Ch 8 - Compounds & Molecules
8.1 Ionic Compounds – Fr 2-288.2 Molecular Compounds - Fr 29-528.3 Intermolecular Forces – Fr 53- 82
8.4 Formula Masses – Fr 83-110
In early chapters we discussed physical and chemical properties
In later chapters we discussed atom structures and chemical bonding
Each chloride ion is attracted to all nearby
sodium ions
Each sodium ion is attracted to all nearby
chloride ions
Structure of NaCl
We have seen that in an ionic salt each atom interacts with all its neighbors.
In any ionic crystal, the ratio of positive ions to negative ions must allow for all of the positive charge to cancel out all of the negative charge.
Lead(II) sulfide(PbS)
Aluminum oxide(Al2O3)
Copper(II) sulfate(CuSO4)
Pb2+ and S2–
(+2) and (–2)2Al3+ and 3O2–
(+6) and (–6)Cu2+ and SO4
2–
(+2) and (–2)
The ionic structure leads to the following physical properties:
Ionic substances are solid at room temperature
Ionic substances have very high melting points
Properties
To be in a liquid state, there must be enough energy for each atom to break free from the attractive forces from its neighbors.
The interconnected network of bonds holds the entire crystal together
The ionic structure leads to the following physical properties:
Ionic substances are solid at room temperature
Ionic substances have very high melting points
Ionic substances are hard
Properties
Ionic substances are brittle, which means they break
The ionic structure leads to the following physical properties:
Ionic substances are solid at room temperature
Ionic substances have very high melting points
Ionic substances are hard but brittle
Properties
When melted or dissolved, ions are free to move around, making it possible for an ionic solution to conduct electricity
Ions dissolved in water move in random directions by colliding
with water molecules
Overall ion movement toward the electrodes creates an
electrical current
Properties
Polyatomic ions
An ion does not have to be a single atom. An ion can be a small molecule with a charge.
a polyatomic ion
a monatomic ion
Polyatomic ions
Calcium carbonate, CaCO3
The calcium ion contributes a +2 charge (Ca2+)
The CO3 molecule needs two extra electrons.
Polyatomic ions
The two oxygens now have a –1 charge,
giving the whole molecule a –2 charge
The carbonate ion switches among these three versions.
Now we will learn how to:
- Write formulas for ionic compounds
- Write names for ionic compounds
- Name ions with transition metals
Write the correct formulas for the compounds that will form using the following ion pairs: Mg2+ and Cl–, Na+ and S2–, Al3+ and O2–.
Writing formulas
Naming transition metal ions
Most transition metals can have different charges
Check whether a Roman numeral is necessary for elements in the highlighted parts of the
periodic table
A Roman numeral indicates the charge on a positive ion
Naming transition metal ions
Consider PbCl2
Check the common ions table
Use the negative ion to determine the charge on lead
With only one Pb ion, and a total charge of +2 needed,
the Pb ion must be lead(II)
Naming transition metal ions
Check the common ions table
Zn only forms a +2 ion
No Roman numeral is needed
Consider ZnSO4
Write the names for each of the following formulas: CaCl2, Zn(NO3)2, Fe3(PO4)2.
Asked: Names for the formulas aboveGiven: Three formulas and a common ion tableRelationships: The name of each formula is
constructed form the name of the ions. If the negative ion is a single atom, modify the name of the element to end in “-ide.” If the positive ion might form multiple charges, make sure to use a Roman numeral in the name.
Write the names for each of the following formulas: CaCl2, Zn(NO3)2, Fe3(PO4)2.
Asked: Names for the formulas aboveGiven: Three formulas and a common ion tableRelationships: The name of each formula is
constructed form the name of the ions. If the negative ion is a single atom, modify the name of the element to end in “-ide.” If the positive ion might form multiple charges, make sure to use a Roman numeral in the name.
Solve: CaCl2 is calcium chloride.Zn(NO3)2 is zinc nitrate.Fe3(PO4)2 is iron(II) phosphate.
