Section 4.1 Understanding Chemical Changes
4.1 Understanding Chemical Changes
Reversible
changes
Physical and chemical change are fundamentally different. For example, consider a
burning candle. Some of the wax in a candle is solid and some is melted. Melting (solid
to liquid) and freezing (liquid to solid) are examples of reversible changes. If you cool
the melted liquid wax down, it becomes solid again. Reversible changes are physical
changes.
Irreversible
changes
What happens inside the candle flame is not just melting, however. Over time, the wax
vanishes! Burning wax is a very different, irreversible kind of change. In the flame, a
chemical reaction changes wax into carbon, water vapor and carbon dioxide, three very
different substances. If you cooled down the smoke from a burning candle, it would not
become wax again. Wax disappears as a candle burns because water vapor and carbon
dioxide are gases that float away into the air.
Physical
changes are
reversible
C20H42, a type of paraffin is a major component of candle wax. Each molecule of
paraffin contains 20 carbon atoms and 42 hydrogen atoms. If you pour the wax into
different shapes, or melt it, or cut it up in tiny pieces, each bit will still be made from
molecules with 20 carbon and 42 hydrogen atoms. That’s because a physical change
leaves the molecules of a substance the same. Physical changes are reversible: melting,
shaping, cutting, bending and freezing are all physical changes.
Chemical
changes are
irreversible
Wax burning in a flame is a chemical change. A
chemical change is a change in the molecules
themselves. In the candle flame, the atoms in paraffin
molecules are rearranged into molecules of water and
carbon dioxide. In burning, wax undergoes an
irreversible change. Chemical changes are irreversible
because they rearrange atoms into different substances.
chemical change - a change that affects the structure or composition of the
molecules that make up a substance, typically turning one substance into another
substance with different physical properties.
irreversible change - is a chemical change that rearranges atoms into different
substances
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Energy and change
Energy
determines
change in
matter
Changes in matter involve an exchange of energy. Whether a change is physical or
chemical fundamentally depends on the amount of energy. If the energy is great enough
to break a molecule apart, then chemical change is possible. If the energy is lower, then
only physical change is possible. To understand why, we have to think about the forces
between and within atoms and molecules. Strong forces take a lot of energy to change;
weaker forces take less energy to change.
Interatomic
forces act
within a
molecule
Consider water as a substance. Interatomic forces hold the two hydrogen atoms
tightly to the one oxygen atom in a single water molecule. Interatomic forces are
relatively strong: they take a lot of energy to break.
Intermolecular
forces act
between
molecules
The fact that water molecules stick together to make ice or liquid means that there are
other forces that act between molecules. These forces are called intermolecular
forces. Physical changes involve only intermolecular forces.
Intermolecular forces are much weaker than interatomic forces
Chemical
changes
involve
interatomic
forces
The graph shows the relationship
between the energy and the types of
changes that occur in water. It takes 333
J of energy to melt 1 gram of solid ice
into liquid water at 0°C. By comparison,
it takes 51,000 J of energy to turn one
gram of water into separated hydrogen
and oxygen gas! To make a chemically
change requires about 100 times more
energy than a physical change.
Chemical
changes can
release energy
The amount of energy needed to break water into hydrogen and oxygen is released if we
let the hydrogen and oxygen come back together again and make water. In fact, that is the
basic principle behind hydrogen fueled cars and trucks! A hydrogen car burns hydrogen
gas along with oxygen from the air and creates harmless water.
interatomic forces - bond atoms together into molecules or ions.
intermolecular forces - act between molecules, typically much weaker than the
forces acting within molecules (interatomic forces).
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Section 4.1 Understanding Chemical Changes
What is NOT a chemical change
Changes in size
or shape are
physical
changes
Any process that changes the shape of a substance is
usually a physical change. That means bending or
deforming are physical changes. It also means
grinding something up into powder is a physical
change. Rock candy, granulated sugar and
confectioners sugar are the same substance, but they
have been ground into different textures.
Mixing and
dissolving are
physical
changes.
Adding food color to water spreads dissolved dye
molecules evenly through the water but does not
change the dye molecules into other molecules. This
is evidence that dissolving is a physical change.
Even vigorous mixing is still a physical change. For
example, mix some corn oil in water and it floats.
Whip it with a beater and the mixture turns cloudy
and white, like milk. Under the microscope however,
you still see a mixture of water and oil. The oil
droplets have become very small, but each droplet
still contains thousands or millions of atoms. Making
bigger drops into smaller drops is definitely a
physical change. Milk and mayonaisse are mixtures
of oils or fats and water.
