Transcript Introductory Chemistry, 2nd Edition Nivaldo Tro

Chapter 5
Molecules
and
Compounds
Molecules and Compounds
• Salt
 Sodium – shiny, reactive, poisonous
 Chlorine – pale yellow gas, reactive, poisonous
 Sodium chloride – table salt
• Sugar
 Carbon – pencil or diamonds
 Hydrogen – flammable gas
 Oxygen – a gas in air
 Combine to form white crystalline sugar
2
Law of Constant Composition
• all pure substances have constant composition
 all samples of a pure substance contain the same elements in
the same percentages (ratios)
 mixtures have variable composition
• If we decompose water by electrolysis, we find 16.0 grams of
oxygen to every 2.00 grams of hydrogen.
Water has a constant Mass Ratio of Oxygen to Hydrogen of 8.0.
mass of oxygen
Mass Ratio 
mass of hydrogen
16.0 g

 8.0
2.0 g
3
Why do Compounds Show
Constant Composition
• smallest piece of a compound is called a molecule
• every molecule of a compound has the same
number and type of atoms as determined by the
electronic structures of the atoms (more on that
later in the year)
• since all the molecules of a compound are
identical…
 every sample will have the same ratio of the
elements
 every sample of the compound will have the
same properties
4
Example; Constant Composition
EXAMPLE 5.1 Constant Composition of Compounds
Two samples of carbon dioxide, obtained from different sources, were decomposed into their constituent
elements. One sample produced 4.8 g of oxygen and 1.8 g of carbon, and the other sample produced 17.1 g
of oxygen and 6.4 g of carbon. Show that these results are consistent with the law of constant composition.
To show this, compute the
mass ratio of one element to
the other by dividing the
larger mass by the smaller
one. For the first sample:
Solution:
For the second sample:
Since the ratios are the same for the two samples, these results are consistent with the law of constant
composition.
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5.3A What is the carbon-hydrogen mass ratio for
methane (CH4)?
1.
2.
3.
4.
5.
5.1B
1.
2.
3.
4.
5.
Examples; Using Mass Ratios
1
0.33
3
4
0.25
If the mass ratio of lead(II) sulfide is 270.0 g lead
and 41.8 g sulfur, how much lead is required to
completely react with 85.6 g of sulfur?
13.3
185
228
312
553
6
Formulas Describe Compounds
water = H2O \ two atoms of
hydrogen and 1 atom of oxygen
table sugar = C12H22O11 \12 atoms
of C, 22 atoms of H and 11 atoms O
7
Order of Elements in a Formula
• metals written first
NaCl
• nonmetals written in order from
Table 5.1
CO2
are occasional exceptions for
historical or informational reasons
H2O, but NaOH
Table 5.1
Order of Listing Nonmetals
in Chemical Formulas
C P N H S
I Br Cl O F
8
Molecules with
Polyatomic Ions
symbol of the polyatomic
ion called nitrate
symbol of the polyatomic
ion called sulfate
Mg(NO3)2
CaSO4
compound called
magnesium nitrate
compound called
calcium sulfate
implied “1” subscript
on magnesium
parentheses to group two NO3’s
implied “1” subscript
on calcium
no parentheses for one SO4
9
Molecules with
Polyatomic Ions
subscript indicating
two NO3 groups
no subscript indicating
one SO4 group
Mg(NO3)2
CaSO4
compound called
magnesium nitrate
compound called
calcium sulfate
implied “1” subscript
on nitrogen, total 2 N
stated “3” subscript
on oxygen, total 6 O
implied “1” subscript
on sulfur, total 1 S
stated “4” subscript
on oxygen, total 4 O
10
Classifying Materials
• 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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Molecular Elements
• Certain elements occur as 2 atom molecules
• Rule of 7’s
 there are 7 common diatomic elements
 find the element with atomic number 7, N
 make a figure 7 by going over to Group 7A, then down
 don’t forget to include H2
VIIA
H2
N2
7
P4
O2
F2
S8
Cl2
Br2
I2
12
Molecular Compounds
• two or more
nonmetals
• smallest unit is a
molecule
13
Ionic Compounds
• metals + nonmetals
• no individual
molecule units, instead
have a 3-dimensional
array of cations and
anions made of
formula units
14
Molecular View of
Elements and Compounds
15
Classify each of the following as either an
atomic element, molecular element, molecular
compound or ionic compound
• aluminum, Al
• aluminum chloride, AlCl3
• chlorine, Cl2
• acetone, C3H6O
• carbon monoxide, CO
• cobalt, Co
16
Classify each of the following as either an
atomic element, molecular element, molecular
compound or ionic compound
• aluminum, Al = atomic element
• aluminum chloride, AlCl3 = ionic compound
• chlorine, Cl2 = molecular element
• acetone, C3H6O = molecular compound
• carbon monoxide, CO = molecular compound
• cobalt, Co = atomic element
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Formula-to-Name
Step 1
Is the compound one of the
exceptions to the rules?
