intermolecular forces

MOLECULES andINTERMOLECULARFORCES

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MOLECULESA molecule is formed when two or more atoms join together

chemically.The smallest bit of each of these

substances.

Diatomic molecules are made of two

atoms of the same element.

Examples• Hydrogen – H2• Oxygen – O2• Water- H2O

INTERMOLECULAR FORCEIntermolecular forces are the attractive forces

between molecules, between ions, or between ions and molecules.

Question: What is the difference between Intramolecular and Intermolecular Forces?

IntramolecularForce that hold atoms

together in a molecule

Intermolecularare attractive

forces between molecules

“Measure” of intermolecular force• boiling point• melting point• DHvap

• DHfus

• DHsub

INTERMOLECULAR FORCE

Definition of Terms• Ions- charged atoms• Cations- positively-charged atoms• Anions- negatively-charged atoms• Dipole- a pair of equal and opposite

electric charges separated by a small distance

• Polar Molecules- have permanent dipoles

• Nonpolar Molecules- has no separation of charge, so no positive or negative poles are formed.

INTERMOLECULAR FORCES1. Dipole-Dipole Forces

Attractive forces between polar molecules.

Polar molecules have permanent dipoles that interact with the permanent dipoles of neighboring molecules. The positive end of one permanent dipole is attracted to the negative end of another permanent dipole.Polar molecules have a higher melting and boiling points than nonpolar molecules of similar molar mass.


Orientation of Polar Molecules in a Solid


The relative magnitude of these forces can also be used to explain trends in melting points and boiling points.

It must be remembered that both melting point and boiling point tend to increase with increasing molar mass, all other factors being equal.

Influence of dipole-dipole forces is seen in the boiling points of simple molecules.Compd Mol. Wt.Boil Point N2 28 -196 oC CO 28 -192 oC Br2 160 59 oC ICl 162 97 oC

INTERMOLECULAR FORCES1A. Hydrogen Bonding

Hydrogen bonding is a special case of dipole - dipole forces, and only exists between hydrogen atoms bonded to F, N, or O, and F, N, and O atoms bonded to hydrogen atoms.


H-bonding is especially strong in biological systems — such as DNA. DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O.Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases. —adenine with thymine —guanine with cytosine

INTERMOLECULAR FORCES2. Ion-Dipole Forces

Attractive forces between an ion and a polar molecule.

• a negative ion attracts the positive dipole of another molecule

• a positive ion attracts the negative dipole of another molecule

INTERMOLECULAR FORCES2. Ion-Dipole Forces

Ion-Dipole Interaction

INTERMOLECULAR FORCES3. Ion-Ion Forces

(Ionic Bond)• The strongest force is the ion - ion force and

is considered later in the section on ionic solids.

• These forces (ion-ion) increase as the size of the ion decreases and as the magnitude of the charge increases.

• Remember that anions are larger than the

atoms they are derived from and cations are smaller than the atoms they are derived from.

INTERMOLECULAR FORCES3. Ion-Ion Forces

Na+ — Cl- in salt.These are the

strongest forces. Lead to solids with

high melting temperatures.

NaCl, mp = 800 oC

INTERMOLECULAR FORCES4. London/ van der Waals Forces

• Also known as the Dispersion Forces.

• Interactions involving Induced Dipoles.

Nonpolar molecules have no permanent dipole moment, but transient dipoles exist due to the random motion of the electrons about the positive charge center.

Induced dipole forces result when an ion or a dipole induces a dipole in an atom or a molecule with no dipole. These are weak forces.

INTERMOLECULAR FORCES4A. London/ van der Waals Forces

(Dipole-Induced Dipole)

INTERMOLECULAR FORCES4B. London/ van der Waals Forces

(Ion-Induced Dipole)

INTERMOLECULAR FORCES4C. London/ van der Waals Forces(Induced Dipole-Induced Dipole)

Molecules that have induced dipoles may also induce neighboring molecules to have dipole moments, so a large network of induced dipole-induced dipole interactions may exist.

INTERMOLECULAR FORCES4C. London/ van der Waals Forces(Induced Dipole-Induced Dipole)

SUMMARY

THANKYOU

joevi

intermolecular forces

Science