Chemistry XXI The central goal of this unit is to help you understand and apply basic ideas that can...
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Transcript of Chemistry XXI The central goal of this unit is to help you understand and apply basic ideas that can...
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I
The central goal of this unit is to help you understand and apply basic ideas that can be used to distinguish
the different substances present in a system.
Unit 1How do we distinguish substances?
In which relevant contexts would be important for humans to distinguish
between different substances?
What would the consequences be of failing to do so?
In your groups, discuss your answers to the following two questions:
Why do we care?
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IUnit 1
How do we distinguish substances?
Food control
Pollution detection and
control
Health monitoring
Crime investigation
Drug development
Resource exploration and
management
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FOUR MAIN MODULES
Unit 1How do we distinguish substances?
M1. Searching for Differences Identifying differences that allow us to separate components.
M2. Modeling MatterUsing the particulate model of matter to explain differences.
M3. Comparing Masses Characterizing differences in particle’s mass and number.
M4. Determining Composition Characterizing differences in particle’s composition.
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Integration
To illustrate the power of chemical ideas and models in distinguishing and identifying chemical substances we will focus our attention in our own
planet’s atmosphere.
What is it made of?
How do we identify them?
How do we explain their properties?
How do we model their behavior?
How do we separate its components?
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IUnit 1
How do we distinguish substances?
Module 1: Searching for Differences
Central goal:
To identify distinctive properties of substances present in a system that can be used to identify
and separate them.
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The Challenge
Imagine that you are interested in investigating
the properties and composition of certain
system, from the Earth’s atmosphere to an unknown
material found in a crime scene or polluting the ocean.
How would you analyze the system?
What properties would you measure?
How would you separate its different components?
AnalysisWhat is this?
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ICentral Problems
Most systems, natural or synthetic, are complex.
They tend to be a combination of hundred or thousands of different substances (mixture vs. pure substance)
Smell > 800
different substances
Their properties change as a function of position (heterogeneous vs. homogeneous).
Their properties change as a function of time (dynamic vs static);
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IHow do we characterize the
state of a system?
We need to identify and measure
properties that allow us to determine how a system changes in
time and space.
Consider, for example, our atmosphere.
What properties could we use to study its state and behavior?
Let′s think!
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Let′s think!
We use properties such as:
Temperature
Pressure
Density
Concentration
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Measurable Properties
In general, it is more useful to measure properties that do not depend on the size of the system
(intensive properties):
Temperature Pressure Density Concentration
For some systems, its is useful to measure properties that are size dependent (extensive properties):
Mass Volume Energy
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IChemical Composition
From a chemical point of view, it is of crucial interest to find the “composition” of the system
(the nature of the substances present in it).
How do we do it?
A first common step is to try to separate the different
components.
Let′s think!
How would you propose to separate the different
substances present in our atmosphere?
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IDifferentiating CharacteristicsAll chemical analysis is based on the assumption
that each substance, no matter how simple or complex, has a “differentiating characteristic” that
can be used to separate/detect/identify it.
For example, in the case of the atmosphere, we can take advantage of
the different boiling/condensation
points of the substances
present in it.
77.36-195.79Nitrogen
87.36-189.35Argon
90.20-182.95Oxygen
373.15100Water
KoCSubstance
T(K) = T(oC) + 273.15
Liquid NitrogenLet′s explore!
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IDifferentiating CharacteristicsWhat other properties of a substance can be used
as “differentiating characteristics”?
Physical Properties
Density Heat CapacityBoiling Point ConductivityMelting Point ViscositySurface Tension Solubility
We look for properties that:
Do not depend on the size of the sample (Intensive Properties);
Can be measured without changing the chemical nature of the substances.
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The separation of the main components of our atmosphere
can be accomplished using boiling/condensation points as “differentiating characteristic.”
Phase Behavior
However, to implement a successful separation we need to understand the phase behavior of
substances.
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IPhase Transitions
A sample of solid water at -20 oC is heated at a constant rate to 120 oC. During the experiment you
measure the temperature of the system as a function of time, as well as the amount of energy absorbed by
the system during the heating process.What would you expect to see?
Let′s think!
T(oC)
t
100
0
0
E (kJ)
T(oC)1000
Absorbed (+)
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IPhysical Change
The observed behavior is characteristic of all substances undergoing phase changes.
During a phase change, the physical properties of the
substance changet
T (oC)
100
0 Constant T
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IEnergy Transfer
Energy addedEnergy added ((kilojoules- kJkilojoules- kJ))
During a phase transition, energy is
transferred between the
system and the surroundings.
Convention
Energy added (+)
Energy released (-)
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GGGGGGGGGGGGG
GGGGGGGGGGGGS
GGGGGGGGGGSSS
GGGGGGLLLSSSS
GGGLLLLLSSSSS
GLLLLLLSSSSSS
LLLLLLSSSSSSS
LLLLLSSSSSSSS
LLLLSSSSSSSSS
LLLSSSSSSSSSS
Phase StabilityFor any given substance, different phases are stable
at different values of temperature and pressure.
P
T
S
L
G
S
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IPhase Changes and
Phase Diagrams
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Phase DiagramsImportant Points
for Water
T(°C) P(mmHg)
Normal boiling point 100 760 =1 atm
Normal freezing point 0 760
Triple point 0.0098 4.58
Vapor Pressure Curve
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Two Different Substances
Carbon dioxide Water
What are the major differences?Let′s think!
Notice differences in scales
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ISeparating Air
A variety of people are interested in separating air components for different purposes:
Purification, environmental analysis, mining.How do they do it?
What differentiating characteristics may they use?
Used to separate
solids from liquids too.
STEP 1
Filtration – Based on differences in particle size.
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Distillation- Based on the difference in boiling points.
Separating AirSTEP 2
Air is normally cooled in stages until it reaches –200°C.
Water vapor condenses, and is removed using absorbent filters; Carbon dioxide freezes at –79 °C, and is removed; Oxygen liquefies at –183 °C; Nitrogen liquefies at –196 °C.
Argon
Fractional Distillation0.934Argon
20.9Oxygen
78.1Nitrogen
% by VSubstance
Air Composition
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I Assess what you know
Let′s apply!
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ILet′s apply!
Boiling of a liquid occurs when its vapor pressure (the
pressure exerted by the vaporize substance) equals the atmospheric pressure.
Notice log scale
ALiquid
Gas
Which liquid, A or B, is more volatile (higher vapor
pressure at same T)?
Which liquid boils at a lower temperature?
What is its normal boiling temperature?
B
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A rich fuel mixture containing the following hydrocarbons has been
extracted from underground:
Propane ButaneNeo-Pentane 2-Heptene
The mixture is at an initial temperature of 5 oC.
Fuel Separation
Based on the data provided, design a procedure to
separate each component.
What differentiating characteristic would you use?
Let′s apply!
PropaneButane
Neo-Pentane 2-Heptene
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Propane (G) Butane (G)
Neo-Pentane (L)
2-Heptene (L)
Fuel Separation
At 5 oC and 1 atm:
We can distill the liquids (heat up)
and the gases (cool down).
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I In your groups, come up with one question about something you don’t understand at this point?
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Searching for Differences
SummaryThe identification of a differentiating characteristics is crucial to: identify, detect, separate, or quantify
the amount of substances is a system.
Phase properties are particularly useful because they can be
measured/induced by changing temperature and pressure.
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For next class,
Investigate what model of matter would allow us to explain why two substances, such as water and carbon dioxide, have different
phase behaviors.
What assumptions about the submicroscopic structure of matter do we need to make
to explain it?