chemistry xxi the central goal of this unit is to help you develop ways of thinking that can be used...

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The central goal of this unit is to help you develop ways of thinking that can be used to

predict the physical properties of chemical compounds based on their submicroscopic structure.

Unit 3How do we predict properties?

How do you think structure determines properties in each of these cases?

Why do we

care?

Light Emitting Diodes

Soft and rigid Plastics

Soaps and Detergents

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IUnit 3

How do we predict properties?

M1. Analyzing Molecular Structure Predicting properties based on molecular structure .

M4. Exploring Electronic Structure Predicting properties based on electron-configurations.

M3. Characterizing Ionic Networks Predicting properties based on ion charge and size.

FOUR MAIN MODULES

M2. Considering Conformations Predicting properties based on spatial conformations.

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1. How do we distinguish

substances?

2. How do we determine structure?

Learning Progression

3. How do we predict properties?

Particulate

Zoom I

nMacro

Molecular

Atomic

Electronic

Zoom

Out

Molecular

Particulate

Macro

Structure-Properties

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Integration

To illustrate the power of chemical ideas and models in predicting the physical properties of

chemical substances we will focus our attention on materials’ analysis and design.

How can we use structural information to predict properties?

How can we use properties to derive structural information?

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Unit 3

How do we predict properties?

Module 1: Analyzing

Molecular Structure

Central goal: To explain and predict the physical properties

of molecular compounds based on the nature and

strength of the intermolecular

interactions among their molecules.

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The Challenge

One of advantages of chemical thinking is that it allows us to explain and predict the physical

and chemical properties of molecular substances based on their molecular structure.

ModelingHow do I predict it?

How can use what we know to make such predictions?

How can we take advantage of this way of thinking to design materials with the

desired properties?

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IThe Power of Classification

Predictions about the properties of substances are greatly facilitated by identifying the types of

elements present in the system.

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IMolecular Compounds

For example, if the substance contains nonmetallic elements,

it is likely to be molecular.

Differences in electronegativities are such that nonmetallic elements combine forming individual

molecules with polar covalent bonds.

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In order to make predictions about the physical properties of a molecular substance, we need to

recognize that these properties “emerge” from the interactions of many molecules.

Emergence

H2O

Atomic Composition

Molecular Structure

Charge Distribution

Intermolecular Forces

Macroscopic Properties(State of matter, density,

boiling and melting points, vapor pressure, heat

capacity, solubility, etc.)

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ICommon Patterns

In general, the stronger the intermolecular

forces (IMFs), the more energy is required to

separate the molecules or displace them from one region to another.

T (oC)

-100

0

100

G

G

L

L

S

S

Stronger IMF

HCl

H2O

List factors that may affect the

type and strength of the IMFs

between particles.Let’s Think

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Intermolecular Forces

Different types of interaction may exist between molecules of the same or different types in a system.

These intermolecular forces arise because atoms, molecules, ands ion have charged particles

(electrons) that are constantly moving

Non polar <> Non polarPolar <> Non polarPolar <> Polar

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Relevant Factors

The strength of the IMFs depends on three main factors:

Molecular Polarity Bond Polarity Polarizability

The more polar the molecule, the stronger the IMFs among its particles.

Dimethyl ether

= 1.30 D, Tb = -23 oC

Acetone

= 2.91 D, Tb = 56.5 oC

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The energy needed to overcome dipole-dipole interactions is smaller than what is normally

required to break a covalent bond:

Covalent bond 200-1000 kJ/molDipole-Dipole 7-40 kJ/mol

Interactions between two polar molecules or sites are called dipole-dipole interactions.

Dipole-Dipole Interactions

INTERMOLECULAR INTERACTIONS

BONDING

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Hydrogen Bonding

The particular dipole-dipole interaction that occurs

between a hydrogen (H) atom bonded to a N, O, or F atom and another N, O, or F atom is called a hydrogen “bond”

(not a real bond, though).

-

-

+

H-bonding in water


Bond polarity is particular relevant in molecules in which H is bonded to N, O, or F.

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ILet’s Think

Analyze the structure of the selected molecules

and indicate what parts of the molecule

can form H-bonds with

a similar molecule or

with H2O. Hydrogen bonds are among the strongest dipole-dipole

interactions (10 - 40 kJ/mol)

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IPolarizability

Electrons are always affected by the presence of other charges, which change their distribution

within a molecule or parts of the molecule.

The presence of other charges polarizes the molecule and creates an “induced dipole moment,” which may

be new or strengthen the one that already exists.


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Dispersion ForcesWhen two atoms or molecules get close together, the

interactions between electrons and protons induce instantaneous dipole moments that cause the

particles to attract each other.

The strength of the “dispersion forces” increases with the total number of e- in the interacting atoms or

molecules (they are more polarizable).

Induced-dipole <> Induced-dipole

DispersionForces

These types of forces are present between any

two particles, polar or non-polar, but are more

relevant in non-polar systems.

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ILet’s Think

How do we explain these

trends in boiling points,

and the exceptions?

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ion <> dipole(40-600 kJ/mol)

Types of Interactions

dipole <> dipoleH-bond

(10-40 kJ/mol)

-A-H ---- B-(A, B N, O, F)

dipole <> dipole(5-25 kJ/mol)

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?

?

Induced dipole <> induced dipole(0.05-40 kJ/mol)

(3-15 kJ/mol)

(2-10 kJ/mol)

Types of Interactions

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IYour Turn

By now you should have developed the knowledge and skills that are needed to explain

and predict the properties of molecular compounds, and even to design

substances with desired properties.

To test your ability to apply chemical thinking, we have

designed a set of challenges based on two very important

set of substances:Compounds of OxygenCompounds of Carbon Are you ready?