“Drying” may
be a chemical
or physical
change
Drying is the opposite of dissolving. In drying, the water is removed from a mixture,
leaving any solutes in their dry form. Like dissolving, drying is usually a physical
change. Drying paint for example is not always just a physical change. Certain molecules
in latex or acrylic paint react chemically with oxygen to link together. Dried latex or
acrylic paint is a solid that does not become liquid again when you heat it, or add water
back to it. In the sense of chemistry, “drying” means the purely physical process of
removing liquid without chemical changes.
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A NATURAL APPROACH TO CHEMISTRY
Atoms and chemical bonds
Chemistry is
explained by
the structure
inside atoms
Before we can understand chemical change, we need to learn why nearly all ordinary
matter exists as compounds and not single atoms. Why does one oxygen atom bond with
two hydrogen atoms to make water? Why not three (H3O) or even four (H4O)? Why are
pure hydrogen and oxygen in diatomic molecules (H2, O2) instead of single atoms? The
answers to these questions can be found by looking inside the atom, at the structure
within. The structure of atoms explains the reason for chemical bonds, and the chemical
bonds are the source of chemical energy.
Atoms are not
hard little balls
We draw atoms as hard, colored balls,
but they are not like that at all! A better
mental image of an atom is of an
extremely tiny, hard core surrounded by
a vast, thin cloud. Atoms really have no
definite “edge” or “surface”.
The nucleus
The core is called the nucleus, and it
contains 99.8% of the mass of the atom.
Compared to the size of a whole atom,
the nucleus is extraordinarily tiny. If the
atom were the size of your classroom, the
nucleus would be the size of a single
grain of sand in the center!
Electrons are in
the space
outside the
nucleus
Around the nucleus are tiny particles called electrons. Hydrogen has one electron,
helium has two electrons and lithium has three. The number of electrons corresponds to
the atomic number of the element, as shown on the periodic table. Chapter 5 will explain
more about the structure of atoms. For now, let’s look at the big ideas to begin
understanding chemical bonds.
nucleus - the tiny, dense core of an atom which contains all the positive charge and
99.8% of the mass.
electron - a tiny particle that fills the outer volume of an atom. Electrons have
negative charge and are responsible for chemical bonds.
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Section 4.1 Understanding Chemical Changes
Electric charge
Electric charge
is a property of
matter
Along with mass and volume, matter has a fundamental property called electric
charge. Electric charge is important because it creates both the forces that hold the
atoms together and the forces that cause atoms to combine into compounds and
molecules.
There are only two kinds of electric charge: we call them positive charge, indicated by
the + sign, and negative charge, indicated by the - sign. A positive and a negative
charge attract each other. Two positive charges repel each other. Two negative charges
also repel each other.
Protons have
positive charge
Positive charge is a property of one of the particles in the nucleus called the proton. All
of the protons, and therefore all the positive charge in an atom are in the nucleus. In fact,
the atomic number is defined as the number of protons in the nucleus. Hydrogen has one
proton in its nucleus, Helium has two protons, lithium has three protons and so on.
Electrons are
attracted to
the nucleus
Electrons are bound to the nucleus by the attractive force between
electrons (-) and protons (+). The electrons don’t fall into the
nucleus because of their kinetic energy. Think of the earth orbiting
the sun. Gravity creates a force that pulls the earth toward the sun.
Earth’s kinetic energy causes it to orbit the sun rather than fall
straight in. While electrons don’t really move in orbits, the energy
analogy is approximately right.
Neutral atoms
have zero total
charge
The force between electric charges is extremely strong. The electrical attraction between
a proton and an electron is approximately 1040 times as strong as gravity. This is a ten
with forty zeros after it! The reason you don’t notice electric charge is that atoms are
perfectly neutral. The positive charge of a proton is exactly the same amount as the
negative charge on an electron. A carbon atom has six protons and six electrons. Its total
electric charge is exactly zero because +6 from the protons and -6 from the electrons add
up to zero.
electric charge - a fundamental property of matter than comes in positive and
negative.
positive, negative - the charge on a proton is defined to be positive and the
charge on an electron is defined to be negative.
proton - a tiny particle in the nucleus that has a positive charge.
neutral - an atom or molecule is neutral when it has zero total electric charge.
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A NATURAL APPROACH TO CHEMISTRY
Covalent bonds
Elements and
electrons
Each element has a different number of electrons
and a unique way that the electrons are arranged
around the nucleus. This different number and
arrangement of electrons is what creates each
element’s unique chemical properties. Hydrogen
has a single electron, and that single electron is
what makes hydrogen combine with oxygen in a
two-to-one ratio to make H2O. Nearly all the
elements readily form chemical bonds. This is
why most of the matter you experience is in the
form of compounds.
