Download Chapter 3 Molecules Molecules, Compounds, and Chemical

Principles of Chemistry: A Molecular Approach, 1st Ed.
Nivaldo Tro
Chapter 3
Molecules,
Molecules
Compounds,
and Chemical
Equations
Roy K
R
Kennedy
d
Massachusetts Bay Community College
Wellesley Hills, MA
Edited by K.M. Hattenhauer
Tro, Principles of Chemistry: A Molecular Approach
1
Elements and Compounds
• Elements combine together to make an almost
limitless number of compounds.
compounds
• The
Th properties
ti off the
th
compound are totally
diff
different
t from
f
the
th
constituent elements.
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2
Chemical Bonds
• Compounds are made of atoms held together by
•
•
chemical bonds
bonds.
Bonds are forces of attraction between atoms.
The bonding attraction comes from attractions
between protons and electrons.
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Bond Types
• Two general types of bonding between atoms are
•
found in compounds, ionic and covalent.
Ionic bonds result when electrons have been
transferred between atoms, resulting in oppositely
charged ions that attract each other.
 generally found when metal atoms bonded to nonmetal
atoms
t
• Covalent bonds result when two atoms share
some of their electrons.
electrons
 generally found when nonmetal atoms bonded together
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Tro, Principles of Chemistry: A Molecular Approach
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Representing Compounds
(Ch i l F
(Chemical
Formula)
l )
• Compounds are generally represented with a
•
chemical formula.
The amount of information about the structure of
the compound varies with the type of formula.
 All formulas and models convey a limited amount of
i f
information—none
ti
are perfect
f t representations.
t ti
• All chemical formulas tell what elements are in the
compound.
compound
 use the letter symbol of the element
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Types
yp of Formula
• Empirical Formulas describe the kinds of elements
found in the compound and the ratio of their atoms.
 They do not describe how many atoms, the order of
attachment, or the shape.
 The formulas for ionic compounds are empirical.
• Molecular Formulas describe the kinds of elements
ffound
d in
i th
the compound
d and
d th
the numbers
b
off th
their
i
atoms.
 does not describe the order of attachment, or the shape
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Types
y
of Formula
• Structural Formulas describe the kinds of
elements found in the compound,
compound the numbers of
their atoms, the order of atom attachment, and the
kind of attachment.
 They do not directly describe the three-dimensional
shape but an experienced chemist can make a good
shape,
guess at it.
 use lines to represent covalent bonds
 Each line describes the number of electrons shared by the
bonded atoms.
single line = 2 shared electrons
electrons, a single covalent bond
double line = 4 shared electrons, a double covalent
bond
triple line = 6 shared electrons, a triple covalent bond
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Representing Compounds
(Molecular Models)
• Models show the three
three-dimensional
dimensional structure along
with all the other information given in the structural
formula.
formula
• Ball-and-Stick Models use
b ll tto representt th
balls
the atoms
t
and sticks to represent the
attachments
tt h
t between
b t
them.
th
• Space-Filling Models use
i t
interconnected
t d spheres
h
tto
show the electron clouds of
atoms
t
connecting
ti together.
t
th
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Types of Formula
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Elements and Compounds
• atomic elements
elements whose particles
are single
i l atoms
t
• molecular elements
particles
elements whose p
are multi-atom molecules
Tro, Principles of Chemistry: A Molecular Approach
• molecular compounds
•
 compounds whose particles are
molecules made of only
nonmetals
ionic compounds
 compounds whose particles are
11
cations
and anions
Molecular Elements
• Certain elements occur as two-atom molecules.
 H2, N2, O2, F2, Cl2, Br2, I2
• Other elements occur as polyatomic molecules.
 P4, S8, Se8
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Formulas Describe Compounds
p
• A compound is a distinct substance that is
•
composed of atoms of two or more elements.
elements
Describe the compound by describing the number
and type of each atom in the simplest unit of the
compound.
 molecules or ions
• Each element is represented by its letter symbol.
