Chapter 2: Compounds and
Chemical Reactions
• Essentially all elements combine to form
compounds
• Compounds are of two types:
– Molecular, which involve shared electrons and
consist of electrically neutral, discrete particles
called molecules
– Ionic compounds, which involve electron
transfer and charged particles called ions
• Chemical formulas are collections of
chemical symbols that are used to describe
elements and compounds
– Free elements are not combined with other
elements in a compound
• Examples: Fe (iron), Na (sodium), and K
(potassium)
– Many nonmetals occur as diatomic molecules
• Chemical formulas specify the composition
of a substance
• NaCl is composed of the elements sodium and
chlorine in a one-to-one (atom) ratio
• Fe2O3 is composed of the elements iron and oxygen
in a two-to-three ratio
• CO(NH2)2 expands to CON2H4, but there are good
reasons to write some compounds with parentheses
• Hydrates are crystals that contain water molecules,
for example plaster: CaSO4 • 2H2O
– When all the water is removed (by heating), the solid that
remains is said to be anhydrous (without water)
• Chemical equations describe what happens
in a chemical reactions
• Hydrogen and oxygen combine to form
water
• Hydrogen and oxygen are called reactants
• Water is called the product
• Reactants are separated from products with “”
2 H2 + O2  2 H2O
• Note that the “” is like an equal sign because both
sides of the equation have the same number of each
type of atom
• This can be represented as:
Note: Mass is conserved because the
number of atoms of each type remains
the same on each side of the arrow.
(Both sides of the arrow show 4 H and
2 O atoms.) This equation is said to be
balanced.
The “2” in front of formulas H2 and H2O are called
coefficients. They indicate the number of molecules of each
type and can change when balancing a chemical equation.
The “2” in the formulas H2 and H2O indicate atom ratios for
the compound and must not change.
• It is sometimes useful to include the
physical state of reactants and products
• For solids use s, liquids use l, gases use g, and for
aqueous solutions use aq.
• For example, the reaction between stomach
acid (an aqueous solution of HCl) and
sodium carbonate (an antacid) can be
written
2 HCl(aq) + CaCO3(s)  CaCl2(aq) + H2O(l) +CO2(g)
Note: You can verify this equation is balanced by checking for
mass balance: each side shows 1 Ca, 1 C, 2 Cl, 2 H, and 3 O.
• Almost all chemical reactions either absorb
or give off energy, often as heat or light
• Kinetic and potential energy are both
important in chemistry
– Kinetic energy is the energy an object has when
moving
– Potential energy is the energy an object has due
to its position
• Potential energy is “stored energy” because
it can be converted into kinetic energy
• Energy must also be conserved
– The Law of Conservation of Energy:
• Energy cannot be created or destroyed; it can only
be converted from one form to another
• Heat and temperature are related to kinetic
energy
• The temperature of an object is proportional to its
average kinetic energy (average speed of its atoms)
• Heat or thermal energy is transferred between
objects with different temperatures
• Heat flow spontaneously from hot to cold objects
• Chemical energy is a form of potential
energy
• The analysis of temperature changes in
chemical reactions can provide information
about the potential energy changes that
occur
– The kinetic molecular theory of matter
provides more details about chemical energy
changes and is discussed in Chapter 7
– Energy can also be transferred as light, which
will be covered later in the book
• As a general rule, molecular compounds are
formed when nonmetallic elements combine
• Many molecular compounds contain
hydrogen:
Period IVA
2
CH4
3
SiH4
4
GeH4
5
Group
VA
VIA
NH3
H2O
PH3
H2S
AsH3
H2Se
SbH3
H2Te
VIIA
HF
HCl
HBr
HI
Noble
Gas
Ne
Ar
Kr
Xe
Note: The number of hydrogens that combined with the nonmetal
equals the number of spaces to the right we have to move to get to
the noble gas
• Organic chemistry is a major specialty that
deals with compounds containing mostly
carbon and hydrogen
• Hydrocarbons contain only hydrogen and
carbon and are organic compounds
• Alkanes are the simplest hydrocarbons
– General formula is CnH2n+2
Space-filling models:
Black atoms = carbon
White atoms = hydrogen
• Other classes of hydrocarbons exist
• Different classes of organic compounds are
derived from hydrocarbons by replacing
hydrogen
• For example alcohols result when a H is
replaced by OH in a hydrocarbon
Methanol (wood alcohol),
CH3OH, is related to methane,
CH4, by replacing one H with
OH.