Electrons form
chemical
bonds
Chemical bonds are created by electrons. Two atoms that are sharing one or more
electrons are chemically bonded and move together. In a water molecule, each hydrogen
atom shares its single electron with the oxygen atom at the center.
A chemical bond is formed by sharing or transferring electrons
Covalent bonds
The strongest chemical bond is called a covalent bond. A covalent bond is formed
when two atoms share a single electron. A water molecule contains two covalent bonds
between oxygen and hydrogen. Each bond represents one electron. In a covalent bond,
electrons are shared between atoms, not transferred.
Molecules
An electrically neutral group of atoms held together by covalent bonds is called a
molecule. Water is a molecule, and so is glucose. Other examples of molecules are
methane (CH4), ammonia (NH3), and carbon dioxide (CO2).
Some elements can share multiple electrons with the same atom. Good examples are
oxygen (O2) and nitrogen (N2). An oxygen molecule contains a double covalent bond (2
shared electrons). A nitrogen molecule has a triple covalent bond (3 shared electrons). In
diagrams, double and triple bonds are represented as double and triple lines connecting
the atoms.
chemical bond - a relatively strong connection between two atoms.
covalent bond - a chemical bond that consists of one shared electron.
molecule - a neutral group of atoms that are covalently bonded together.
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Section 4.1 Understanding Chemical Changes
Why do chemical bonds form?
Bonds give an
energy
advantage to
atoms
Chemical bonds form when attractive forces create an advantage in energy. For example,
think about two strong magnets as they are brought near each other. When they get close
enough the magnets snap quickly together. Clearly, a force pulls the magnets together,
but there is another way to look at the situation. The total energy of the two magnets is
lower when they are together compared to when they are apart.
Energy is
released when
bonds form
If you think about pulling the magnets apart, you need a force and it takes energy to
create forces. If it takes energy to pull them apart, the same energy is released when the
magnets come together. In fact, the force that pulls the magnets together is created by the
energy difference between being apart and being together. The same is true of chemical
bonds. Energy is released when chemical bonds form. Energy is released because
chemically bonded atoms have less total energy than free atoms.
Atoms form
bonds to reach
a lower energy
state
A general principle of chemistry is that
atoms arrange themselves so they have the
lowest possible energy. Like a ball rolling
downhill, atoms form compounds because
the atoms have lower energy when they are
together in compounds compared to when
they are separate. Consider water for
example, one oxygen and two hydrogen
atoms have more total energy apart than
they do when combined in a water
molecule.
The enthalphy
of formation
When hydrogen and oxygen combine to
make water, 285,000 joules of energy is released for every mole of water molecules
created. This energy is called the enthalpy of formation shown as ΔHf. Table 4.1 lists
the enthalpy of formation for water as negative because energy is given off instead of
absorbed. By convention, pure elements are assigned an energy of zero.
TABLE 4.1. Enthalpy of formation (ΔHf) for some common substances
Substance
ΔHf (kJ/mol)
Substance
ΔHf (kJ/mol)
Hydrogen (H2)
0
H2O
-285.5
Carbon (C)
0
-393.5
Oxygen (O2)
0
CO2
CH4
Nitrogen (N2)
0
C6H12O6
-1266.5
-74.6
enthalpy of formation - the change in energy when one mole of a compound is
assembled from pure elements.
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A NATURAL APPROACH TO CHEMISTRY
Ionic bonds
An ion is a
charged atom
Not all compounds are made of molecules. For example, sodium chloride (NaCl) is a
compound of sodium (Na) and chlorine (Cl) in a ratio of one sodium atom per chlorine
atom. The difference is that in sodium chloride, the electron is essentially transferred
from the sodium atom to the chlorine atom. When atoms gain or lose an electron they
become ions. An ion is an atom which either lost one or more electrons or gained one or
more electrons. By losing an electron, the sodium atom becomes a sodium ion with a
charge of +1. By gaining an electron, the chlorine atom becomes a chloride ion with a
charge of -1 (when chlorine becomes an ion, the name changes to chloride).
An ionic bond forms when one or more electrons is transferred from one
atom to another
Ionic bonds
Sodium and chlorine form an ionic bond because the positive sodium ion is attracted to
the negative chloride ion. Ionic bonds are bonds in which electrons are transferred from
one atom to another. In general, ionic bonds are slightly weaker than covalent bonds.
Ionic
compounds do
not form
molecules
Ionic bonds are not limited to a single pair of
atoms like covalent bonds. In sodium
chloride, each positive sodium ion is attracted
to all of the neighboring chloride ions.
