Lecture 5.6 - Chemical Bonding 4- Polarity & BE

8/14/2019 Lecture 5.6 - Chemical Bonding 4- Polarity & BE

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Chemical Bonding IV:

Molecular Polarity

& Bond Energy


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gy

Polar Molecules Dipole - A molecule such as HF which has

a positive and a negative end. This dipolarcharacter is often represented by an arrowpointing towards the negative charge.

Dipole moments the measure of the

net molecular polarity (overall pull ofe-towards one direction in a molecule)

Measure of separation of charge in units of

Debyes (D) = Qr (charge x separation) ()

H - F

+ -


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3

Dipole Moments and Polar Molecules

H F

electron rich

regionelectron poor

region

+

= Q x rQ is the charge

r is the distance between charges

1 D = 3.36 x 10-30 C m


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Which Molecules Have aDipole Moment?

Dipole moment forms when all dipoles do not canceleach other completely

Any two atom molecule with a polar bond

With three or more atoms there are three considerations.

1) There must be at least one dipole (polar bond)

2) There are significant strength differences among the dipoles

3) There is molecular asymmetrical geometry


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Cl is more electronegative than Be

Polar covalent bonds dipoles

Molecule is symmetrical dipole forces cancel

Molecule has no dipole moment nonpolar

Cl Be Cl

on-po ar o ecu es wPolar Bonds


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H Be FF & H are more electronegative than Be

Polar covalent bonds dipoles

F is more electronegative than H

stronger dipoleMolecule is symmetrical but dipole

forces do not completely cancel

Molecule has a dipole moment polar

dipole moment


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Electronegativity of F > B

Dipoles (but of equal strength)

Trigonal planar is symmetrical

All dipoles completely cancel

No dipole moment

not polar

Ex: Is BF3 polar?

F

FFB


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Electronegativity of Cl > C

Dipoles (but of equal strength) Tetrahedron is symmetrical

All dipoles completely cancel

No dipole moment

not polar

Ex: Is CCl4 polar?

Cl

ClCl

Cl

C


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Electronegativity of H < N

Dipoles Trigonal pyramidal

is asymmetrical

All dipoles are additive

Dipole moment polar

Ex: Is NH3 polar ?

HH

H

N

+

+

+

-

Resultant dipole = 1.47 D


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Electronegativity of F > N

Dipoles Trigonal pyramidal

is asymmetrical

All dipoles do not cancel

Dipole moment polar

Ex: Is NF3 polar?

FF

F

N

-

-

-

+



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Ex: NH3 is more polar than NF3

HH

H

N

+

+

+

-


FF

F

N

-

-

+

-



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14

Bond moments and resultant dipole moments in NH3 and NF3.


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Water is even more polar


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Lone e- pairs

DO NOT

make amolecule polar


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Electronegativity of F > Br > C

Dipoles (not of equal strength)

Tetrahedron is symmetrical

But all dipoles do not completely cancel

Dipole moment

polar

Ex: Is CBr3F polar?

F

BrBr

Br

C

+

+

+

-


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Symmetrical Molecules

Linear

Trigonal

planar

Tetrahedral

Trigonal

bipyramidal

Octahedral Square

planar


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Asymmetrical Molecules

Bent or V-shaped Trigonal

pyramidal

Seesaw T-shapedSquare pyramidal


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Ex: Is CH3Cl polar?


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Bond DissociationEnergy (Enthalpy)

Bonded atoms are more stable (less PE)

than singular atoms

Forming bonds exothermic H < 0 Breaking bonds endothermic H > 0 Bond Dissociation Energy: H required to

break a particular bond in 1 mole of gaseousmolecules


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Interaction of Two H Atoms and the

Energy Profile

B d l th th ti


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Bond length - the optimumdistance between nuclei in a

covalent bond.


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next


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Change in electron density as two hydrogen atoms

approach each other.


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2( ) ( ) ( )H H H g g g +1

436.4 H kJmol =

2( ) ( ) ( )Cl Cl Cl g g g +1242.7 H kJmol =

Bond Energy of H2 > Bond Energy of Cl2

H-H bond is more stable than Cl-Cl


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Why?

Size

H H

Cl Cl

With smaller radius, bonding e- can be closer

and more attracted to the nucleus


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Bond energy is simplified, for example:

It takes 1652 kJ to dissociate a mole of CH4

into its ions

Each C-H bond has a different energy.