Naming simple molecular compounds
P2S3
Consider the following molecular compound:
two phosphorous atomswith three sulfur atoms
diphosphorous trisulfide
Naming simple molecular compounds
H2O
Consider the following molecular compound:
two hydrogen atomswith one oxygen atom
dihydrogen monoxide
(another name for water)
Naming simple molecular compounds
CO
Consider the following molecular compound:
one carbon atomwith one oxygen atom
carbon monoxide
Drop “mono” for the first element
Naming simple molecular compounds
Write the name for each of the following formulas:
N2O4,
S2F10,
SO3.
Naming simple molecular compounds
Write the name for each of the following formulas: N2O4, S2F10, and SO3.
Solve: N2O4 is dinitrogen tetraoxide.
S2F10 is disulfur decafluoride.
SO3 is sulfur trioxide.
Properties of molecular compounds vary widely
HardnessSome are hard and brittle; some are flexible,
soft or mushyState of matter
They can be solid, liquid or gas at room temperatureBoiling points
Their boiling points can vary from –253oC to over 1,000oCAbility to conduct electricity
Most do not conduct electricity well
Water
Water is a covalent compoundAbout eight million billion billion (1024) water molecules make up
this cup of water
Properties of molecular substances depend
on:
- the structure of the individual molecule
- the attractions between molecules
Small
Caffeine, found in tea, coffee,
and soda
Medium
Wax, one of the main hydrocarbons
in beeswax
Molecules can be classified into different categories
Large: polymer Large: network
Protein,molecule on the right shows a thick line
tracing the linear chain with red and blue marking either end
Graphene,layers of which make
graphite
Molecules can be classified into different categories
Small molecules
Acetaminophen(Tylenol)
is a liquid at room temperature
Most small molecules (no more than a dozen atoms) are liquids or gases at room temperature
Some of the most important molecules related to our ecosystem are small molecules
Water
If ice did not float, ponds would freeze from the bottom up, killing everything inside.
Most small molecules (no more than a dozen atoms) are liquids or gases at room temperature
Some of the most important molecules related to our ecosystem are small molecules
Small molecules
Ethene(or ethylene)
Nonpolar (gas at room temperature)Boiling point = –103.7oC
Ethanol
Polar (liquid at room temperature)Boiling point = 78.4oC
Medium-sized molecules
Candle wax is a mixture of medium-sized hydrocarbons
Medium-sized molecules (no more than 100 atoms) tend to be liquids or soft solids at room temperature.
They are often long-chain hydrocarbons or lipids
hydrocarbon: a molecule made entirely from carbon and hydrogen atoms.
Medium-sized molecules
Linoleic acid, a major component of vegetable oil
Lipids are typically fats or steroids
Fats are nonpolar and insoluble in water
Medium-sized molecules
Testosterone(a steroid)
Most steroids have four rings of carbon atoms.
They can act as hormones, drugs, vitamins, or poisons
1
2
3
4
Polymers
Polyvinyl chloride (PVC) is made by bonding
many vinyl chloride monomers together
A single monomer of vinyl chloride
A single polymer chain typically contains hundreds to thousands of atoms
Recycling symbols and polymer codes
Polymers
polymer: a long chain molecule formed by connecting small repeating units with covalent bonds.
monomer: a small molecule that is a building block of larger molecules called polymers.
Polyvinyl chloride
Polymers
Glucosea natural monomer
Cellulose fragment
Starch fragment
Homopolymers are made from only one type of monomer
Networks
network covalent: a type of large structure, usually made from hundreds to billions of atoms, in which each atom is covalently bonded to multiple neighboring atoms, forming a web of connections.
Silicon dioxide (or quartz)is a mineral in sand
Since every atom in quartz is covalently bonded together, the entire grain can be considered a single network.
Networks
Carbon forms many types of network covalent substances
Diamond BuckyballsCarbon nanotubes
Graphite
Chemical formulas
Use the empirical formula to describe the simplest ratio of elements of that substance
a molecule of water
Use the molecular formula to indicate the exact type and number of each atom in a single molecule of that substanceA sample of ionic or network covalent
substance is a single bonded unit of material
Use the molecular formula to indicate the exact type and number of each atom in a single molecule of that substance
Chemical formulas
Benzene
Empirical formula: Molecular formula:
CH C6H6
a molecule of water
Write the molecular formula for each of the following molecules:
Chemical formulas
Asked: The molecular formulaRelationships: The formula should reflect the
exact number and type of atoms in an individual molecule of the substance.
