Download Chapter 3 Molecules, Compounds, and Chemical Equations

9/27/2011
Principles of Chemistry: A Molecular Approach, 1st Ed.
Nivaldo Tro
Chapter 3
Molecules,
Compounds,
and Chemical
Equations
Roy Kennedy
Massachusetts Bay Community College
Wellesley Hills, MA
Edited by K.M. Hattenhauer
Tro, Principles of Chemistry: A Molecular Approach
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Elements and Compounds
- elements combine together to make an almost limitless
number of compounds.
- the properties of the compound are totally different from the
constituent elements
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Chemical Bonds
chemical bonds
- hold atoms together to form compounds
- are forces of attraction between atoms.
- the bonding attraction comes from attractions between
protons and electrons.
i.) Ionic bonds
- result when electrons have been transferred
between atoms, resulting in oppositely charged ions
that attract each other
- generally formed when metal atoms bonded to
nonmetal atoms
ii.) Covalent bonds
– result when two atoms share some of their
electrons.
– generally found when nonmetal atoms bond together
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Chemical Formula
Chemical formula
- way to represent compounds
- amount of information about structure of compound varies
with the type of formula.
all formulas and models convey a limited amount of
information—none are perfect representations.
- all chemical formulas tell what elements are in the
compound
compound.
use the letter symbol of the element
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Types of Formula
Empirical Formulas
- describe the kinds of elements found in the compound and
the ratio of their atoms
- do not describe how many atoms, the order of attachment,
or the shape
- formulas for ionic compounds are empirical
Molecular Formulas
- describe the kinds of elements found in the compound and
the numbers of their atoms
- does not describe the order of attachment, or the shape
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Types of Formula
Structural Formulas
- describes the kinds of elements found in the compound, the
numbers of their atoms, the order of atom attachment, and
the kind of attachment
- do not directly describe the three-dimensional shape (yet
experienced chemist can make a good guess at it)
- use lines to represent covalent bonds
Note: - each line describes the number of electrons shared by
the bonded atoms.
single line = 2 shared electrons (single covalent bond)
double line = 4 shared electrons (double covalent bond)
triple line = 6 shared electrons (triple covalent bond)
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Types of Formula
Molecular Models
- show the three-dimensional structure along with all the other
information given in the structural formula.
Two types:
i.) Ball-and-Stick Models
- use balls to represent the
atoms and sticks to represent
the attachments between them
ii.) Space-Filling Models
- use interconnected spheres to
show the electron clouds of
atoms connecting together.
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Formulas Describe Compounds
- a compound is a distinct substance that is composed of atoms
of two or more 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.
subscript
 If there is only one atom, the 1 subscript is not written.
- polyatomic groups are placed in parentheses.
 if more than one
Table salt
- contains an array
of Na+ ions and
Cl– ions
Propane
- contains
individual C3H8
molecules
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Types of Formula
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Elements and Compounds
atomic elements
- elements whose particles
are single atoms
molecular elements
- elements whose particles
are multi-atom molecules
molecular compounds
- compounds whose particles are
molecules made of only
nonmetals
ionic compounds
- compounds whose particles are
cations and anions
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Elements and Compounds
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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Elements and Compounds
Question:
Classify each of the following as either a (n):
atomic element
molecular element
molecular compound
p
ionic compound
a)
b)
c)
d)
e)
f)
Aluminum, Al
Aluminum chloride, AlCl3
Chlorine, Cl2
Acetone, C3H6O
Carbon monoxide, CO
Cobalt, Co
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Elements and Compounds
ionic compounds
- compounds of metals with nonmetals are made of ions
 Metal atoms: cations; nonmetal atoms: anions.
- no individual molecule units, instead they have a threedimensional 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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Elements and Compounds
Ionic Compounds: Writing Formulas
1. Write the symbol for the metal cation and its charge.
2. Write the symbol for the nonmetal anion and its charge.
3. Charge (without sign) becomes subscript for other ion.
4. Reduce subscripts to smallest whole number ratio.
5. Check that the sum of the charges of the cation cancels the
sum of the anions.