CH4 CH3 + H H = 435 kJ/mol

CH3

CH2 + H

H = 453 kJ/mol CH2 CH + H H = 425 kJ/mol

CH C + H H = 339 kJ/mol

Each bond is sensitive to its environment. Since each hydrogen is hooked to the carbon,

we get the average energy = 413 kJ/mol


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More bondsshorter bond lengthmore B.E

C C 347

C C 620

C C 812

Ho (kJmol-1)C H 413


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Bond Energy(kJ/mol)

Bond Length(pm)

C-C 347 154

C=C 611 134

CC 837 120

C-N 305 147

C=N 615 128

CN 891 116

N-N 163 145

N=N 418 123

NN

946 110

O-O 142 145

O=O 498 121

It is experimentallyfound that there isa direct correlation

between the bondlength and the bondstrength.

As the bond length

decreases, the bondstrength increases.


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Using Bond Energies We can estimate H for a reaction.

H = total Einput total Eoutput Energy and Enthalpy are state functions.

reactants productsBE BEo

rxnH =

BE BEorxn broken formed H =

or


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The enthalpy change required to break a particular bond in one mole of gaseous

molecules is the bond enthalpy.

H2( g) H (g) + H (g) H0 = 436.4 kJ

Cl2( g) Cl (g) + Cl (g) H0 = 242.7 kJ

HCl (g) H (g) + Cl (g)

H

0

= 431.9 kJ

O2( g) O (g) + O (g) H0 = 498.7 kJ O O

N2( g) N (g) + N (g) H0 = 941.4 kJ N N

Bond Enthalpy

Bond Enthalpies

Single bond < Double bond < Triple bond

B d E th l i (BE) d E th l h i ti


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Bond Enthalpies (BE) and Enthalpy changes in reactions

H0 = total energy input total energy released

= BE(reactants) BE(products)

Imagine reaction proceeding by breaking all bonds in the reactants and

then using the gaseous atoms to form all the bonds in the products.

endothermic

exothermic


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H2 (g) + Cl2 (g) 2HCl (g) 2H2 (g) + O2 (g) 2H2O (g)


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Use bond enthalpies to calculate the enthalpy change for:

H2( g) + F2( g) 2HF (g)

H0 = BE(reactants) BE(products)

Type of

bonds broken

Number of

bonds broken

Bond enthalpy

(kJ/mol)

Enthalpy

change (kJ/mol)

H H 1 436.4 436.4

F F 1 156.9 156.9

Type of

bonds formed

Number of

bonds formed

Bond enthalpy

(kJ/mol)

Enthalpy change

(kJ/mol)

H F 2 568.2 1136.4

H0 = 436.4 + 156.9 2 x 568.2 = -543.1 kJ/mol

Ex: Calculate the Ho for the combustion of H (g)


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Ex: Calculate the H rxn for the combustion of H2(g)using the table below.

Bond HoBE

H-H 436.4

H-N 393

H-O 460

O-O 142

O=O 498.7

Rxn: 2 H2 + O2 2 H2O

2 (H-H) O=O 2 (H-O-H)

4 (H-O)

BE BEo

rxn broken formed H =

( ) ( ) ( )2 H-H O=O 4 H-OorxnH = +


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Bond HoBE

H-H 436.4

H-N 393

H-O 460

O-O 142

O=O 498.7

( ) ( ) ( )2 H-H O=O 4 H-OorxnH = +

( ) ( )2 436.4 498.7orxnH = +

( )4 460 1484 kJmolorxnH

=


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BREAK

HW:p. 404 # 47 49

p. 405 # 50 54

p. 407 # 85 88


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The bond is a human invention.

It is a method of explaining theenergy change associated withforming molecules.

Bonds dont exist in nature, but areuseful.

We have a model of a bond.


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What is a Model?

Explains how nature operates.

Derived from observations.

It simplifies them and categorizes theinformation.

A model must be sensible, but it haslimitations.


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Properties of a Model A human invention, not a blown up

picture of nature. Models can be wrong, because they are

based on speculations and

oversimplification. Become more complicated with age.

You must understand the assumptions

in the model, and look for weaknesses. We learn more when the model is

wrong than when it is right.


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Find the energy for this

2 CH2 = CHCH3

+

2NH3 O2

+

2 CH2 = CHC N

+

6 H2O

C-H 413 kJ/mol

C=C 614kJ/mol

N-H 391 kJ/mol

O-H 467 kJ/mol

O=O 495 kJ/mol

C N 891 kJ/molC-C 347 kJ/mol

Lecture 5.6 - Chemical Bonding 4- Polarity & BE

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