•H2O = water, steam, ice
•NH3 = ammonia
Formula-to-Name
Step 2
What major class of compound is it?
Ionic or Molecular
Major Classes
• Ionic
 metal + nonmetal
 metal first in formula
 Binary Ionic
 compounds with polyatomic ions
• Molecular
 2 nonmetals
 Binary Molecular (or Binary Covalent)
 Acids – formula starts with H
 though acids are molecular, they behave as ionic when
dissolved in water
 may be binary or oxyacid
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Formula-to-Name
Step 3
What major subclass of compound is it?
Binary Ionic, Ionic with Polyatomic Ions,
Binary Molecular,
Binary Acid, Oxyacid
Classifying Compounds
• Compounds containing a metal and a nonmetal =
binary ionic
 Type I and II
• Compounds containing a polyatomic ion =
ionic with polyatomic ion
• Compounds containing two nonmetals =
binary molecular compounds
• Compounds containing H and a nonmetal =
binary acids
• Compounds containing H and a polyatomic ion =
oxyacids
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Formula-to-Name
Step 4
Apply Rules for the Class and Subclass
Formula-to-Name
Rules for Ionic
• Made of cation and anion
• Name by simply naming the ions
If cation is:
Type I metal = metal name
Type II metal = 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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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
S = sulfide
Cl = chloride
Si = silicide P = phosphide
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Metal Cations
• Type I
 metals whose ions can only have one
possible charge
 IA, IIA, (Al, Ga, In)
 determine charge by position on the
Periodic Table
 IA = +1, IIA = +2, (Al, Ga, In = +3)
• Type II
 metals whose ions can have more than How do you know a
one possible charge
metal cation is Type II?
 determine charge by charge on anion
its not Type I !!!
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Determine if the following metals are Type I
or Type II. If Type I, determine the charge on
the cation it forms.
•
•
•
•
•
lithium, Li
copper, Cu
gallium, Ga
tin, Sn
strontium, Sr
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Determine if the following metals are Type I
or Type II. If Type I, determine the charge on
the cation it forms.
•
•
•
•
•
lithium, Li
copper, Cu
gallium, Ga
tin, Sn
strontium, Sr
Type I
Type II
Type I
Type II
Type I
+1
+3
+2
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Type I Binary Ionic Compounds
• Contain Metal Cation + Nonmetal Anion
• Metal listed first in formula & name
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
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Type II Binary Ionic Compounds
•
•
Contain Metal Cation + Nonmetal Anion
Metal listed first in formula & name
1. name metal cation first, name nonmetal anion second
2. metal cation name is the metal name followed by a
Roman Numeral in parentheses to indicate its charge
 determine charge from anion charge
 Common Type II cations in Table 5.5
3. nonmetal anion named by changing the ending on the
nonmetal name to -ide
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Examples
•
•
•
•
•
•
•
LiCl = lithium chloride
AlCl3 = aluminum chloride
PbO = lead(II) oxide
PbO2 = lead(IV) oxide
Mn2O3 = manganese(III) oxide
ZnCl2 = zinc(II) chloride or zinc chloride
AgCl = silver(I) chloride or silver chloride
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Compounds Containing
Polyatomic Ions
• Polyatomic ions are single ions that contain
more than one atom
• Name any ionic compound by naming cation
first and then anion
Non-polyatomic cations named like Type I and II
Non-polyatomic anions named with -ide
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Fixed Charge Metals and Nonmetals
IA
IIA
IIIA
Li+1 Be+2
Na+1 Mg+2
K+1 Ca+2
Rb+1 Sr+2
Al+3
Zn+2 Ga+3
Ag+1 Cd+2 In+3
VA VIA VIIA
N-3 O-2
F-1
P-3 S-2
Cl-1
As-3 Se-2 Br-1
Te-2 I-1
Cs+1 Ba+2
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Some Common Polyatomic Ions
Name
Formula
Name
Formula
acetate
carbonate
hydrogen carbonate
(aka bicarbonate)
hydroxide
nitrate
nitrite
chromate
dichromate
ammonium
C2H3O2–
CO32–
hypochlorite
chlorite
chlorate
perchlorate
sulfate
sulfite
hydrogensulfate
(aka bisulfate)
hydrogensulfite
(aka bisulfite)
ClO–
ClO2–
ClO3–
ClO4–
SO42–
SO32–
HCO3–
OH–
NO3–
NO2–
CrO42–
Cr2O72–
NH4+
HSO4–
HSO3–
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Patterns for Polyatomic Ions
1. elements in the same column form similar
polyatomic ions
 same number of O’s and same charge
