Download 24 Sept 08 - Seattle Central College

Plan for Wed, 24 Sept 08
• Lecture
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Classifying and separating mixtures (1.9)
How do we know that there are atoms? (2.2)
Ok, there are atoms. What do they look like? (2.5)
Re-introducing the periodic table (2.6-7)
Chemical nomenclature (2.8)
• Make sure to read:
– 2.3 (Dalton’s atomic theory)
• Sections you can skip:
– 2.1 (early history of chemistry), 2.4 (early experiments
to characterize the atom)
Pure Substances vs Mixtures
Pure substances cannot
be separated into
different components by
physical means.
Mixtures can be
separated into
different components.
The unique
properties of the
components of a
mixture can be used
to separate the
components.
Homogeneous vs Heterogeneous Mixtures
A Homogeneous
Mixture is a mixture that
has a uniform
appearance and
composition throughout
A Heterogeneous
Mixture is a mixture in
which you can identify
the component parts just
by looking at it
Homogeneous mixtures are often
indistinguishable from pure substances on the
macroscopic level. You have to consider an
unknown sample on the molecular level to
determine if it is a mixture or pure substance.
Separation Techniques
• Since components in a mixture retain their
identities, we can exploit the unique
properties of the components to separate
them.
• The more similar the properties are, the
more difficult it is to separate the
components.
Separation of Heterogeneous Mixtures
...by particle size
Cacao beans grow in
the jungles of
equatorial countries,
many of which are
rather dangerous.
Before chocolate
manufacturers can
roast the cacao beans
from their suppliers,
they must remove
sticks, stones, and
sometimes bullets!!
freshly harvested cacao beans
plus some other junk
chocolate fountain!
roasted cacao beans
Separation of Heterogeneous Mixtures
…by magnetism
In this example, the magnetic property of the iron filings is used to
separate it from the non-magnetic sulfur powder.
Physical Properties of
Homogeneous Mixtures
• Pure substances have a
unique set of physical
properties that are different
from any other pure
substance
• The physical properties of
mixtures are not
unique...they differ with the
relative amounts of the
components in the mixture
Change in Boiling Point of a
Solution vs. a Pure Liquid
Separating Liquid-Liquid Mixtures
Change in Boiling Point of a
Solution vs. a Pure Liquid
Ch 2 – Atoms Molecules and Ions
What did we know in the 1800’s?
• Most natural materials are mixtures of
pure substances.
• Pure substances are either elements or
combinations of elements called
compounds.
• A given compound always contains the
same proportions (by mass) of the
elements...this is the law of definite
composition
Law of Definite Composition
A given compound always has the same
composition, regardless of where it comes from.
...or, a chemical compound always contains
exactly the same proportion of elements by
mass.
Water:
8 g oxygen (O) to 1 g hydrogen (H)
Carbon dioxide:
2.7 g oxygen (O) to 1 g carbon (C)
Law of Multiple Proportions
When two elements combine to
form more than one
compound, the different
weights of one element that
combine with the same weight
of the other element are in a
simple ratio of whole numbers.
What this means at the particulate
level is that when elements
combine, they do so in the
ratio of small whole numbers.
For example: carbon and
oxygen react to form CO or
CO2, but not CO1.8.
Fig. 5-2, p. 121
Ok, there are atoms. What do they
look like? Negatively-charged
• It was known that atoms were
neutrally-charged, but that they
contained negatively-charged
particles called electrons.
• Lord Kelvin (yes, that Lord
Kelvin) proposed the “PlumPudding” Model (we will call it
the “Chocolate-Chip Cookie”
Model) of atomic structure.
• In this model, the atom is
composed of discrete,
negatively-charged electrons
embedded in a cloud of uniform
positive charge.
electrons
Uniform positive
charge
The Nuclear Atom
1911 – Ernest Rutherford demonstrated
the nuclear nature of the atom in which
the empty space is 10,000 to 100,000
times larger than the size of the
nucleus.
Atomic Structure
1. Every atom contains small, dense nucleus.
2. All of the positive charge and most of the
mass are concentrated in the nucleus.
3. The nucleus is surrounded by a large volume
of nearly empty space that makes up the rest
of the atom.
4. The rest of the atom is thinly populated by
electrons, the total charge of which exactly
balances the positive charge of the nucleus.
If an atom was the
size of a baseball
stadium, the
nucleus would be
the size of a fly on
home plate.
What’s in an Atom?