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Elements and Compounds
Ionic Compounds: Formula-to-Name
Two situations
1.) common names - some have one or more nicknames
that are only learned by experience
2.) systematic names –simply obtained by naming the ions.
if cation is:
i ) metal
i.)
t l with
ith iinvariant
i t charge
h
= metal
t l name
ii.) metal with variable charge = metal name (charge)
iii.) polyatomic ion = name polyatomic ion
if anion is:
i.) nonmetal = stem of nonmetal name + -ide
ii.) polyatomic ion = name polyatomic ion
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Elements and Compounds
Ionic Compounds:
Metal Cations with invariant charge
- metals whose ions can have only one
possible charge
(Groups 1A1+ & 2A2+, Al3+,Ag1+, Zn2+, Sc3+ )
- cation name = metal name
Metal Cations with variable charge
- metals whose ions can have more
than one possible charge
- determine charge of cation by charge
on anion
- name = metal name with Roman
numeral charge in parentheses
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Elements and Compounds
Ionic Compounds:
Naming Binary Ionic Compounds
for Metals with Invariant Charge
1. name metal cation first, name nonmetal anion second
2. cation name is the metal name
3. nonmetal anion named by changing the ending on the
nonmetal name to –ide
Naming Binary Ionic Compounds
for Metals with Variable Charge
1.
Name metal cation first, name nonmetal anion second.
2. Metal cation name is metal name followed by a Roman
numeral in parentheses to indicate its charge.

determine charge from anion charge
3. nonmetal anion named by changing the ending on the
nonmetal name to -ide
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Elements and Compounds
Ionic Compounds: Naming Monatomic Nonmetal Anion
- determine the charge from position on the Periodic Table.
- to name anion, change ending on the element name to -ide.
4A = 4−
5A = 3−
6A = 2−
7A = 1−
C = carbide
N = nitride
O = oxide
F = fluoride
Si = silicide P = phosphide S = sulfide Cl = chloride
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Elements and Compounds
Ionic Compounds: Compounds Containing Polyatomic Ions
polyatomic ions
- single ions that contain more than one atom.
often identified by (ion) in formula
- name and charge of polyatomic ion do not change.
- name any ionic compound by naming cation first and then anion.
Name
Formula
Name
Formula
acetate
C2H3O2–
hypochlorite
ClO–
carbonate
CO32–
chlorite
ClO2–
hydrogen carbonate
(aka bicarbonate)
HCO3–
chlorate
ClO3–
hydroxide
OH–
perchlorate
ClO4–
sulfate
SO42–
sulfite
SO32–
hydrogen sulfate
(aka bisulfate)
HSO4–
hydrogen sulfite
(aka bisulfite)
HSO3–
nitrate
NO3–
nitrite
NO2–
chromate
CrO42–
dichromate
Cr2O72–
ammonium
NH4+
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Elements and Compounds
Patterns for Polyatomic Ions
1. Elements in the same column form similar polyatomic ions.
 same number of O’s and same charge
2. If the polyatomic ion starts with H, add hydrogen- prefix
before name and add +1 to the charge.
Periodic Pattern of Polyatomic Ions: -ate groups
3A
4A
5A
BO33−
borate
CO32−
carbonate
NO3−
nitrate
6A
7A
SiO32−
silicate
PO43−
phosphate
SO42−
sulfate
ClO3−
chlorate
AsO43−
arsenate
SeO42−
selenate
BrO3−
bromate
TeO42−
tellurate
IO3−
iodate
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Elements and Compounds
Patterns for Polyatomic Ions
1.) -ate ion
chlorate = ClO3–
2.) -ate ion + 1 O  same charge, per- prefix
perchlorate = ClO4–
3.) -ate ion – 1 O  same charge, -ite suffix
chlorite = ClO2–
4.) -ate ion – 2 O  same charge, hypo- prefix, -ite suffix
hypochlorite = ClO–
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Elements and Compounds
Hydrates
- are ionic compounds containing a specific
number of waters for each formula unit.