• Inorganic compounds are substances not
considered to be derived from hydrocarbons
• The rules for naming, or nomenclature, of
simple inorganic compound is covered now
(organic nomenclature is covered later)
• Binary compounds are compounds
comprised of two different elements
• The goal is to be able to convert between
the chemical formula and the name
• The first element in the formula is identified
by its English name, the second by
appending the suffix –ide to its stem
Chemical
Symbol
O
S
N
P
F
Cl
Br
I
Name as
Stem
First Element
oxoxygen
sulfsulfur
nitrnitrogen
phosph- phosphorus
fluorfluorine
chlorchlorine
brombromine
iodiodine
Name as
Second Element
oxide
sulfide
nitride
phosphide
fluoride
chloride
bromide
iodide
• The number of each type of atom is
specified with Greek prefixes
Greek Prefixes
mono- = 1 (often omitted)
di=2
tri=3
tetra- = 4
penta- = 5
hexaheptaoctanonadeca-
Examples:
PF5
= phosphorus pentafluoride
HCl = hydrogen chloride
N2O5 = dinitrogen tetraoxide
=6
=7
= 8
= 9
= 10
Note: many
compounds have
common names, like
water for H2O.
• The subscripts in the formula of an ionic
compound always specifies the smallest
whole-number ratio of the ions because
molecules don’t exist in ionic compounds
• The smallest unit of a compound is called
the formula unit
• Positively charged ions have more protons than
electrons and are called cations
• Negatively charged ions have more electrons than
protons and are called anions
• The formula unit of an ionic compound always
contains both cations and anions
• Ionic compounds are composed of charged
particles (ions)
– Ions can be formed from the reaction of metal
with a nonmetal
– The metals form cations and the nonmetals
form anions
• The charges on many representative
elements can be predicted:
– Metals form cations
• The positive charge on the cation is the same as the
“A” group number of the metal
– Nonmetals form anions
• The negative charge on the anion is equal to the
number of spaces to the right we have to move in
the periodic table to get to a noble
• Ionic compounds must be electrically
neutral
• Rules for writing Formulas of Ionic
Compounds:
1) The positive ion is given first in the formula.
2) The subscripts in the formula must produce an
electrically neutral formula unit.
3) The subscripts should be the set of smallest
whole numbers possible.
4) The charges on the ions are not included in the
finished formula of the substance.
• Ions formed by transition metals (Group
IIIB – VIIIB) and post-transition metals:
Transition Metals
Chromium
Cr2+, Cr3+
Manganese
Mn2+, Mn3+
Iron
Fe2+, Fe3+
Cobalt
Co2+, Co3+
Nickel
Ni2+
Copper
Cu+, Cu2+
Post-transition Metals
Tin
Sn2+, Sn4+
Lead
Pb2+, Pb4+
Bismuth
Bi3+
Zinc
Silver
Cadmium
Gold
Mercury
Zn2+
Ag+
Cd2+
Au+, Au3+
Hg22+, Hg2+
• Some polyatomic ions (ions with two or
more atoms):
Ion
NH4+
OHNO2NO3ClO2ClO3PO43-
Name
ammonium ion
hydroxide ion
nitrite ion
nitrate ion
chlorite ion
chlorate ion
phosphate ion
Ion
CO32H3O+
SO32SO42CrO42Cr2O72-
Name
carbonate ion
hydronium ion
sulfite ion
sulfate ion
chromate ion
dichromate ion
See Table 2.5 for a more polyatomic ions
• Naming ionic compounds
– The name of the cation is given first followed by
the name of the anion
• Cations:
– If the metal forms only one positive ion, the cation name is
the English name for the metal
– If the metal forms more than one positive ion, the cation
name is the English name followed, without a space, by the
numerical value of the charge written as a Roman numeral
in parentheses (this is for the Stock system)
• Anions:
– For monoatomic anions, the name is created by adding the
“–ide” suffix to the stem name for the element.
– For polyatomic ions, use the names in Table 2.5
• To name a compound, you can use this
flowchart:
• Summary of Properties
– Hardness and brittleness
• Molecular compounds tend to be soft and easily
crushed because the attractions between molecules
are weak and molecules can slide past each other
• Ionic compounds are hard and brittle because of the
strong attractions and repulsions between ions
• Melting points
– To melt the particles in the solid must have
sufficient kinetic energy to overcome the
attractions between particles
• Molecular compounds tend to have weak attractions
between particles and so tend to have low melting
points
– Many molecular compounds are gases at room
temperature
• Ionic compound tend to have strong attractions so
they have high melting points
– Nearly all ionic compounds are solids at room temperature
• Electrical conductivity requires the
movement of electrical charge
• Ionic compounds:
– Do not conduct electricity in the solid state
– Do conduct electricity in the liquid state
• The ions are free to move in the liquid state
• Molecular compounds:
– Do not conduct electricity in the solid or liquid
state
• Molecules are comprised of uncharged particles