• The number of atoms of each element is written to
the right of the element as a subscript.
 If there is only one atom
atom, the 1 subscript is not written
written.
• Polyatomic groups are placed in parentheses.
 if more than one
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Ionic vs. Molecular Compounds
Propane—contains
i di id l C3H8
individual
molecules
Tro, Principles of Chemistry: A Molecular Approach
Table salt—contains
an array off Na
N + ions
i
and Cl– ions
14
Ionic Compounds
• Compounds of metals with nonmetals are made of
•
•
ions.
Metal atoms: cations; nonmetal atoms: anions.
no individual molecule units
units, instead they have a
three-dimensional array of cations and anions
made of formula units
many contain polyatomic ions
several atoms attached together in one ion
Note: Compound must have no total charge;
therefore, we must balance the numbers of
cations and anions in a compound to get 0
charge.
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Writing Formulas: Ionic Compounds
1. Write the symbol for the metal cation and its
2.
3
3.
4
4.
5
5.
charge.
Write the symbol
y
for the nonmetal anion and its
charge.
Charge (without sign) becomes subscript for other
ion.
Reduce subscripts to smallest whole number
ratio.
Ch k that
Check
th t the
th sum off the
th charges
h
off the
th cation
ti
cancels the sum of the anions.
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Formula-to-Name
R l ffor IIonic
Rules
i C
Compounds
d
• made of cation and anion
• Some have one or more nicknames that are only
learned by experience.
experience
 NaCl = table salt, NaHCO3 = baking soda
• Write systematic name by simply naming the ions
ions.
 if cation is:
 metal with invariant charge = metal name
 metal with variable charge = metal name(charge)
 polyatomic ion = name of polyatomic ion
 if anion is:
 nonmetal = stem of nonmetal name + -ide
 polyatomic ion = name of polyatomic ion
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Metal Cations
• metals with invariant
charge
 metals whose ions can
have only one possible
charge
 Groups
p 1A1+ & 2A2+, Al3+,
Ag1+, Zn2+, Sc3+
 cation name = metal name
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Metal Cations
• metals with variable
g
charges
 metals whose ions
can have more
than one possible
g
charge
 determine charge
g
of cation byy charge
on anion
 name = metal
name with Roman
g in
numeral charge
parentheses
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Naming Monatomic Nonmetal Anion
• Determine the charge from position on the Periodic
•
Table
Table.
To name anion, change ending on the element
name to
t -ide.
id
4A = 4−
5A = 3−
6A = 2−
7A = 1−
C = carbide
bid
N = nitride
it id
O = oxide
id
F = fluoride
fl id
Si = silicide
ili id P = phosphide
h
hid S = sulfide
lfid Cl = chloride
hl id
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Naming Binary Ionic Compounds for
M t l with
Metals
ith IInvariant
i t Charge
Ch
• contain metal cation + nonmetal anion
• metal listed first in formula and name
1. name metal cation first, name nonmetal anion
2.
3.
second
cation name is the metal name
nonmetal anion named by changing the ending on
the nonmetal name to -ide
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Naming Binary Ionic Compounds for
Metals with Variable Charge
• contain metal cation + nonmetal anion
• metal
t l listed
li t d fi
firstt iin fformula
l and
d name
1. Name metal cation first, name nonmetal anion
2.
second.