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ILet’s Think

As we have seen, oxygen and its compounds play a central role in the atmosphere of our planet.

Sub Tb(K)

N2 77.4

O2 90.2

O3 ?

How do you explain these results? What do you predict

for O3 and why?

Arrange these NOx in order of increasing boiling point.

Justify your decision.

NO NO2 N2O

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IOxygen Compounds

In general, the physical properties of molecular compounds containing oxygen are determined by

the polarity of the molecule, or the present functional groups, and the possibility of H-bonding.

R

R

R1

R2Carbonyl

Carboxyl

Hydroxyl

Are they polar?Can they H-bond?

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ILet’s Think

Water, H2O, helps regulate the temperature in the troposphere. This is due to its high “heat capacity.”

The specific heat capacity is a measure

of the amount of energy required to

change the temperature of 1.0 g of substance by 1 oC.

Substance Heat Capacity(J/(g oC)

H2O, liquid 4.184

H2O, vapor 2.0

CH3CH2OH (Ethanol)

2.44

C6H6 (Benzene) 1.67

How would you explain these differences? What would be the environmental impact of water

having a smaller heat capacity?

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ILet’s Think

The importance of water for life in our planet is also due to its capacity to

dissolve, or not, a variety of substances.

If the solubility of O2 in H2O at 20 oC is close to 9 mg/L, would the solubility of N2

and CO2 be lower or higher?

What would you expect to be the effect of global

warming on the solubility of these substances in water?

The main atmospheric gases

N2, O2, and CO2 dissolve only slightly in water, but enough

to sustain aquatic life.

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ILet’s Think

Imagine for a second, that the water molecule was linear and not bent.

What would be the impact of this change on a) the physical properties of water,

b) Earth’s climate, and d) life in our planet?

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IAmazing Carbon

Carbon is, perhaps, the most multifacetic element.

C [He] 2s2p2 CThe electron configuration of the carbon atom allows for many bonding possibilities.

For example, as an element, Carbon exists in several allotropic forms.

Covalent Networks (Macromolecular)

Diamond Graphite

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ILet’s Think

What is the stable form of

carbon at STP?

If you could look at the system at the triple point, what would you see?

Where would each phase be located?

Could graphite be transformed into diamonds?

How?

Diamond

Graphite

Liq.

1 atm = 105 Pa ; 1 GPa = 109 Pa

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INano Carbon

Nanotubes

Buckyballs

C60

C20

C540

Fullerenes exhibit a wide range of properties.

Flat screens

Solar panelsPhoto

detectors

1985

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ICarbon Chemistry

Carbon atoms can bond to themselves and other types of atoms to form chains and rings of different lengths and shapes. That opens the possibility of

creating millions of different compounds.

The physical properties of these

compounds are mainly determined by:

SizeShape

Functionality (Substituents)

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ILet’s Think

Linear Alkanes and Alcohols are commonly

used as solvents in

many labs and industries.

How would you explain the

trends in their boiling points?

020406080

100120140160180200

4 5 6 7 8

# of Carbons

Bo

ilin

g T

em

pe

ratu

re (

oC

)

What boiling points would you predict for these compounds?

C5H11BrC6H13Br

C7H15Br

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ILet’s Think

Liquid alkanes are commonly used as lubricants, both in the automotive and the cosmetic industries.

In general, lubricants are expected to have high

viscosities and low volatilities.

Which of these isomers of decane do you think is the best

lubricant? C10H22

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IBranching

In general, the presence of “branches” or “ramifications” reduces the effective area through

which molecules can interact, decreasing IMFs.

Lower IMFs lead to lower:

Melting PointsBoiling Points

Viscosities

and higher

Volatilities.

BUT:

(g/mL)

BP (oC)

MP (oC)

(cP)

n-Pentane

0.645 36 -130 0.283

Isopentane

0.639 28 -160 0.277

Neopentane

0.613 10 -18 0.327

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I Assess what you know

Let′s apply!

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IFats

Fats belong to a group of compounds called

“triglycerides,” which have the following general

structure:

Ester Functional Group

The hydrocarbon chains can be

saturated (no double bonds

between carbons) or unsaturated (at least

one double bond).Unsaturated

Saturated

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ILet′s apply! Predictions

Around each unsaturation the

molecule can have one of two

configurations:TransCis

Apply your knowledge to predict the effect that:

a) the length of the hydrocarbon chains;b) the degree of unsaturation;

(how many double bonds in a chain);c) the types of configurations (cis- versus trans-),

will have on fats’ density and melting point.

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I In your groups, come up with one idea from this module that you

understood well and one idea that is still confusing.

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Summary

The physical properties of a molecular compound, “emerge” from the intermolecular interactions among the millions of molecules present in a

macroscopic sample of the material.

Analyzing Molecular Structure

Intermolecular forces arise because atoms, molecules, and ions have charged particles that

are unevenly distributed and constantly moving .

-

-

+

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SummaryThe stronger the intermolecular forces, the more energy is required to separate the molecules or

displace them from one region to another.


Thus, the physical properties of many molecular compounds are determined by the polarity and

polarizability of their molecules and the possibility of H-bonding.

Ind. Dipole-Ind. DipoleDipole-Ind. DipoleDipole-Dipole

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The element carbon reacts with other nonmetallic elements to

form millions of different molecular compounds. Carbon atoms can bond to themselves

and other types of atoms to form chains and rings of different

lengths and shapes. The physical properties of these

carbon-based compounds are mainly determined by molecular

size, shape, and functionality.

Summary


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For next class,

Investigate what types of compounds are formed when metals react with nonmetals .

In which ways fundamental ways are these types of compounds different from molecular

compounds?

chemistry xxi the central goal of this unit is to help you develop ways of thinking that can be used...

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