Solve: A. C2H2 B. NH3 C. C2H5O
We have seen in Chapter 3 that molecules in solids and liquids are held together by intermolecular forces
What are these forces? Where do they come from? Do all molecules feel them?
A tiny drop of waterIntermolecular attraction
As a liquid, water molecules can move around but intermolecular forces keep them from separating completely to become a gas.
A quick look at water
strong
London dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
Intermolecular attractionsweak
strong
London dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
Intermolecular attractionsweak
Dipole-dipole attractions
Like water, formaldehyde is a polar molecule
The polar covalent C=O bond makes the entire molecule polar
We say the molecule has a dipole
dipole-dipole attraction: the attractions between the positive part of one polar molecule and the negative part of another polar molecule.
Dipole-dipole attractions
Like water, formaldehyde is a polar molecule
Dipole-dipole attractions
Dipole-dipole attractions cause formaldehyde to condense into a liquid at room temperature
Dipole-dipole attractions
more
less
Dipole-dipole attractions
Molecules that are more polar will attract strongly.
Dipole-dipole attractions
more
less
Dipole-dipole attractions
Molecules that are more polar will attract strongly.
Boiling point
Dipole-dipole attractions
Molecules that are more polar will attract strongly. more
less
Molecules that attract more strongly
will have a boiling point. higher
lower
Boiling point
Dipole-dipole attractions
Molecules that attract more strongly
will have a boiling point.
higher
lower
Molecules that are more polar will attract strongly. more
less
Dipole-dipole attractions
Higher polarity molecules attract more strongly and have a higher boiling point
propane 1-propanol 1,3-propanediol
least polar
most polar
–42oC 97oC 214oCBoiling points
strong
London dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
Intermolecular attractionsweak
Hydrogen bonding
Electronegativity
There is a moderate difference in electronegativity between H and F, O and N (0.94 to 1.88)
Hydrogen bonding
Iceberg photo courtesy of NOAA
One special property of water:Ice is less dense than water in the liquid form
There is more space in between water molecules in ice Water in the liquid form
Hydrogen bonding
surface tension: a force acting to pull a liquid surface into the smallest possible area.
Why a drop of water doesn’t “lie flat” on a hard surface:
In reality water molecules are much, much smaller than on the drawing!
H-bonds keep the water molecules together
Hydrogen bonding
DNA uses hydrogen bonds to hold the two strands together
Hydrogen bonds
Hydrogen bonding plays a crucial role in DNA and protein structures
Hydrogen bonding
Hydrogen bonding plays a crucial role in DNA and protein structures
The protein structure is stabilized with H bonds
Hydrogen bonding
Paper glue is a mixture of polyvinyl acetate (PVA) and water
In “wet” glue, polymer molecules are lubricated by water
Hydrogen bonding
Paper glue is a mixture of polyvinyl acetate (PVA) and water
In “wet” glue, polymer molecules are lubricated by water
As glue dries, many more H-bonds form between the polymer molecules, so the glue hardens
strong
London dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
Intermolecular attractionsweak
London dispersion
Isolated hydrogen molecules are nonpolar
A temporary, very small polarity can be induced when nonpolar molecules are close enough
Molecules with a larger surface area
will attract ____strongly. more
less
London dispersion
propane
pentane
A temporary, very small polarity can be induced when nonpolar molecules are close enough
Molecules with a larger surface area
will attract ____strongly. more
less
London dispersion
propane
pentane
A temporary, very small polarity can be induced when nonpolar molecules are close enough
Molecules with a larger surface area
will attract ____strongly.
Boiling point
Molecules that attract more strongly
will have a boiling point. higher
lower
more
less
London dispersion
Boiling point
Molecules that attract more strongly
will have a boiling point. higher
lower
London dispersion
(It takes more energy to overcome the intermolecular forces.)
more
Molecules with a larger surface area
will attract ____strongly. less
Molecules with larger surface area attract more strongly and have a higher boiling point
propane butane pentane
least surface area
–42oC 0oC 36oCBoiling points
most surface area
London dispersion
London dispersion
The shape of the molecule also matters!