Metal cation name is metal name followed by a
Roman numeral in parentheses to indicate its
charge.
 determine charge from anion charge
 common ions Table 3.4
3. nonmetal anion named byy changing
g g the ending
g on
the nonmetal name to -ide
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Compounds
p
Containing
g Polyatomic
y
Ions
• Polyatomic ions are single ions that contain
•
•
•
more than one atom
atom.
often identified by (ion) in formula
N
Name
and
d charge
h
off polyatomic
l t i iion d
do nott change.
h
Name any ionic compound by naming cation first
and
d th
then anion.
i
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Some Common Polyatomic Ions
Name
Formula Name
Formula
acetate
carbonate
hydrogen carbonate
(aka bicarbonate)
hydroxide
nitrate
nitrite
C2H3O2–
CO32–
hypochlorite
chlorite
chlorate
ClO–
ClO2–
ClO3–
ClO4–
SO42–
SO32–
chromate
dichromate
ammonium
CrO42–
Cr2O72–
NH4+
p
perchlorate
sulfate
sulfite
hydrogen sulfate
(aka bisulfate)
HCO3–
OH–
NO3–
NO2–
Tro, Principles of Chemistry: A Molecular Approach
hydrogen sulfite
((aka bisulfite))
24
HSO4–
HSO3–
Patterns for Polyatomic
y
Ions
1. Elements in the same column form similar
polyatomic
l t i iions.
 same number of O’s and same charge
ClO3– = chlorate  BrO3– = bromate
2. If the polyatomic ion starts with H, add
hydrogen- prefix before name and add 1 to the
charge.
CO32– = carbonate  HCO3– = hydrogen carbonate
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Periodic Pattern of Polyatomic
y
Ions
-ate groups
3A
4A
5A
6A
7A
BO333−
borate
CO322−
carbonate
NO3−
nitrate
SiO32−
silicate
PO43−
phosphate
SO42−
sulfate
ClO3−
chlorate
AsO43−
arsenate
SeO42−
selenate
BrO3−
bromate
TeO42−
tellurate
IO3−
iodate
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Patterns for Polyatomic Ions
• -ate ion
 chlorate = ClO3–
• -ate ion + 1 O  same charge, per- prefix
 perchlorate = ClO4–
• -ate
ate ion – 1 O  same charge, -ite
ite suffix
 chlorite = ClO2–
• -ate ion – 2 O  same charge,
charge hypo- prefix,
prefix -ite
suffix
 hypochlorite = ClO–
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Hydrates
• Hydrates are ionic compounds
•
containing a specific number of waters
for each formula unit.
water of hydration often “driven off” by
g
heating
• in formula, attached waters follow ·
 CoCl2·6H2O
• in name, attached waters indicated by
prefix+hydrate after name of ionic
compound
 CoCl2·6H2O = cobalt(II) chloride hexahydrate
 CaSO
C SO4·½H
½H2O = calcium
l i
sulfate
lf
h
hemihydrate
ih d
Hydrate
CoCl2·6H2O
Tro, Principles of Chemistry: A Molecular Approach
Anhydrous
CoCl2
28
Prefix
No. of
Waters
hemi
mono
di
tri
tetra
penta
hexa
hepta
½
1
2
3
4
5
6
7
octa
8
Writing Names of Binary Molecular
Compounds of Two Nonmetals
1. Write the name of the first element in the formula.
2.
 element furthest left and down on the Periodic Table
 Use the full name of the element,
Write the name of the second element in the formula
with an -ide suffix.

as if it were an anion—However,, remember these compounds
p
do not contain ions!
3. Use a prefix in front of each name to indicate the
number of atoms.
a) Never use the prefix mono- on the first element.
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Subscript—Prefixes
p
• 1 = mono-
• 6 = hexa
hexa-
 not used on first
nonmetal
•
•
•
•
•
•
•
•
•
2 = di3 = ttrii
4 = tetra5 = penta-
7 = hepta8 = octa9 = nona10 = deca
deca-
Drop last “a” if name begins with a vowel.
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Acids
• Acids are molecular compounds that form H+
when dissolved in water.
 contain H+ cation and anion (in aqueous sol’n)
 To indicate the compound is dissolved in water, (aq) is
written after the formula.
not named as acid if not dissolved in water
• sour taste
• dissolve many metals
 like Zn, Fe, Mg; but not Au, Ag, Pt
• formula generally starts with H (eg. HCl, H2SO4)
• Two Types:
 Binary acids have H+ cation and nonmetal anion.
