Transcript Chapter 1

Jeffrey Mack
California State University,
Sacramento
Chemistry and Its Methods
• Hypothesis: A tentative explanation or
prediction based on experimental
observations.
• Law: A concise verbal or mathematical
statement of a behavior or a relation that
seems always to be the same under the
same conditions.
• Theory: a well-tested, unifying principle that
explains a body of facts and the laws based
on them. It is capable of suggesting new
hypotheses that can be tested
experimentally.
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Chemistry and Its Methods
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• Experimental results should be reproducible.
• Furthermore, these results should be
reported in the scientific literature in sufficient
detail that they can be used or reproduced by
others.
• Conclusions should be reasonable and
unbiased.
• Credit should be given where it is due.
Qualitative Observations
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• No numbers involved
• Color, appearance, statements like “large” or
“small:
• Stating that something is hot or cold without
specifying a temperature.
• Identifying something by smell
• No measurements
Qualitative Observations
• A quantity or attribute that is measureable is
specified.
• Numbers with units are expressed from
measurements.
• Dimensions are given such as mass, time,
distance, volume, density, temperature, color
specified as a wavelength etc...
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Classifying Matter: States of Matter
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Classifying Matter: States of Matter
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• In solids these particles are packed closely together, usually in a
regular array. The particles vibrate back and forth about their
average positions, but seldom does a particle in a solid squeeze
past its immediate neighbors to come into contact with a new set
of particles.
• The atoms or molecules of liquids are arranged randomly rather
than in the regular patterns found in solids. Liquids and gases are
fluid because the particles are not confined to specific locations
and can move past one another.
• Under normal conditions, the particles in a gas are far apart. Gas
molecules move extremely rapidly and are not constrained by their
neighbors. The molecules of a gas fly about, colliding with one
another and with the container walls. This random motion allows
gas molecules to fill their container, so the volume of the gas
sample is the volume of the container.
States of Matter
• SOLIDS — have rigid shape, fixed volume.
External shape may reflect the atomic and
molecular arrangement.
–Reasonably well understood.
• LIQUIDS — have no fixed shape and may
not fill a container completely.
–Structure not well understood.
• GASES — expand to fill their container
completely.
–Well defined theoretical understanding.
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Classifying Matter
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Classifying Matter
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Mixtures: Homogeneous and Heterogeneous
• A homogeneous mixture consists of two or
more substances in the same phase. No amount
of optical magnification will reveal a
homogeneous mixture to have different
properties in different regions.
• A heterogeneous mixture does not have
uniform composition. Its components are easily
visually distinguishable.
• When separated, the components of both types
of mixtures yields pure substances.
Classifying Matter
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Classifying Matter
Pure Substances
• A pure substance has well defined physical
and chemical properties.
• Pure substances can be classified as
elements or compounds.
• Compounds can be further reduced into two
or more elements.
• Elements consist of only one type of atom.
They cannot be decomposed or further
simplified by ordinary means.
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Matter and its Representation
What we observe…
To what
we can’t
see!
Chemical symbols allow us to connect…
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The Representation of Matter
In chemistry we use chemical formulas and
symbols to represent matter.
Why?
We are “macroscopic”:
large in size on the order of 100’s of cm.
Atoms and molecules are “microscopic”:
on the order of 10-12 cm
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Elements
• The elements are recorded on the PERIODIC TABLE
• There are 117 recorded elements at this time.
• The Periodic table will be discussed further in chapter 2.
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Chemical Compounds
Chemical compounds are composed of two or more
atoms.
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Chemical Compounds
Molecule:
Ammonia (NH3)
Ionic Compound
Iron pyrite (FeS2)
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Chemical Compounds
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• All Compounds are made up of molecules or
ions.
• A molecule is the is the smallest unit of a
compound that retains its chemical
characteristics.
• Ionic compounds are described by a “formula
unit”.
• Molecules are described by a “molecular
formula”.
Molecular Formula
• A molecule is the smallest unit of a compound
that retains the chemical characteristics of the
compound.
• Composition of molecules is given by a
molecular formula.
H2O
C8H10N4O2 - caffeine
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Physical Properties
Some physical properties:
− Color
− State (s, g or liq)
− Melting and Boiling point
− Density (mass/unit volume)
Extensive properties (mass)
depend upon the amount of
substance.
Intensive properties (density)
do not.
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Physical Properties
Physical properties are a function of intermolecular
forces.
H
Water (18 g/mol)
liquid at 25oC
Methane (16 g/mol)
gas at 25oC
H
C
H
H
O
H
H
• Water molecules are attracted to one another by
“hydrogen bonds”.
• Methane molecules only exhibit week “London
Forces”.
Physical Properties
Physical properties are
affected by
temperature
(molecular motion).
The density of water is
seen to change with
temperature.
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Physical Properties
Mixtures may be separated by physical properties:
Physical Property
Means of Separation
Density
Decantation,
centrifugation
Boiling point
Distillation
State of Matter
Filtration
Intermolecular Forces
Chromatography
Vapor pressure
Evaporation
Magnetism
Magnets
Solubility
Filtration
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Chemical Properties
• Chemical properties are really chemical changes.
• The chemical properties of elements and
compounds are related to periodic trends and
molecular structure.
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Chemical Properties
A chemical property indicates whether and
sometimes how readily a material undergoes
a chemical change with another material.
For example, a chemical property of hydrogen
gas is that it reacts vigorously with oxygen
gas.
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The Nature of Matter
Gold
Mercury
Chemists are interested in the nature of matter
and how this is related to its atoms and
molecules.
A Chemist’s
View of Water
Macroscopic
H 2O
(gas, liquid, solid)
Particulate
Symbolic
2 H2(g) + O2 (g)  2 H2O(g)
Energy: Some Basic Principles
Energy can be classified as Kinetic or Potential.
• Kinetic energy is energy associated with motion such
as:
• The motion at the particulate level (thermal energy).
• The motion of macroscopic objects like a thrown
baseball, falling water.
• The movement of electrons in a conductor (electrical
energy).
• Wave motion, transverse (water) and compression
(acoustic).
Matter consists of atoms and molecules in motion.
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Energy: Some Basic Principles
Potential energy results from an object’s
position:
• Gravitational: An object held at a height,
waterfalls.
• Energy stored in an extended spring.
• Energy stored in molecules (chemical energy,
food)
• The energy associated with charged or partially
charged particles (electrostatic energy)
• Nuclear energy (fission, fusion).
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