About the same surface area
Stronger attraction Weaker attractionHigher boiling point Lower boiling point
Intermolecular attractionsweak strong
London dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
Molar mass
Calculating molar mass
H2SO4
Subscripts apply only to the element or group they follow
For H: 2 x 1.0079 g/mole = 2.02 g/mole
For S: 1 x 32.065 g/mole = 32.07 g/mole
For O: 4 x 15.999 g/mole = 64.00 g/mole
For H2SO4: 98.08 g/mole
Molar mass
Calculating molar mass
(NH4)3PO4
If the subscript follows a parenthesis, then multiply everything inside the parentheses by the subscript
For N: 3 x 14.007 g/mole = 2.02 g/mole
For H: 12 x 1.0079 g/mole = 32.07 g/mole
For P: 1 x 30.974 g/mole = 30.97 g/mole
For O: 4 x 15.999 g/mole = 64.00 g/mole
For (NH4)3PO4: 149.08 g/mole
What is the molar mass of aluminum carbonate, Al2(CO3)3?
Molar mass
Asked: The molar mass
Given: The name of the formula
Relationships: The name of the formula can be used to determine the ratios of elements in the formula. Then the periodic table can be used to get the molar mass of each of the elements used.
Solve: Aluminum carbonate is ionic, so the formula is Al2(CO3)3.
Percent composition
You can identify a compound using the percent mass of each element.
NaCl
% by atom numberNa: 50% Cl: 50%
% by massNa: 39% Cl: 61%
1 mole of Cl is heavier than 1 mole of Na
Calculate % composition using a chemical formula
Calculate % composition with a measured sample
Two types of problems
Find the % composition of Ca3(PO4)2.
We know the % composition of a white powder. Determine if this powder is common table sugar.
Find the % composition of Ca3(PO4)2.1. Calculate the molar mass
2. Calculate the percent of each element in a mole of the substance
For Ca: 3 x 40.078 = 120.23 g/mole
For P: 2 x 30.974 = 61.95 g/mole
For O: 8 x 15.999 = 127.99 g/mole
For Ca3PO4: 310.17 g/mole
Percent composition
Find the % composition of Ca3(PO4)2.1. Calculate the molar mass
2. Calculate the percent of each element in a mole of the substance
Ca: 120.24 g/mole
P: 30.97 g/mole
O: 64.00 g/mole
Ca3PO4:
310.17 g/mole
120.24100
310.17
30.97100
310.
%
%
%
38.77%
9.98%
20.6
17
64.00100
310.173%
Ca
P
O
Percent composition
Calculate % composition using a chemical formula
Calculate % composition with a measured sample
Two types of problems
Find the % composition of Ca3PO4.
We know the % composition of a white powder. Determine if this powder is common table sugar.
1.05 g0.16 g1.29 g
2.50 g
C:H2:O2:
Total of unknown:
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
1. Add up the individual mass measurements to get the total mass
2. Calculate the percent composition of the unknown
3. Compare with the percent composition of sucrose
Percent composition
Total of unknown: 2.50 g
1.05100
2.50
0.16100
2.50
42.0%
6.4%
%
%
%1.29
100 515
.6%2. 0
C
H
O
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
1. Add up the individual mass measurements to get the total mass
2. Calculate the percent composition of the unknown
3. Compare with the percent composition of sucrose
Percent composition
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
1. Add up the individual mass measurements to get the total mass
2. Calculate the percent composition of the unknown
3. Compare with the percent composition of sucrose
12.01 144.13 /
1.0079 22.17 /
15.999 175.99 /
: 342
:
:
:
12
22
11
.29 /
C
H
O
g mole
g mole
g mole
Molar mass g mole
Percent composition
For sucrose:
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
1. Add up the individual mass measurements to get the total mass
2. Calculate the percent composition of the unknown
3. Compare with the percent composition of sucrose
144.13100
342.29
22.17100
342.
%
%
%
42.1%
6.5%
51.
29
175.99100
342.294%
C
H
O
For sucrose:
Percent composition
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
144.13100
342.29
22.17100
342.
%
%
%
42.1%
6.5%
51.
