Download Chemistry 1 Lectures

Molecules and
Compounds
ed. Brad Collins
Some images Copyright © The McGraw-Hill Companies, Inc.
Sunday, August 18, 13
A molecule is an aggregate of two or more atoms in a
definite arrangement held together by chemical bonds
Chemical Bond
H2
H 2O
NH3
CH4
A diatomic molecule contains only two atoms
H2, N2, O2, Br2, HCl, CO
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4
3.1
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Molecules vs. Compounds
Molecules are composed of 2
or more atoms
Compounds are composed of
2 or more elements
Molecules have a definite
arrangement of atoms
Compounds have a definite ratio
of elements, but may or may not
have a definite arrangement of
those elements
Molecules are held together by
chemical bonds
Compounds may be held
together by chemical bonds or
by other attractive forces.
Examples of molecules:
H2, O3 (ozone), NH3, H2O, C6H12O6
Examples of compounds:
H2O, CH4, Al2O3, NaCl
3.1
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3.1
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Molecular mass (or molecular weight) is the sum of
the average atomic masses (in amu) in a molecule.
1S
SO2
2O
SO2
32.07 amu
+ 2 x 16.00 amu
64.07 amu
For any molecule
molecular mass (amu) = molar mass (grams)
1 molecule SO2 = 64.07 amu
1 mole SO2 = 64.07 g SO2
3.1
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Do You Understand Molecular Mass?
How many H atoms are in 72.5 g of C3H8O ?
1 mol C3H8O = (3 x 12) + (8 x 1) + 16 = 60 g C3H8O
1 mol C3H8O molecules = 8 mol H atoms
1 mol H = 6.022 x 1023 atoms H
1 mol C3H8O 8 mol H atoms 6.022 x 1023 H atoms
72.5 g C3H8O x
x
x
=
1 mol H atoms
60 g C3H8O
1 mol C3H8O
5.82 x 1024 atoms H
3.1
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Organic vs. Inorganic
Compounds
• Organic compounds are carbon-containing substances
• Do not include carbon-oxides, e.g., carbon dioxide (CO2),
carbon monoxide (CO) or carbonate (CO32–)
• 20-30 million known organic compounds
• Carbon can concatenate (C–C– C–C– C–C– C–C–C)
• Inorganic compounds are substances based on all the other
elements.
• ~500,000 known inorganic compounds
3.1
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Ionic Compounds
• Compounds composed of ions
• Ionic compounds don’t exist as molecules.
• Exist as 3-dimensional networks of ions: crystal lattice
A monatomic ion contains only one atom
Na+, Cl–, Ca2+, O2–, Al3+, N3–
A polyatomic ion contains more than one atom
OH–, CN–, NH4+, NO3–
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3.2
Examples of Monatomic Ions
3.2
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Do You Understand Ions?
How many protons and electrons are in
27 3+
13 Al ?
13 protons, 10 (13 – 3) electrons
For cations subtract the charge from the number of protons
How many protons and electrons are in
78
234 Se
?
34 protons, 36 (34 + 2) electrons
For anions add the charge to the number of protons
3.2
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ionic compounds consist of a combination of
cations and anions
• the formula is always the same as the empirical formula
• the sum of the charges on the cation(s) and anion(s) in
each formula unit must equal zero
The ionic compound NaCl
3.2
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Formulas of Ionic Compounds
2 x +3 = +6
3 x -2 = -6
Al2O3
Al3+
1 x +2 = +2
Ca2+
1 x +2 = +2
Na+
O22 x -1 = -2
CaBr2
Br–
1 x -2 = -2
Na2CO3
CO323.2
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A molecular formula shows the exact number of
atoms of each element in the smallest unit of a
molecule.
An empirical formula shows the simplest
whole-number ratio of the atoms in a substance
molecular
empirical
H 2O
H2O
C6H12O6
CH2O
O3
O
N2H4
NH2
Formulas for ionic compounds always reflect
their empirical formulas.
Sunday, August 18, 13
3.2
Hydrated Compounds
Definition: Ionic compounds (usually) that have a
specific number of water molecules trapped in their
crystal lattice.
CuSO4 • 5 H2O
copper sulfate pentahydrate
Anhydrous (anhydride) - a hydrate with the water
molecules driven off (usually upon heating)
CuSO4
copper sulfate
Molecular mass of hydrates: Include the mass
of water given in the formula.
3.2
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Covalent Compounds
An atomic force
microscopy image of a
graphene molecule
A ball-and-stick
model of graphene
Image credit: IBM Research
3.3
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Covalent Compounds
• Electrons are shared between 2 atoms in a covalent bond.
• Do not give up or take electrons - no ions formed
• Usually between 2 non-metals, or a non-metal and metaloid
• Formulas for covalent compounds are molecular formulas.