Likewise, each chloride ion is attracted to all
the neighboring sodium atoms. Since the
bonds are not just between pairs of atoms,
ionic compounds do not form molecules! In
an ionic compound, each atom bonds with all
of its neighbors through attraction between
positive and negative charge.
The chemical
formula for
ionic
compounds
The chemical formula for an ionic compound like salt is used in the exact same way as
the formula for a molecular compound like water. The chemical formula for salt (NaCl)
means that there is one sodium atom per chlorine atom. You calculate the formula mass
of salt (58.5 g/mole) by adding the atomic masses of sodium (23.0 g/mole) and chlorine
(35.5 g/mole).
Ions may be
multiply
charged
Sodium chloride involves the transfer of one electron. However, ionic compounds may
also be formed by the transfer of two or more electrons. A good example is magnesium
chloride (MgCl2). The magnesium atom gives up two electrons to become a magnesium
ion with a charge of +2 (Mg2+). Each chlorine atom gains one electron to become a
chloride ion with a charge of -1 (Cl-). The ion charge is written as a superscript after the
element (Mg2+, Fe3+,Cl-, etc.).
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Section 4.1 Understanding Chemical Changes
Reactivity
Elements that
react strongly
are rarely found
in pure form
Some elements react so strongly that they are never
found in nature by themselves as pure elements.
Good examples are lithium (Li), sodium (Na), and
potassium (K). These are very common elements,
but are always in compounds such as salt (NaCl),
minerals in rocks such as petalite (LiAlSi4O10) and
even chemicals in your body.
Some elements
do not react
Other elements such as helium, neon and argon are always found as pure elements. These
elements are not reactive and are called noble gases. The noble gases are not very
reactive due to their electronic structure. Noble gases do not form chemical bonds with
other elements.
Reactivity and
the periodic
table
What does it mean for an element to be “reactive?” In a literal sense it means that it
forms bonds with anything it touches! If you look at the periodic table, you see that the
three most reactive metals (Li, Na, K) are in the same group (column). The unreactive
elements (He, Ne, Ar, Kr, Xe) are also in a group, but on the far right of the table.
The halogens
are reactive
elements
To the immediate left of the noble gases is another group of very reactive elements - the
halogens which include fluorine (F), chlorine (Cl) and bromine (Br).
If we look at the two example compounds of salt (NaCl) and the mineral petalite
(LiAlSi4O10) we see an important pattern. Sodium and lithium from the left of the
periodic table are bonded with elements from the right half of the periodic table: chlorine,
aluminum, oxygen and silicon.
Elements on the far left and far right of the periodic table are more likely
to form chemical bonds
Reactivity and
the periodic
table
The more reactive elements in the periodic table tend to form chemical bonds more
easily. For example, sodium readily bonds with chlorine and the reaction releases a lot of
energy. Sodium and chlorine are on opposite sides of the periodic table.
reactivity - the tendency of elements to form chemical bonds. A reactive element
forms bonds easily therefore tends to have many reactions.
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A NATURAL APPROACH TO CHEMISTRY
Reactivity and the periodic table
The two main classes of chemicals that we work with in general chemistry are ionic and
molecular compounds. The periodic table helps us tell the difference between these two
types of compounds.
Ionic
compounds
contain a
metal and a
nonmetal
Ionic compounds, which are often referred to as salts, form between metals and nonmetals. You have learned that ionic compounds transfer electrons. In the case of an ionic
compound the metal loses the electron(s) and the nonmetal(s) gains the electrons. Lets
look at the periodic table below to see how this pattern works.
When dealing with a chemical compound, you can look at the periodic table and see if
there is a metal bonded to a nonmetal; if so, you can classify the compound as ionic.
Molecular
compounds
contain two or
more
nonmetals
Molecular compounds are made up of two or more nonmetals bonded together. You can
see from the periodic table that these elements appear on the far right upper corner.
Molecular compounds share electrons. For example carbon dioxide, CO2, is made of
carbon and oxygen and both are nonmetals.
Use the periodic table to help determine whether a compound is ionic or
molecular
Hydrogen (H) is ambidextrous and has metallic and nonmetallic behavior. The fact that
it only has one electron and one proton makes hydrogen small in size. This is why it is a
unique element on the periodic table. In most compounds, hydrogen shares it’s one
electron and forms a molecular compound. This is because hydrogen holds it’s one
electron very tightly to the nucleus.
Is the compound CF4 ionic or molecular? We start by locating carbon (C) and fluorine
(F) on the periodic table, and find that they are both nonmetals. This indicates that CF4 is
a molecule! We know it shares its electrons in the bonds it makes.
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