 Oxyacids have H+ cation and polyatomic anion.
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Acids
Naming Binary Acids
•
•
•
•
Write a hydro prefix.
prefix
Follow with the nonmetal name.
Change ending on nonmetal name to -ic.
Write the word acid at the end of the name.
Naming Oxyacids
• If the
h polyatomic
l
i iion name ends
d iin -ate, then
h change
h
•
•
ending to -ic suffix.
If the polyatomic ion name ends in -ite, then change
ending to -ous suffix.
Write word acid at the end of all names.
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Writing Formulas for Acids
• when name ends in acid, formula starts with H
• Write
W it formulas
f
l as if ionic,
i i even th
though
h it iis
•
•
molecular.
H d prefix
Hydro
fi means it iis a bi
binary acid,
id no prefix
fi
means it is an oxyacid.
f oxyacid,
for
id if ending
di iis -ic,
i polyatomic
l t i iion ends
d iin ate; if ending is -ous, polyatomic ion ends in -ite
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Mass
Formula Mass
• the mass of an individual molecule or formula unit
 also known as molecular mass or molecular weight
• sum of the masses of the atoms in a single
g
molecule or formula unit
 whole = sum of the parts!
Molar Mass of Compounds
• The
Th relative
l ti masses off molecules
l
l can b
be calculated
l l t d
from atomic masses.
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Percent Composition
•
percentage of each element in a compound
 by mass
•
can be determined from
1.
1
2.
•
the formula of the compound
the experimental mass analysis of the compound
The percentages may not always total to 100%
due to rounding.
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Mass Percent as a
C
Conversion
i F
Factor
t
• The mass percent tells you the mass off a
constituent element in 100 g of the compound.
The fact that CCl2F2 is 58.64% Cl by mass means
that 100 g of CCl2F2 contains 58.64 g Cl.
• This can be used as a conversion factor.
100 g CCl2F2 : 58.64
58 64 g Cl
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Conversion Factors in Chemical
Formulas
• Chemical
Ch i l fformulas
l h
have iinherent
h
t iin th
them
relationships between numbers of atoms and
molecules.
l
l
 or moles of atoms and molecules
• These relationships can be used to convert
between amounts of constituent elements and
molecules.
 such as percent composition
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Empirical
p
Formula
• simplest, whole-number ratio of the atoms of
•
elements
l
t iin a compound
d
can be determined from elemental analysis
 masses of elements formed when decompose or react
combustion analysis
percent composition
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Finding an Empirical Formula
1) Convert the percentages to grams.
a)
b)
assume you start with 100 g of the compound
skip if already grams
2)) Convert g
grams to moles.
a)
use molar mass of each element
3) Write a pseudoformula using moles as subscripts.
4) Divide all by smallest number of moles.
a)
If result is within 0.1 of whole number, round to whole
number.
number
5) Multiply all mole ratios by number to make all
whole numbers.
numbers
a)
b))
if ratio ?.5, multiply all by 2; if ratio ?.33 or ?.67, multiply
all by 3; if ratio 0.25 or 0.75, multiply all by 4; etc.
skip iff already whole numbers
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Molecular Formulas
•
•
The molecular formula is a multiple
p of the
empirical formula.
To determine the molecular formula
formula, you need to
know the empirical formula and the molar mass
of the compound.
compound
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Combustion Analysis
y
• A common technique for analyzing compounds is to
burn a known mass of compound and weigh the
amounts of product made.
 generally used for organic compounds containing C,
C H,
H O
• By knowing the mass of the product and
composition of constituent elements in the product
product,
the original amount of constituent elements can be
dete
determined.
ed
 All the original C forms CO2, the original H forms H2O,
the original mass of O is found by subtraction.