• Examples:
Sunday, August 18, 13
• NH3
ammonia
• H2O
water (dihydrogen oxide)
• CO2
carbon dioxide
3.3
Naming Compounds
Organic Compounds:
Contain carbon (except oxides, sulfides, cyanide)
Inorganic Compounds:
Everything else, divided into 4 classes
Ionic compounds
Molecular compounds
Acids and Bases
Hydrates
3.4
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Chemical Nomenclature
• Ionic Compounds
– often a metal + nonmetal (binary compounds)
– anion (nonmetal), add “ide” to element name
BaCl2
barium chloride
K2O
potassium oxide
MgCl2
magnesium chloride
K3N
potassium nitride
3.4
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3.4
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Naming Polyatomic Ions: 8 “-ates” + 5
Names for all the polyatomic ions and the acids
derived from them can be derived from the names
of 12 core ions: 8 “-ates” + 4
8 “-ates”
“+ 5”
carbonate
CO32–
cyanide
CN–
chlorate
ClO3–
thiocyanate
SCN–
chromate
CrO42–
hydroxide
OH–
dichromate
Cr2O72–
peroxide
O2–
nitrate
NO3–
ammonium
phosphate
PO43–
NH4+
manganate
MnO3–
sulfate
SO42–
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per- ____ -ate
hypo- ____ -ite
Hydrogen in front of an “-ate” ion
is named hydrogen ____ -ate, except
sulfate, which is bisulfate.
Naming Ionic Compounds
• With Polyatomic Ions
– sometimes a metal + polyatomic ion
– Use the metal and the polyatomic ion naming rules
Ba(ClO2)2
barium chlorite
KNO3
potassium nitrate
Mg(OH)2
magnesium hydroxide
KClO4
potassium perchlorate
3.4
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Naming Ionic Compounds
•
Transition metal ionic compounds have variable charges
– Iron can be +2 or +3
– Smaller charge is sometimes named as an ‘ic’ ion higher charge as an ‘ous’ ion
– So in ferric chloride (FeCl2) iron ion is Fe2+
– Modern method is to indicate charge on the metal with Roman numerals
– So FeCl2 is now named iron(II) chloride
FeCl2 2 Cl- are -2 so Fe is +2
iron(II) chloride
FeCl3 3 Cl- are -3 so Fe is +3
iron(III) chloride
Cr2S3 3 S-2 are -6 so Cr is +3 (6/2)
chromium(III) sulfide
3.4
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• Covalent compounds
• Between 2 (or more) nonmetals or
nonmetals + metalloids
• Naming:
• Some have common names:
• H2O, NH3, CH4, C60
• Otherwise:
• Element further left in periodic table is 1st
• Element closest to bottom of group is 1st
• Last element ends in ide (for inorganics)
• If more than one compound can be formed from
the same elements, use prefixes to indicate
number of each kind of atom
3.4
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Covalent Compounds
HI
hydrogen iodide
NF3
nitrogen trifluoride
SO2
sulfur dioxide
N2Cl4
dinitrogen tetrachloride
NO2
nitrogen dioxide
N2O
dinitrogen monoxide
TOXIC!
Laughing Gas
3.4
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Exceptions: Molecular Compounds
B2H6
diborane
CH4
methane
SiH4
silane
NH3
ammonia
PH3
phosphine
H2O
water
H2S
hydrogen sulfide
3.4
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Naming Compounds
Compound
Ionic
Molecular
NH4+
Metal or
plus mono or
polyatomic anion
Yes
Alkali metal ions
Alkaline earth metal ions
Ag+, Al3+, Cd2+, Zn2+
✤ Name
metal ion first,
then anion
✤ If monatomic anion,
add “ide” to the end
of the root of the
element name
✤ If polyatomic ion, use
8 “-ates” + 4 rules
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Cation
has only one
charge
Binary compounds of nonmetals
or nonmetals and metalloids
No
Transition metal cations
✤ Name
metal ion first
✤ Specify charge on
cation with a Roman
numberal
✤ If monatomic anion, add
“ide” to the end of the
root of the element
name
✤ If polyatomic ion, use 8
“-ates” + 4 rules
✤ Name
element furthest
left or furthest down in a
group first.
✤ Add “ide” to the end of
the root of the second
element’s name
✤ Use prefixes to
designate the numbers
of each element present
✤ The mono prefix is
usually omitted
3.4
Practice
• Name the following compounds:
• SiCl4
• P4O10
• What are the formulas for the following compounds?
• carbon dioxide
• disilicon hexabromide
3.4
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Acids and Bases
An acid can be defined as a substance that yields
hydrogen ions (H+) when dissolved in water.
HCl
•Pure substance, hydrogen chloride
•Dissolved in water (H+ Cl-), hydrochloric acid
Binary acids consist of hydrogen and one other atom:
•Examples: HF, HCl, HBr, HI, H2S
3.4
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Naming Binary Acids
“-ides” are “-ic”-y
3.4
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An oxoacid is an acid that contains hydrogen,
oxygen, and another element.
HNO3
nitric acid
H2CO3
carbonic acid
H2SO4
sulfuric acid
HNO3
“-ates” are “-ic”-y and “-ites” “-ous”
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3.4
Naming Oxo-acids
Per______ate
______ate
______ite
hypo______ite
Add H+
Add H+
Per______ic acid
______ic acid
“Representative” -ic acid
Add H+
Add H+
______ous acid
hypo______ous
3.4
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Worked Examples 2.8
3.4
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Acids and Bases
A base can be defined as a substance that yields
hydroxide ions (OH–) when dissolved in water.
NaOH
KOH
Ba(OH)2
sodium hydroxide
potassium hydroxide
barium hydroxide
Name the cation, then add “hydroxide”
Don’t use prefixes with “hydroxide”
3.4
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3.4
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