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Unit 1How do we distinguish substances?

M1. Searching for Differences Identifying differences that allow us to separate components.

M2. Modeling Matter Using 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.

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.

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Unit 1How do we distinguish

substances?

Module 2: Modeling Matter

Central goal: To explain the diversity in

properties and behaviors of the different substances in a

system based on the particulate model of matter.

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The Challenge ModelingHow do I explain it?

As we have seen, each substance has at least one differentiating characteristic that makes it unique.

What causes the differences?

How could we explain, predict, and even design,

these differences?

Carbon dioxide

Nitrogen

Water

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The central task of differentiating substances has been greatly simplified by the development

models about their internal structure.

Models of Matter

These models allow us to explain and

predict the properties of matter, and to develop better

techniques to detect and identify them.

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Particulate Model of MatterOne of the most powerful models we have to explain and predict the physical properties and behavior of

substances is the particulate model of matter.

1 mL of water =33444444444444444444444 particles =

3.34 x 1022 particles

Solid Liquid Gas

Basic Assumptions:

1. Any macroscopic sample of a substance is composed of a large number of very small particles;

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0.0000001 m = 10-7 m0.00001 m = 10-5 m0.001 m = 10-3 m0.01 m = 10-2 m0.1 m = 10-1 m

How Small?

1 m

0.000,000,001 m = 1 x 10-9 m = 1 nm (1 nanometer)

Most substances are

made of particles of

“nanometer” size.

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Solid Liquid

Gas

What do these “particles” represent?

What are the limitations of these representations?

Being Cautious

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2. Particles are constantly moving;

Dynamic NatureBasic Assumptions:

What determines the speed (v) of the

particles?

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2. Particles are constantly moving;

Dynamic NatureBasic Assumptions:

What is the “average pressure”

in this model?

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InteractionsBasic Assumptions:

3. Particles interact with each other. The nature and strength of the interactions depend on the distance between particles;

Distance

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Let’s ExploreGo to: http://www.chem.arizona.edu/chemt/C21/sim

(Ideal Gas)

Explore the properties of the particulate model of

matter when interactions among particles are

neglected:

How does pressure depend on temperature, volume, and number of particles?

Build graphs of the type P vs. T, P vs. V, and P vs. N

as part of your analysis.

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Let’s Explore

Molecular Dynamics Simulation

F(r) a(t) v(t) r(t)

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Predictions

P

T

P

N

P

V

The model predicts the following type of behavior:

This behavior is observed in all gases at high temperatures and low pressures.

Why?

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Let’s ExploreIn the absence of interactions among particles

(intermolecular forces) the model does not predict the existence of phase transitions as we change T.

Go to: http://www.chem.arizona.edu/chemt/C21/sim(Real Gas)

Analyze the behavior of the model when intermolecular forces (IMF)among particles are introduced.

Discuss:What is the effect of the on the behavior of the particles and the

system?

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Let’s Explore

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Phase TransitionsTo explain the existence of

phase transitions we have to assume that there are

intermolecular forces among particles.

When temperature decreases, the average kinetic energy per particles decreases.

Attractive forces between particles are then able to hold them together.

Force strength does not change; Size of particles does not change;

Let′s think! Why does the temperature remain constant during a phase transition?

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Particles that attract each other are said to have negative potential energy compared to free particles.

Potential Energy

0

Ep

r

r1

r2

r3

r4

More Negativ

e

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Potential vs. Kinetic Energy

0

Ep

r

r1

r4

Need to add energy to separate

Need to extract energy to get them closer

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XXIExplain and Predict

Let′s think!Why does the boiling temperature of

water decrease with decreasing external pressure?

Let′s think! Why do liquids get colder when they evaporate?

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XXIExplain and Predict

T1 < T2

Let′s think!How do you explain that nitrogen condenses at a lower temperature

than oxygen in terms of IMF?

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Substance A

Substance B

Explain and Predict

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Understanding DifferencesIn the particulate model of matter, many differences

between substances are attributed to the presence of different intermolecular forces among particles.

Why are the intermolecular forces different?

We assume the composition and the structure of

the “particles” are different.