• Once the masses of all the constituent elements in
the original compound have been determined, the
empirical
i i l fformula
l can b
be ffound.
d
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Chemical Reactions
• Reactions involve chemical changes in matter
•
resulting in new substances
substances.
Reactions involve rearrangement and exchange of
atoms
t
to
t produce
d
new molecules.
l
l
 Elements are not transmuted during a reaction.
Reactants
Tro, Principles of Chemistry: A Molecular Approach

Products
42
Chemical Equations
q
• shorthand way of describing a reaction
• provides information about the reaction
 formulas of reactants and products
 states
t t off reactants
t t and
d products
d t
 relative numbers of reactant and product molecules that
are required
 can be used to determine weights of reactants used and
products that can be made
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Combustion of Methane
• Methane gas burns to produce carbon dioxide gas
and g
gaseous water.
 Whenever something burns, it combines with O2(g).
CH4(g) + O2(g)  CO2(g) + H2O(g)
(g)
This equation reads “1 molecule of CH4 gas combines with 1
molecule
l
l off O2 gas to
t make
k 1 molecule
l
l off CO2 gas and
d1
molecule of H2O gas”.
O
H
H
C
H
H
+
O
C
O
Tro, Principles of Chemistry: A Molecular Approach
O
44
+
H
O
H
Combustion of Methane, Balanced
• To show the reaction obeys the Law of Conservation
of Mass,
Mass the equation must be balanced.
balanced
 We adjust the numbers of molecules so there are equal
numbers
u be s o
of ato
atomss of
o each
eac e
element
e e to
on bot
both ssides
des o
of tthe
e
arrow.
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
(g)
H
H
C
H
H
O
+
O
O
+
O
C
O
Tro, Principles of Chemistry: A Molecular Approach
O
45
+
H
H
O
+
O
H
H
Chemical Equations
q
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
• CH4 and O2 are the reactants, and CO2 and H2O
•
•
are the
th products.
d t
The (g) after the formulas tells us the state of the
chemical.
The number in front of each substance tells us the
number of those molecules in the reaction.
 called the coefficients
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46
Chemical Equations
q
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
• This equation is balanced, meaning that there are
equal numbers of atoms of each element on the
reactant and product sides.
 To obtain the number of atoms of an element
element, multiply
the subscript by the coefficient.
1C1
4H4
4O2+2
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Symbols
y
Used in Equations
q
• symbols used to indicate state after chemical
 (g) = gas; (l) = liquid; (s) = solid
 (aq) = aqueous = dissolved in water
• energy symbols used above the arrow for
p
reactions
decomposition
  = heat
 h = light
g
 shock = mechanical
e
elec
ec = e
electrical
ec ca
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48
Classifying Compounds
Organic vs. Inorganic
• In the18th century, compounds from living things
•
•
were called organic; compounds from the nonliving
environment were called inorganic.
Organic compounds easily decomposed and could
not be made in an 18th-century lab.
Inorganic compounds are very difficult to
decompose, but can be synthesized.
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49
Modern Classifying Compounds
Organic vs. Inorganic
• Today, we commonly make organic compounds in
•
•
the lab and find them all around us.
Organic compounds are made mainly of C and H,
sometimes with O,
O N,
N P,
P S,
S and trace amounts of
other elements.
The main element that is the focus of organic
chemistry is carbon.
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50
Carbon Bonding
g
• Carbon atoms bond almost exclusively covalently.
 Compounds with ionic bonding C are generally inorganic.
• When C bonds, it forms four covalent bonds.
 4 single bonds, 2 double bonds, 1 triple + 1 single, etc.
• Carbon is unique in that it can form limitless chains
of C atoms, both straight and branched, and rings of
C atoms
atoms.
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51
Classifying
y g Organic
g
Compounds
p
• There are two main categories of organic
•
•
compounds, hydrocarbons and functionalized
hydrocarbons.
Hydrocarbons contain only C and H.
Most fuels are mixtures of hydrocarbons.
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