ClO3- = chlorate \ BrO3- = bromate
2. if the polyatomic ion starts with H, the name
adds hydrogen- prefix before name and add
1 to the charge
CO32- = carbonate \ HCO3-1 = hydrogencarbonate
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Periodic Pattern of Polyatomic Ions
-ate groups
IIIA
-3
BO3
IVA
VA
VIA
VIIA
-2
CO3
-1
NO3
-2
SiO3
-3
PO4
-2
SO4
-1
ClO3
-3
AsO4
-2
SeO4
-1
BrO3
-2
TeO 4
-1
IO3
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Binary Molecular Compounds
of 2 Nonmetals
1. Name first element in formula first
 use the full name of the element
2. Name the second element in the formula with an -ide
 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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Subscript - Prefixes
•
•
•
•
•
•
•
•
•
•
1 = mono 2 = di3 = tri4 = tetra5 = penta6 = hexa7 = hepta8 = octa9 = nona10 = deca-
not used on first nonmetal
drop last “a” if name
begins with vowel
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Acids
• Contain H+1 cation and anion
in aqueous solution
• Binary acids have H+1 cation
and nonmetal anion
• Oxyacids have H+1 cation
and polyatomic anion
39
Formula-to-Name
Acids
• acids are molecular compounds that often behave
like they are made of ions
• All names have acid at end
• Binary Acids = hydro prefix + stem of the name of
the nonmetal + ic suffix
• Oxyacids
 if polyatomic ion ends in –ate = name of polyatomic
ion with –ic suffix
 if polyatomic ion ends in –ite = name of polyatomic ion
with –ous suffix
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Example – Naming Binary Acids
HCl
1. Is it one of the common exceptions?
H2O, NH3, CH4, NaCl, C12H22O11 = No!
2. Identify Major Class
first element listed is H, \ Acid
3. Identify the Subclass
2 elements, \ Binary Acid
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Sample - Naming Binary
Acids – HCl
4. Identify the anion
Cl = Cl-, chloride because Group 7A
5. Name the anion with an –ic suffix
Cl- = chloride  chloric
6. Add a hydro- prefix to the anion name
hydrochloric
7. Add the word acid to the end
hydrochloric acid
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Example – Naming Oxyacids
H2SO4
1. Is it one of the common exceptions?
H2O, NH3, CH4, NaCl, C12H22O11 = No!
2. Identify Major Class
first element listed is H, \ Acid
3. Identify the Subclass
3 elements in the formula, \ Oxyacid
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Example – Naming Oxyacids
H2SO4
4. Identify the anion
SO4 = SO42- = sulfate
5. If the anion has –ate suffix, change it to –ic. If the
anion has –ite suffix, change it to -ous
SO42- = sulfate  sulfuric
6. Write the name of the anion followed by the word acid
sulfuric acid
(kind of an exception, to make it sound nicer!)
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Example – Naming Oxyacids
H2SO3
1. Is it one of the common exceptions?
H2O, NH3, CH4, NaCl, C12H22O11 = No!
2. Identify Major Class
first element listed is H, \ Acid
3. Identify the Subclass
3 elements in the formula, \ Oxyacid
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Example – Naming Oxyacids
H2SO3
4. Identify the anion
SO3 = SO32- = sulfite
5. If the anion has –ate suffix, change it to –ic. If
the anion has –ite suffix, change it to -ous
SO32- = sulfite  sulfurous
6. Write the name of the anion followed by the
word acid
sulfurous acid
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Formula-to-Name Flow Chart
47
Name – to – Formula
Writing the Formulas from the Names
•
•
For binary molecular compounds, use the
prefixes to determine the subscripts
For Type I, Type II, Ternary Compounds
and Acids
1. Determine the ions present
2. Determine the charges on the cation and anion
3. Balance the charges to get the subscripts
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Example – Binary Molecular
dinitrogen pentoxide
• Identify the symbols of the elements
nitrogen = N
oxide = oxygen = O
• Write the formula using prefix number for
subscript
di = 2, penta = 5
N2O5
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Compounds that Contain Ions
• compounds of metals with nonmetals are made
of ions
metal atoms form cations, nonmetal atoms for anions
• compound must have no total charge, therefore
we must balance the numbers of cations and
anions in a compound to get 0 charge
• if Na+ is combined with S2-, you will need 2 Na+
ions for every S2- ion to balance the charges,
therefore the formula must be Na2S
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Writing Formulas for
Ionic Compounds
1.