• Electron
– mass = 9.11 x 10-28 g
– charge = -4.8080 x 10-10 esu
• Proton
– mass = 1.67 x 10-24 g
– charge = +4.8080 x 10-10 esu
• Neutron
– mass = 1.68 x 10-24 g
– charge = 0 esu
0.000549 amu (call this 0 amu)
-1
1.00728 amu (call this 1 amu)
+1
1.00867 amu (call this 1 amu)
0
(where esu = electrostatic unit; amu = atomic mass unit)
1 amu = 1.66 x 10-24 g
1 amu = 1/12 the mass of one carbon atom
What do the numbers of different
particles mean?
•
# Protons = chemical identity of the atom (which element is it?)
– In an electrically-neutral atom, the number of protons in the nucleus is exactly
balanced by the number of electrons.
•
# Electrons = ionic character of the atom. An ion has either more or fewer
electrons than the electrically-neutral atom.
– anion = more electrons, so ion is negatively-charged
– cation = fewer electrons, so ion is positively-charged
•
# Neutrons = isotopic character of the atom
– an atom of an element usually comes in at least 2 or 3 different isotopes
(sometimes more)
– usually there will be one isotope that is far more abundant than the others
•
If the number of protons is changed, the chemical identity of the atom is
changed.
A few definitions. . .
• Atomic number (Z): the number of protons in the nucleus of an atom
• Mass number (A): the sum of the numbers of protons and neutrons in
the nucleus of an atom
• Atomic Mass: the mass of one atom, expressed in amu
• Atomic Weight: an average of the atomic masses of the most common
isotopes
Atomic
Symbol
A
X
Z
Element
symbol (X)
8
In the periodic
table...
O
16.00
16
For example:
O
8
or 16O
Atomic number (Z)
Atomic symbol (X)
Atomic Weight
(related to A)
Let’s count some particles
# protons
# electrons
# neutrons
Cobalt-60
60
Co
27
27
27
60 – 27 = 33
Chlorine-37
37
Cl
17
17
17
37 – 17 = 20
Uranium-238
238
U
92
92
92
238 – 92 = 146
QUESTION
Of the following three choices, which would have the greatest
number of neutrons?
1.
137Ba2+
A = 137
Z = 56
# neutrons = 137 – 56 = 81
2.
128Te2–
A = 128
Z = 52
# neutrons = 128 – 52 = 76
3.
133Cs
A = 133
Z = 55
# neutrons = 133 – 55 = 78
QUESTION
Of the following, which would NOT qualify as an isotope of
35Cl?
1.
36Cl
2.
35Cl–
3.
37Cl–
Not an isotope because # neutrons is the same.
QUESTION
Calcium plays several critical roles in the functioning of human
cells. However, this form of calcium is the ion made with 20
protons and 18 electrons. Therefore the ion would be…
1. positive and called an anion.
2. positive and called a cation.
3. negative and called an anion.
4. negative and called a cation.
Meet the Periodic Table
1A
2A
Hello!!
8A
3A 4A 5A 6A 7A
Group
Period
• Alkali Metals ... soft, shiny metals; react vigorously with water;
rarely found in elemental form
• Alkaline Earth Metals ... soft, shiny metals; react less vigorously
with water than alkali metals; rarely found in elemental form
• Halogens ... gases: F, Cl; liquid: Br; solid: I; highly reactive; F is the
most reactive element; all quite toxic; not found in elemental form
• Noble Gases ... all gases; largely unreactive, although Kr and Xe
can form compounds; found in minute quantities in the atmosphere
1A
8A
3A 4A 5A 6A 7A
2A
B
Non-metals
Si
Ge As
Metals
Sb Te
Po At
• Metals … good conductors of heat, electricity; malleable
solids. Tend to lose electrons in reactions to form cations.
• Non-metals … poor conductors; not malleable. Tend to
gain electrons in reactions to form anions.
• Metalloids … both metallic and nonmetallic properties
QUESTION
From the following list select the element that is most likely to
become an anion during a chemical reaction.
1. Hydrogen
2. Tungsten
3. Germanium
4. Bromine
Nonmetals tend to gain electrons in reactions.
QUESTION
Of the following, which is most likely to become a cation as a
result of a chemical reaction? What would be the charge on that
cation?
1. N; –3
2. Ne; +1
3. Na: +1
Metals tend to lose electrons in reactions.