29
175.99100
342.294%
C
H
O
% composition of unknown:
% C: 42.0%% H: 6.4%% O: 51.6%
% composition of sucrose:
YES!The percent composition is very close,
so the unknown is probably sugar
Percent composition
Empirical formulas
ClNa
NaCl Cl
Na
The empirical formula is the one with the simplest ratio
Na6Cl6ClNa
NaCl Cl
NaClNa NaCl
1:1 ratio6:6 ratio
Calculating an empirical formula1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formulaAssume you have a sample that is 0.504 g hydrogen and 4.00 g oxygen.
Empirical formulas
2
1
:0.5
:
00
0.250
0.250
0.250
H
O
10.504
1.00.500
0.
1
14.00
16.000
:
25:
H
O
moleg
g
moleg
g
moles
moles
smallest number of moles
H2O
2.0
1.
0.04:
:
66
0.023
0.0230
0.023
Cu
O
1: 2.96 0.0466
63.55
1: 4.00 0.023
15.999
moleCu g moles
g
moleO g moles
g
Empirical formulas
A jar of powder is labeled copper oxide, but you do not know whether the copper is Cu+ or Cu2+. After taking a small sample you find that it is made from 2.96 g of copper and 0.37 g of oxygen. What is the correct name and formula for the ionic compound in that jar?
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula
Cu2O
Empirical formulas
A jar of powder is labeled copper oxide, but you do not know whether the copper is Cu+ or Cu2+. After taking a small sample you find that it is made from 2.96 g of copper and 0.37 g of oxygen. What is the correct name and formula for the ionic compound in that jar?
Cu2OCu+
Cu+
O2–
+2 –2
Cu2+
Cu2+
O2–
+4 –2
Copper(I) oxide
Calculating an empirical formula1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formulaAssume you have a sample that is 0.504 g hydrogen and 4.00 g oxygen.
Empirical formulas
2
1
:0.5
:
00
0.250
0.250
0.250
H
O
10.504
1.00.500
0.
1
14.00
16.000
:
25:
H
O
moleg
g
moleg
g
moles
moles
smallest number of moles
H2O
2.0
1.
0.04:
:
66
0.023
0.0230
0.023
Cu
O
1: 2.96 0.0466
63.55
1: 4.00 0.023
15.999
moleCu g moles
g
moleO g moles
g
Empirical formulas
A jar of powder is labeled copper oxide, but you do not know whether the copper is Cu+ or Cu2+. After taking a small sample you find that it is made from 2.96 g of copper and 0.37 g of oxygen. What is the correct name and formula for the ionic compound in that jar?
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula
Cu2O
Empirical formulas
A jar of powder is labeled copper oxide, but you do not know whether the copper is Cu+ or Cu2+. After taking a small sample you find that it is made from 2.96 g of copper and 0.37 g of oxygen. What is the correct name and formula for the ionic compound in that jar?
Cu2OCu+
Cu+
O2–
+2 –2
Cu2+
Cu2+
O2–
+4 –2
Copper(I) oxide
Molecular formulas
The empirical formula can be the same as the molecular formula…
CH2O Formaldehyde
…but not always.
C6H12O6Glucose
CH2O
CH2O
Molecular formula
Empiricalformula
Molecular formulas
C6H12O6 GlucoseCH2O
Molecular formula
Empiricalformula
The molecular mass will always be equal to, or a multiple of, the empirical formula mass.
180.16 g/mole30.03 g/mole
x 6
x 6
Molecular formulas
CH
Molecular formula
Empiricalformula
13.018 g/mole
C?H?
78.11 g/mole
For CH: 12.011 + 1.0079 = 13.018 g/mole
Given the following empirical formula and molar mass, determine the molecular formula for CH (78.11 g/mole).
Molecular formulas
CH
Molecular formula
Empiricalformula
13.018 g/mole
C6H6
78.11 g/molex 6
The molecular formula is C6H6
Given the following empirical formula and molar mass, determine the molecular formula for CH (78.11 g/mole).
Calculate % composition using a chemical formula
Calculate % composition with a measured sample
Two types of problems:
Find the % composition of Ca3(PO4)2.
We know the % composition of a white powder. Determine if this powder is common table sugar.
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