2.
3.
4.
5.
Write the symbol for the metal cation and its charge
Write the symbol for the nonmetal anion and its charge
Charge (without sign) becomes subscript for other ion
Reduce subscripts to smallest whole number ratio
Check that the sum of the charges of the cation cancels
the sum of the anions
52
Write the formula of a compound made from
aluminum ions and oxide ions
1.
2.
3.
4.
5.
Write the symbol for the metal
cation and its charge
Write the symbol for the
nonmetal anion and its charge
Charge (without sign) becomes
subscript for other ion
Reduce subscripts to smallest
whole number ratio
Check that the total charge of
the cations cancels the total
charge of the anions
Al+3 column IIIA
O2- column VIA
Al+3 O2Al2 O3
Al = (2)∙(+3) = +6
O = (3)∙(-2) = -6
53
Practice - What are the formulas for
compounds made from the following ions?
• potassium ion with a nitride ion
• calcium ion with a bromide ion
• aluminum ion with a sulfide ion
54
Practice - What are the formulas for
compounds made from the following ions?
• K+ with N3-
K3N
• Ca+2 with Br-
CaBr2
• Al+3 with S2-
Al2S3
55
Example – Ionic Compounds
manganese(IV) sulfide
1.
2.
3.
4.
5.
Write the symbol for the cation
and its charge
Write the symbol for the anion
and its charge
Charge (without sign) becomes
subscript for other ion
Reduce subscripts to smallest
whole number ratio
Check that the total charge of
the cations cancels the total
charge of the anions
Mn+4
S2Mn+4 S2-
Mn2S4
MnS2
Mn = (1)∙(+4) = +4
S = (2)∙(-2) = -4
56
Example – Ionic Compounds
Iron(III) phosphate
1.
2.
3.
4.
5.
Write the symbol for the cation
and its charge
Write the symbol for the anion
and its charge
Charge (without sign) becomes
subscript for other ion
Reduce subscripts to smallest
whole number ratio
Check that the total charge of
the cations cancels the total
charge of the anions
Fe+3
PO43Fe+3 PO43- Fe3(PO4)3
FePO4
Fe = (1)∙(+3) = +3
PO4 = (1)∙(-3) = -3
57
Example – Ionic Compounds
ammonium carbonate
1.
2.
3.
4.
5.
Write the symbol for the cation
and its charge
Write the symbol for the anion
and its charge
Charge (without sign) becomes
subscript for other ion
Reduce subscripts to smallest
whole number ratio
Check that the total charge of
the cations cancels the total
charge of the anions
NH4+
CO32NH4+ CO32- (NH4)2CO3
(NH4)2CO3
NH4 = (2)∙(+1) = +2
CO3 = (1)∙(-2) = -2
58
Practice - What are the formulas for
compounds made from the following ions?
• copper(II) ion with a nitride ion
• iron(III) ion with a bromide ion
• aluminum ion with a sulfate ion
59
Practice - What are the formulas for
compounds made from the following ions?
• Cu2+ with N3-
Cu3N2
• Fe+3 with Br-
FeBr3
• Al+3 with SO42-
Al2(SO4)3
60
Example – Binary Acids
hydrosulfuric acid
1.
2.
3.
4.
5.
Write the symbol for the cation
and its charge
Write the symbol for the anion
and its charge
Charge (without sign) becomes
subscript for other ion
Reduce subscripts to smallest
whole number ratio
Check that the total charge of
the cations cancels the total
charge of the anions
H+
S2H+ S2-
in all acids the
cation is H+
hydro means
binary
H2S
H 2S
H = (2)∙(+1) = +2
S = (1)∙(-2) = -2
61
Example – Oxyacids
carbonic acid
1.
2.
3.
4.
5.
Write the symbol for the cation
and its charge
Write the symbol for the anion
and its charge
Charge (without sign) becomes
subscript for other ion
Reduce subscripts to smallest
whole number ratio
Check that the total charge of
the cations cancels the total
charge of the anions
H+
CO32-
in all acids the
cation is H+
no hydro means
polyatomic ion
-ic means -ate ion
H+ CO32-
H2CO3
H2CO3
H = (2)∙(+1) = +2
CO3 = (1)∙(-2) = -2
62
Practice - What are the formulas for the
following acids?
• chlorous acid
• phosphoric acid
• hydrobromic acid
63
Practice - What are the formulas for the
following acids?
• H+ with ClO2–
HClO2
• H+ with PO43–
H3PO4
• H+ with Br–
HBr
64
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
molecule or formula unit
whole = sum of the parts!
mass of 1 molecule of H2O
= 2(1.01 amu H) + 16.00 amu O = 18.02 amu
65