate of decay, or half-life. This method
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Radioac've Isotope
Half-­‐Life
Rubidium-­‐87
48.8 Billion Years
Uranium-­‐238
4.468 Billion Years
Potassium-­‐40
1.26 Billion Years
Uranium-­‐235
703.8 Million Years
Carbon-­‐14
5730 Years
Thorium-­‐234
24.1 Days
Radon-­‐222
3.82 Days
of dating specimens is called radioactive dating.
Radioisotopes with longer half-lives are used to date older
specimens, and those with shorter half-lives are used to date
younger ones. Carbon-14 dating is used to date specimens
younger than about 60,000 years old. It is commonly used to
date fossils of living things and human artifacts.
What holds the nucleus together? Remember that the protons
in the nucleus are positively charged, so they tend to repel one
another.
The strong nuclear force is a powerful attractive force that
binds protons and neutrons together in the nucleus. The
range over which the strong nuclear forces acts is
approximately equal to the diameter of a proton. Although
this force acts over only extremely short distance, it is 100
times stronger than the electric force of repulsion at these
distances.
The other powerful force in the nucleus is the weak nuclear
force. As the name implies, the weak force is weaker in
strength than the strong nuclear force. The weak nuclear
force is an attractive force that acts only over a short range
and affects all particles, not just protons and neutrons.
Einstein’s equation, E = mc2, shows that matter and energy
are two forms of the same thing. It also shows that there is a
tremendous amount of energy (E) in a small mass (m) of
matter. In nuclear reactions, matter changes to energy, but
the total amount of mass and energy together does not
change.
Nuclear fission is the splitting of the nucleus of an atom
into two smaller nuclei. This type of reaction releases a great
deal of energy from a very small amount of matter. It begins
when the nucleus of a radioactive atom gains a neutron. In
uncontrolled nuclear fission, one fission reaction starts a
chain reaction, in which neutrons produced in one reaction
cause other reactions, which cause more reactions, and so on.
Energy released by nuclear fission is used to produce
electrical energy in nuclear power plants. Production of
nuclear energy doesn’t produce air pollution but it poses the
risk of accidents that release harmful radiation.
In nuclear fusion, two or more small nuclei combine to
form a single larger nucleus, a neutron, and a tremendous
amount of energy. Nuclear fusion of hydrogen to form
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helium occurs naturally in the sun and other stars. It takes
place only at extremely high temperatures. Scientists are
searching for ways to create controlled nuclear fusion
reactions in order to produce safe nuclear power. Fusion
involves only harmless, plentiful elements but requires
extremely high temperatures.
Section Review:
1.Why are some nuclei unstable?
2.List the types of nuclear radiation from least penetrating
to most penetrating.
3.What contributes to background radiation?
4.How can radiation damage cells?
5.What devices are used to detect nuclear radiation?
6.How can radioactivity be useful?
7.Why do some radioisotopes decay at a faster rate?
8.Which radioactive isotope would be used to date a rock
from about 10,000 years ago?
9.What holds the nucleus together?
10.What changes in a nuclear reaction?
11.What is uncontrolled nuclear fission?
12.What is an advantage of nuclear fission?
13.What are advantages and disadvantages to nuclear
fusion?
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