What does this mean?

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XXIModeling Substances

This is a typical chemical representation at the particulate level of the main components of

“pure” air.

Let′s think!

How many different substances are included in this representation?

What similarities and differences do you

observe between the different types of particles

present in the system?

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The representation conveys the idea that we model air as a “mixture”:

a system composed of two or more types of independent particles present in proportions

that may vary from sample to sample.

Individual particles of different substances are

modeled as made of free or bonded atoms of

different types.

Free atom of argon

Molecule of oxygen

Bonded atom of nitrogen

Molecule of carbon dioxide

Modeling Substances

Molecules are made of two or more bonded atoms

Atom or molecule?

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Classifying Substances

Nitrogen (liquid)

Carbon dioxide(Solid, Dry ice)

Argon (gas)

Chemists classify substances as “elements” or “compounds”

based on particle composition.

As we have seen, the different components of a mixture can be separated by physical means (filtration, distillation):

Water

S L G

Let′s think!

Which of these are elements/compounds?

What makes the difference?

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XXIChemical Elements

Elements are the most simple substances in Nature. They are composed of identical particles made of free or bonded atoms of the same type.

Atomic Element

Macroscopic SymbolicParticulate

Molecular Element

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XXIChemical Elements

There are relatively few elements in Nature (fewer than 100), and a few more have been

synthesized in lab.

None of them can be decomposed in

simpler substances by

physical or chemical means.

Cl2

Chlorine

Phosphorus

P4

Na

Sodium

Carbon

C

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XXIAtoms vs. Elements

We need to differentiate between the elements, as real substances, and the atoms they are made of. The Periodic Table summarizes the properties of the individual atoms, not of the actual elements.

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Chemical CompoundsMost substances in nature are chemical compounds.

They are composed of identical particles made of bonded atoms of two or more different types.

Macroscopic SymbolicParticulate

Carbon dioxide

CO2

Water

H2O

C

H

O

N

Color Code

Molecular Formula

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Molecular CompoundsH2O and CO2 belong to a group of compounds

called “molecular compounds”: They are made of molecules.

There is a wide variety of molecular

compounds in Nature.

This diversity is due to the possibility of

having molecules with different compositions, sizes, and structures.

MethaneCH4

CaffeineC8H10N4O2

Hemoglobin

C2952H4664N812O832S8Fe4

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XXIModels and Formulas

Formaldehyde(Air Pollutant)

Keep in mind that molecules are represented in a variety of ways:

Models

Formulas CCHH22OO

Molecular FormulaMolecular Formula

Space-fillingSpace-filling Ball-and-stickBall-and-stick

Structural formula Structural formula

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Let’s ExploreDecide whether these particulate models correspond

to an element, a compound or a mixture.

C E M

E M C

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XXI Assess what you know

Let′s apply!

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T = 280 K

P = 2 atm

AnalyzeLet′s apply!

What does this system represent?

How many phases are present in this system?

How many substances are in each phase?

How many elements? How many compounds?

How would you separate the different components

in the system?

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PredictIn which of the phases:

Are the intermolecular forces strongest?

Is the average potential energy per particle the

lowest (most negative)?Is the average particle speed

the highest?Is the average kinetic energy

per particle the lowest?

Which of the two main substances, water or hexane,

has the greater vapor pressure?

T = 280 K

P = 2 atm

Let′s apply!

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XXI Write down one central idea that you learned

in this module.

Share your idea with the members of your group.

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Modeling MatterSummary:

The particulate model of matter allows us to explain and predict

the properties of chemical substances.

Useful to analyze, synthesize, and transform chemical substances.

Differences in the intermolecular forces among the particles of different substances can be used to

explain their different physical properties.

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Modeling Matter

Summary:

The nature and strength of these interactions depend on the atomic

composition and structure of a substance’s particles.

Based on the composition of their particles, substances are classified

as elements or compounds.

N2 (Element)

CO2 (Compound)

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For next class,Investigate how we can determine how many

times heavier is one type of atom than another.

How can we use this knowledge to quantify, for example, how many molecules of O2

we breathe in a day?