Chm361_chapter 1 Valence Bond Theory 1.pdf

CHM361 CHAPTER 1: INTERMOLECULAR FORCES IN COVALENT COMPOUNDS

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LEARNING OUTCOMES Student should be able to: 

Explain the intermolecular forces.



Identify the intermolecular forces.



Differentiate the intermolecular forces.



Explain the effects of the intermolecular forces to the boiling point, melting point & solubility.

INTERMOLECULAR FORCES Weak attractive forces between molecules. Forces that hold molecules together in solids, liquids and gas.

Weak forces compared to the intramolecular forces that bond atoms together within a molecule.

INTERMOLECULAR FORCES

INTERMOLECULAR FORCES

VAN DER WAALS (LONDON DISPERSION)

DIPOLE DIPOLE

HYDROGEN BONDING

ION DIPOLE

LONDON DISPERSION FORCES •

Attraction between instantaneous dipole with induced dipole.

•

Also known as instantaneous dipole-dipole forces.

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The weakest intermolecular forces.

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Occurs in ALL molecules (polar & non polar molecules).

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In nonpolar molecules, this is the only intermolecular forces that operates.

Instantaneous dipole and induced dipole attraction •

At any instant, movement of atoms/molecules, together with their electrons, may result in the electrons gathering at one side of the molecule.

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One side of the molecule will has higher electron density (partially negative) while other side will has lower electron density (partially positive).

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The molecules becomes a small instantaneous dipole.

Instantaneous dipole and induced dipole attraction •

If this molecule is close enough with another molecule, it will induce a dipole on the neighboring molecule (induced dipole).

Uneven distribution of electron

Instantaneous dipole

Induced dipole

DIPOLE-DIPOLE FORCES •

Attractive forces between polar molecules.

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Exist between molecules that have dipole moment.

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Usually for molecules from two atoms of different element.

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The larger the dipole moment, the greater the force

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Or the more polar the molecule, the stronger the dipoledipole forces.

DIPOLE-DIPOLE FORCES •

Molecules will have permanent partially positive & partially negative charges.

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Dipole-dipole forces arise from the attraction between positive end with negative end of another molecule. Orientation of Polar Molecules in a Solid

DETERMINETHEPOLARITY 1) Draw the Lewis structure. 2) If there are no lone pairs on the central atom, and if all the bonds to the central atom are the same, the molecule is nonpolar. 3) If the central atom has at least one lone pair and if the groups bonded to the central atom are not all identical, the molecule is probably polar.

DETERMINETHEPOLARITY Molecule CH4 AlF3 H2O NH3 NO2F

Lewis structure

Polarity

HYDROGEN BONDING •

Attraction between Hydrogen with highly electronegative atoms (Fluorine or Oxygen or Nitrogen {FON}).

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H-bonding is stronger than VDW forces.

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The strength is proportional to the polarity of the bond.

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H-F > H2O > NH3 (order of electronegativity of the element).

HYDROGEN BONDING •

N, O and F all have at least one lone pair to interact with H atom in other molecules.

ION DIPOLE FORCES •

Attraction between an ion (cation or anion) with a polar molecule.

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The strength of this interaction depends on the charge of the ion, the dipole moment of the molecule & the size of both species.

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Cations interacts more strongly compare to anion having a charge of the same magnitude.

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Cations with higher charge and smaller in size, interacts strongly with water

ION DIPOLE FORCES

Mg2+ ion, with higher charge and smaller, interacts strongly with water

SUMMARY FORCE London Dispersion

Dipole - Dipole Hydrogen Bonding Ion Dipole

INTERACTION

EFFECT OF FORCES ON BOILING POINT &MELTING POINT Dispersion Forces: •

Increase with the increasing of molar mass of the molecule.

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Increase with the increasing of size of the molecule (isomer).

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Increase with the increasing of polarisability



Polarisability increased with increase of molar mass. -the ease with which the distribution of electron cloud can be distorted -The larger the electron cloud (or the higher number of electrons), the easier it is to be polarised

EFFECT OF FORCES ON BOILING POINT &MELTING POINT Dipole-Dipole Forces: •

Increase with the increasing of polarity.

EFFECT OF FORCES ON BOILING POINT &MELTING POINT Dispersion vs Dipole-Dipole forces Size and Shape

Intermolecular Forces

Comparable

Dipole-dipole

One is larger than another

Dispersion



Trend in boiling points of H Halides:



HF



HCl



HBr



HI

Arrange the boing point in increasing order.



Trend in boiling points of H Halides:



HCl, - 85˚C; HBr, -67˚C; HI, -15˚C



Stronger intermolecular forces result in higher boiling points. The dipole moments increase with the polarity of the H-X bond

-Cl is more EN than I so HCl should have a greater boiling point compare to HBR and HI due to stronger dipole-dipole forces.  but this data suggests that HI has higher boiling point. It shows that, London forces dominate dipole-dipole interactions for these molecules since Iodine is larger than Cl.  HF is an exception because of the stronger force of attraction between HF molecules resulting from hydrogen bonds acting between the HF molecules. So HF has higher boiling point than the other molecules in this series. 

EFFECT OF FORCES ON BOILING POINT &MELTING POINT Hydrogen Bonding •

Increases with the increasing of the electronegativities of the elements. (Normality) H-F > H2O > NH3 (order of electronegativities of the element)

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Higher if there (Abnormality)

are

more

H-bonding

per

molecule.

H2O > H-F > NH3 •

Although F is more electronegative than O, but H2O can form 4 intermolecular hydrogen bonds. HF can only form 2 hydrogen bonds.

Hydroge n Bonding Per Molecule H2O

HF

NH3

4 Hydrogen bonding

2 Hydrogen bonding

2 Hydrogen bonding

 Two with the lone pairs on oxygen  Another two with two hydrogen atoms. More hydrogen bonding.

 one with the lone pair on nitrogen  the other with one of the three hydrogen atoms. Fluorine is more EN than Nitrogen.

there will not be enough lone pairs to form hydrogen bonds with all the three partial positive hydrogens.

EFFECT OF FORCES

ON SOLUBILITY

• +ve ion of the solute is attracted th e - charged atom in polar solvent molecule, and the –ve ion of the solute is attracted to the + charged atom the solvent molecule. on Solute Solvent Non-polar

Non-polar

Polar such as glucose (C6H12O6)

polar

Ionic such as sodium chloride (NaCl)

polar

SUMMARY •

In general, the strength of intermolecular forces increase in this order: dispersion < dipole-dipole < hydrogen bond < ion-dipole

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Dispersion forces operates on all molecules.

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Dipole-dipole operates on polar molecules, but compete

with

dispersion. •

If hydrogen bond is present, it makes a significant contribution to intermolecular interaction between molecules.

REFERENCES •

Chang, R. (2005). Chemistry. Iowa, United State: McGrawHill College.

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Silberberg, M. S.. (2006). Chemistry: The Molecular Nature of Matter and Change. Iowa, United State: McGraw-Hill Science.

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Intermolecular Forces. (2013). Retrieved from https://www.coursehero.com/file/6556560/IntermolecularFor ces/