- water of hydration often “driven off” by
heating
- in formula, attached waters follow ·
e.g. CoCl2·6H2O
- in name, attached waters indicated by
prefix+hydrate after name of ionic
compound
Prefix
No. of
Waters
hemi
½
mono
1
di
2
tri
3
tetra
4
penta
5
hexa
6
hepta
7
octa
8
e.g. CoCl2·6H2O = cobalt(II) chloride hexahydrate
e.g CaSO4·½H2O = calcium sulfate hemihydrate
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Hydrate
CoCl2·6H2O
Anhydrous
CoCl2
Hydrates
Question:
1.) What is the formula of iridium (III) bromide?
2.) What is the name of Mg(BrO3)2▪6 H2O?
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Elements and Compounds
Writing Names of Binary Molecular Compounds of Two
Nonmetals
1. Write the name of the first element in the formula.
 element furthest left and down on the Periodic Table
 use the full name of the element
2. Write the name of the second element in the formula with an
-ide
id suffix.
ffi
 as if it were an anion—However, remember these
compounds 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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Elements and Compounds
Subscript—Prefixes
• 1 = mono•
•
•
•
• 6 = hexa-
 not used on first nonmetal
2 = di3 = tri4 = ttetrat
5 = penta-
•
•
•
•
7 = hepta8 = octa9 = nona
nona10 = deca-
**Drop last “a” if name begins with a vowel.
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Elements and Compounds
Acids
- 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
eg. like Zn, Fe, Mg; but not Au, Ag, Pt
- formula generally starts with H (eg. HCl, H2SO4)
Two types:
i.) binary acids - have H+ cation and nonmetal anion.
ii.) oxyacids - have H+ cation and polyatomic anion.
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Elements and Compounds
Naming Acids
i.) Naming Binary Acids
1.) write a hydro prefix.
2.) follow with the nonmetal name.
3.) change ending on nonmetal name to -ic.
4.) write the word acid at the end of the name.
ii.) Naming Oxyacids
1.) if the polyatomic ion name ends in -ate, then change
ending to -ic suffix.
2.) if the polyatomic ion name ends in -ite, then change
ending to -ous suffix.
3.) write word acid at the end of all names.
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Elements and Compounds
Writing Formulas for Acids
1.) when name ends in acid, formula starts with H
2.) write formulas as if ionic, even though it is molecular.
3.) Hydro prefix means it is a binary acid, no prefix means it
is an oxyacid.
for oxyacid, if ending is -ic, polyatomic ion ends in -ate;
if ending is -ous,
ous, polyatomic ion ends in -ite
ite
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Mass
Formula Mass (molecular mass/molecular weight)
- the mass of an individual molecule or formula unit
- sum of the masses of the atoms in a single molecule or
formula unit
 whole = sum of the parts!
Molar Mass of Compounds
p
- the relative masses of molecules can be calculated from
atomic masses.
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Mass
Question:
How many moles are in 50.0 g of PbO2?
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Mass
Question:
Calculate the mass (in g) of 9.85 x 1019 CCl2F2
molecules.
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Percent Composition
percent composition
- percentage of each element in a compound (by mass)
- determination of percent composition:
1. the formula of the compound
2. the experimental mass analysis of the compound
Note: - percentages may not always total to 100% due to
rounding
rounding.
mass percent as a conversion factor
- mass percent tells you the mass of a constituent element
in 100 g of the compound.
- can be used as a conversion factor.
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Percent Composition
Question:
Determine the mass percent composition of C2H6.
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Percent Composition
Question:
The element M forms the chloride MCl4 containing 75.0%
Cl by weight. What is the atomic weight of M?
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Mass Percent as a Conversion Factor
Question:
Silver chloride, often used in silver plating, contains 75.27%
silver. Calculate the mass of silver chloride required to plate
155 mg of pure silver.
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Conversion Factors in Chemical Formulas
- chemical formulas have inherent in them relationships
between numbers of atoms and molecules.
 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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Conversion Factors in Chemical Formulas
Question:
Calculate the number of grams of sodium in 8.5 g of sodium
phosphate.
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Empirical and Molecular Formulas
empirical formula
- simplest, whole-number ratio of the atoms of elements in a
compound
- can be determined from elemental analysis:
i.) percent composition
ii.) combustion/elemental analysis
- masses of elements formed when decompose or
react
Molecular formula
- a multiple of the empirical formula.
