1 molecular polarity why do ionic compounds dissolve in water? boiling point = 100 ˚c boiling point...

1

Molecular Molecular PolarityPolarity

Why do ionic compounds dissolve in Why do ionic compounds dissolve in water?water?

Boiling point = 100 ˚C

Boiling point = -161 ˚C

Why do water and methane differ so

much in their boiling points?

2Why do CuCl2, acetic acid, and

ethanol dissolve in water?

3

FLOW CHART

• COVALENT BOND (MOLECULES)

POLARPOLAR NONPOLARNONPOLAR

ALL NONMETALSALL NONMETALS

4

Polar Molecules

• POLAR Molecule have a positive and negative end called POLES

• Polar molecules HAVE UNEQUAL SHARING

• NONpolar molecules are EQUAL SHARING OF VALENCE ELECTRONS

5

Due to the bond polarity, the H—Cl bond energy is GREATER than expected for a

“pure” covalent bond.

BONDBOND ENERGYENERGY

““pure” bondpure” bond339 kJ/mol calc’d339 kJ/mol calc’d

real bondreal bond 432 kJ/mol measured432 kJ/mol measured

BONDBOND ENERGYENERGY

““pure” bondpure” bond339 kJ/mol calc’d339 kJ/mol calc’d

real bondreal bond 432 kJ/mol measured432 kJ/mol measured

Difference = 92 kJ. This difference is Difference = 92 kJ. This difference is proportional to the difference in proportional to the difference in

ELECTRONEGATIVITYELECTRONEGATIVITY, , ..

Difference = 92 kJ. This difference is Difference = 92 kJ. This difference is proportional to the difference in proportional to the difference in

ELECTRONEGATIVITYELECTRONEGATIVITY, , ..

Bond Bond PolarityPolarityBond Bond PolarityPolarityH Cl

••

••

+ -••

6

Electronegativity, Electronegativity, is a measure of the ability of an

atom in a molecule to attract electrons to itself.

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

Basic Chemistry Copyright © 2011 Pearson Education, Inc.Basic Chemistry Copyright © 2011 Pearson Education, Inc.77

Some Electronegativity Values for Group A Elements


Electronegativity and Bond Types


Comparing Nonpolar and Polar Covalent Bonds

10

Bond PolarityBond PolarityBond PolarityBond Polarity

HCl is POLAR because it has a positive end and a negative end.

Cl has a greater share in bonding electrons than does H.

Cl has a greater share in bonding electrons than does H.

Cl has slight negative charge (-) H has slight positive charge (+ )

H Cl••

••

+ -••H Cl

••

••

+ -••

11

Molecular PolarityMolecular Polarity

The magnitude of the

dipole is given in Debye

units.

Named for Peter Debye

(1884 - 1966). Rec’d 1936

Nobel prize for dipole

moments.

12

Dipole MomentsDipole Moments

Why are some molecules polar but others Why are some molecules polar but others are not?are not?

13

Molecular PolarityMolecular PolarityMolecules—such as HCl and H2O— can be POLAR (or

dipolar).

They have a DIPOLE MOMENT. The polar HCl molecule will turn to align with an electric field.

Figure 9.15Figure 9.15


• share electrons unequally between nonmetal atoms

• has an electronegativity difference

range of 0.5 to 1.9

Examples:

Atoms Dipole Type of Bond

Moment

OCl 3.5 - 3.0 = 0.5 Polar covalent

ClC 3.0 - 2.5 = 0.5 Polar covalent

OS 3.5 - 2.5 = 1.0 Polar covalent

Polar Covalent Bonds

15

Bond PolarityBond PolarityBond PolarityBond Polarity Which bond is more polar (or DIPOLAR)?

O—H O—F

3.5 - 2.1 3.5 - 4.0

1.4 0.5

OH is more polar than OF

O H+-

O F+ -

O H+-

O F+ -

Basic Chemistry Copyright © 2011 Pearson Education, Inc.Basic Chemistry Copyright © 2011 Pearson Education, Inc.