4. Not enough information given to predict.
Chemical Nomenclature Outline
• Chemical formulas of elements
• Naming binary compounds (two elements)
– compounds containing a metal and a nonmetal, aka ionic
compounds
• Type I: metal forms one kind of cation (we have seen these before)
• Type II: metal can form more than one kind of cation
– compounds containing two nonmetals
• Naming compounds that contain polyatomic ions
• Naming Acids and their anions
– acids that do not contain oxygen typically give monatomic ions
(exceptions include CN-)
– acids that contain oxygen (“oxyacids”) always give polyatomic ions
Formulas of Elements
• The chemical formula of most elements is
the atomic symbol, e.g. Li, Os, Xe, Pu
• Some elements form diatomic molecules.
Memorize these: H2, N2, O2, F2, Cl2, Br2, I2
Horses Need Oats For Clear Brown I’s
• Other exceptions: P4, S8, C60
Binary Ionic Cmpds (Type I)
Metal can form only one kind of cation
Note: These cations are not
isoelectronic with any noble gas!!
Ni2+
Zn2+ Ga3+
Ag+ Cd2+
Binary Ionic Cmpds (Type I)
Cations (Mn+): name of atom + cation
– Magnesium: Mg2+ ... magnesium cation
– Cesium: Cs+ ... cesium cation
Anions (Xm-): root of atom name + -ide
– Fluorine: F- ... fluoride anion
– Sulfur: S2- ... sulfide anion
– Selenium: Se2- ... selenide anion
Compound Name: <cation name> <anion name>
Chemical Formula: MmXn
Example
• Name the following compound: KCl
potassium chloride
• Name the following compound: CaBr2
calcium bromide
• Write the formula for: barium hydride
BaH2
• Write the formula for: aluminum sulfide
Al2S3
Binary Ionic Cmpds (Type II)
Metal can form more than one kind of cation (see table 6.7)
These are a few common examples
of metals that can form different
cations. This is not an exhaustive list.
Binary Ionic Cmpds (Type II)
Cations (Mn+): name of atom + (oxidation state) + cation
– Oxidation state = charge. Use Roman numerals.
– Iron:
• Fe2+ ... iron(II) cation
• Fe3+ ... iron(III) cation
– Chromium:
• Cr2+ ... chromium(II) cation
• Cr3+ ... chromium(III) cation
Anions (Xm-): root of atom name + -ide (same as before)
Compound Name: <cation name> <anion name>
Chemical Formula: MmXn
Example
• Name the following compound: CuCl
copper(I) chloride
• Name the following compound: MnO2
manganese(IV) oxide
• Write the formula for: vanadium(V) fluoride
VF5
• Write the formula for: tin(IV) bromide
SnBr4
Binary Covalent Cmpds (Type III)
(two nonmetals, or a nonmetal and a metalloid)
1. The first element in the
formula is named first, and the
full element name is used.
2. The second element is named
as though it were an anion.
3. Prefixes are used to indicate
the number of atoms present.
4. The prefix mono- is never
used for naming the first
element.
• i.e., CO is carbon monoxide, not
monocarbon monoxide.
Prefix
# Indicated
mono-
1
di-
2
tri-
3
tetra-
4
penta-
5
hexa-
6
hepta-
7
octa-
8
nona-
9
deca-
10
Example
• Name the following compound: BF3
boron trifluoride
• Name the following compound: I2O7
diiodine heptoxide
• Write the formula for: phosphorus trichloride
PCl3
• Write the formula for: dinitrogen trioxide
N2O3
Table 2.5 Common Polyatomic Ions
Ionic Compounds with Polyatomic Ions
• Writing names and formulas is pretty much the same as
for binary compounds…
– cation is named first, anion is named second
– multiples of the ions are taken to ensure charge neutrality
• Example:
– Na2SO4 … 2Na+ + SO42-  sodium sulfate
– manganese(II) hydroxide … Mn2+ + 2OH-  Mn(OH)2
• The tricky part is learning where one ion ends and
the next begins.
• Example:
– KHSO4 … K+ + HSO4-  potassium hydrogen sulfate
– NH4C2H3O2 … NH4+ + C2H3O2-  ammonium acetate
– NaH2PO3 ... Na+ + H2PO3-  sodium dihydrogen phosphite
• Lucky for you guys, there are only a few common
polyatomic cations: NH4+ and Hg22+
Flowchart for Naming Compounds
No
Polyatomic ion(s)
present?
No
Yes
Naming procedure is
similar to naming binary
ionic compounds.
Take organic
chemistry