- determined by knowing the empirical formula and the
molar mass of the compound.
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Empirical and Molecular Formulas
Question:
Consider the empirical formula and molar mass of C3H2N
(312.29 g▪mol-1). Determine the molecular formula of the
compound.
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Finding an Empirical Formula
i.) finding the empirical formula from percent composition
1.) Convert the percentages to grams.
a) assume you start with 100 g of the compound
b) skip if already grams
2.) Convert 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.
5.) Multiply all mole ratios by number to make all whole
numbers.
a) 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.
b) skip if already whole numbers
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Empirical Formulas from Percent
Composition
Question:
Calculate the empirical formula of methyl butyrate
(component of apple taste and smell) based on the following
percent composition:
C 58.80%
H 9.87%
O 31.33%
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Finding an Empirical Formula
ii.) finding the empirical formula from combustion/elemental
analysis
- 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,, H,, O
- by knowing the mass of the product and composition of
constituent elements in the product, the original amount of
constituent elements can be determined.
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
formula can be found.
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Empirical Formulas from
Combustion/Elemental Analysis
Question:
A 45.2 mg sample of phosphorus reacts with selenium to
form 131.5 mg of the selenide. What is the empirical formula
of the phosphorus selenide?
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Empirical Formulas from
Combustion/Elemental Analysis
Question:
Combustion analysis of a 12.01 g sample of tartaric acid
which contains only C, H and O produced 14.08 g CO2 and
4.32 g H2O. Find the empirical formula of tartaric acid.
H2O = 18.016 g/mol
Note: CO2 = 44.011 g/mol
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Chemical Reactions
- reactions involve chemical changes in matter resulting in new
substances.
- reactions involve rearrangement and exchange of atoms to
produce new molecules.
 Elements are not transmuted during a reaction.
Reactants
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
Products
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Chemical Equations
chemical equations
- shorthand way of describing a reaction
- provides information about the reaction
formulas and states of reactants and products
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
CH4(g) + O2(g)  CO2(g) + H2O(g)
O
H
H
+
C
H
O
H
O
+
C
O
H
H
O
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Combustion of Methane, Balanced
- to show the reaction obeys the Law of Conservation of Mass,
the equation must be balanced.
balanced
- meaning that there are equal numbers of atoms of each
element on the reactant and product sides
coefficients
- number in front of each substance tells us the number of
those molecules in the reaction.
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
H
H
C
H +
H
O
O
O
O
O
+
C
+
H
O
O
Tro, Principles of Chemistry: A Molecular Approach
H
+
O
H
H
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Chemical Equations
Symbols Used in Equations
- several symbols are used in chemical equations
i.) symbols used to indicate state after chemical
 (g) = gas; (l) = liquid; (s) = solid
 (aq) = aqueous = dissolved in water
gy symbols
y
used above the arrow for decomposition
p
ii.)) energy
reactions
  = heat
 h = light
 shock = mechanical
 elec = electrical
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Chemical Equations
Question:
1.) In a popular classroom demonstration, solid sodium is
added to liquid water and reacts to produce hydrogen
gas and aqueous sodium hydroxide. Write a balanced
chemical equation for this reaction.
2.) Potassium superoxide (KO2) can simulate a plant-type
action by consuming carbon dioxide gas and releasing
oxygen gas. The other product is potassium
carbonate. Write a balanced equation for this process.
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Classifying Compounds - Organic vs. Inorganic
In the18th century,
- organic compounds
- compounds from living things
- easily decomposed and could not be made in the lab
- inorganic compounds
- from the nonliving environment
- are very difficult to decompose,
decompose but can be synthesized
Present day,
- organic compounds
- commonly make organic compounds in lab and find
them all around us.
- made mainly of C and H, sometimes with O, N, P, S, and
trace amounts of other elements.
- main element of interest is carbon.
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Carbon Bonding
carbon
- bonds 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.
classifying organic compounds
- two main categories:
i.) hydrocarbons
- contain only C and H
- mixtures of these make up most fuels
ii.) functionalized hydrocarbons
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