• equal sharing of electrons between nonmetal

• has an electronegativity difference of 0.0 to 0.4

• Called dipole momentExamples:

Atoms Dipole Type of Bond Moment

NN 3.0 - 3.0 = 0.0 Nonpolar covalent

ClBr 3.0 - 2.8 = 0.2 Nonpolar covalent

HSi 2.1 - 1.8 = 0.3 Nonpolar covalent

Nonpolar Covalent Bonds

Basic Chemistry Copyright © 2011 Pearson Education, Inc.Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Ionic BondsIonic Bonds• electron transfer between metal and nonmetal

• has a large electronegativity difference (>2 or more)

Examples:

Atoms Electronegativity Type of Bond

Difference

ClK 3.0 – 0.8 = 2.2 Ionic

NNa 3.0 – 0.9 = 2.1 Ionic

SCs 2.5 – 0.7 = 1.8 Ionic


Predicting Bond Types


Use electronegativity differences to classify each of the following bonds as nonpolar covalent (NP), polar covalent (P), or ionic (I):

A. K and N

B. N and O

C. Cl and Cl

D. H and Cl

Learning Check

Metal / Non (I)

0.5 (P)

0.0 (NP)

0.9 (P)

20

• F has maximum .

• Atom with lowest is the center atom in most molecules.

• Relative values of determine BOND POLARITY

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 180

0.5

1

1.5

2

2.5

3

3.5

4

H

FCl

CN

O

SP

Si

Electronegativity,Electronegativity, Electronegativity,Electronegativity,

See Figure 9.9See Figure 9.9

21

Microwave Microwave ovenoven

Consequences of HConsequences of H22O O PolarityPolarity

22

Molecular PolarityMolecular PolarityMolecular PolarityMolecular PolarityMolecules will be polar if

a) bonds are polar

ANDb)the molecule is NOT

“symmetric”


Nonpolar Molecules

A nonpolar molecule • may contain identical atoms (nonpolar bonds)

• may have a symmetrical arrangement of polar bonds that cancel dipoles

24

All below are NOT polar

If If Symmetric = nonpolarSymmetric = nonpolar


Determining Molecular Polarity

The polarity of a molecule is determined from its• shape • polarity of the bonds• dipole addition / cancellation


Polar Molecules• contains polar bonds

• has a separation of positive and negative charge called a dipole indicated by a dipole arrow

• has dipoles that do not cancel

27Polar or Polar or Nonpolar?Nonpolar?

Compare CO2 and H2O.

Which one is polar?

28

Carbon DioxideCarbon Dioxide• CO2 is NOT polar even though

the CO bonds are polar.• symmetrical..

••O OC

•• ••

••

+1.5+1.5 -0.75-0.75-0.75-0.75

29

Polar or Nonpolar?Polar or Nonpolar?• Consider AB3 molecules:

• BF3, Cl2CO, and NH3.

30Molecular Polarity, Molecular Polarity, BFBF33

Molecular Polarity, Molecular Polarity, BFBF33

F

F FB

F

F FB

B atom is positive and F atoms are

negative..

B—F bonds in BFB—F bonds in BF33 are polar. are polar. B—F bonds in BFB—F bonds in BF33 are polar. are polar.

But molecule is symmetrical and But molecule is symmetrical and NOTNOT polar polar

But molecule is symmetrical and But molecule is symmetrical and NOTNOT polar polar

31Molecular Polarity, Molecular Polarity, HBFHBF22

Molecular Polarity, Molecular Polarity, HBFHBF22

H & F atoms are more electronegative than B..

H

F FB

H

F FB

B—F and B—H bonds both are polar.

NOT symmetrical = POLAR

B—F and B—H bonds both are polar.

NOT symmetrical = POLAR

32

Is Methane, CHIs Methane, CH44, Polar?, Polar?Is Methane, CHIs Methane, CH44, Polar?, Polar?

Methane is symmetrical

and is NOT polar.

33

Is CH3F Polar?Is CH3F Polar?

C—F bond is very polar.

NOT symmetrical

= POLAR.

C—F bond is very polar.

NOT symmetrical

= POLAR.

34

Substituted EthyleneSubstituted EthyleneSubstituted EthyleneSubstituted Ethylene• C—F bonds are MUCH

more polar than C—H bonds.

(HYDROCARON = NON)

• both C—F bonds are on same side of molecule =

ASYMMETRIC

• POLAR.

35

Substituted EthyleneSubstituted EthyleneSubstituted EthyleneSubstituted Ethylene• C—F bonds are MUCH more polar than C—H bonds.

• HOWEVER both C—F bonds are on opposite ends so

molecule is symmetric

• NOT POLAR.


Identify each of the following as (P) polar or (NP) nonpolar:

A. PBr3

B. HBr

C. Br2

D. SiBr4

(P); dipoles don’t cancel; (P); dipoles don’t cancel;

(P) one polar bond (dipole); (P) one polar bond (dipole);

(NP) element; nonpolar bond; (NP) element; nonpolar bond;

(NP) symmetric; dipoles cancel (NP) symmetric; dipoles cancel

1 molecular polarity why do ionic compounds dissolve in water? boiling point = 100 ˚c boiling point...

Documents