Lecture 5Intermolecular ForcesAdopted from various sources including: Lectures from David P. WhiteUniversity of North Carolina, Wilmington; Presentation slides from Prentice Hall; Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Second Edition Copyright 2004 Brooks/Cole; & Presentation slides to accompany Silberberg Chemistry 4th edition The McGraw Hill Companies as well as Zumdahl Chemistry 6th edition Houghton Mifflin

A Molecular Comparison of Gases, Liquids and SolidsPhysical properties of substances understood in terms of kinetic molecular theory:Gases are highly compressible, assumes shape and volume of container: Gas molecules are far apart and do not interact much with each other.Liquids are almost incompressible, assume the shape but not the volume of container:Liquids molecules are held closer together than gas molecules, but not so rigidly that the molecules cannot slide past each other.Solids are incompressible and have a definite shape and volume:Solid molecules are packed closely together. The molecules are so rigidly packed that they cannot easily slide past each other.

A Molecular Comparison of Gases, Liquids and Solids

A Molecular Comparison of Gases, Liquids and SolidsConverting a gas into a liquid or solid requires the molecules to get closer to each other:cool or compress.Converting a solid into a liquid or gas requires the molecules to move further apart: heat or reduce pressure.The forces holding solids and liquids together are called intermolecular forces.

Intermolecular ForcesThe covalent bond holding a molecule together is an intramolecular force.The attraction between molecules is an intermolecular force.Intermolecular forces are much weaker than intramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/mol for HCl).When a substance melts or boils the intermolecular forces are broken (not the covalent bonds).When a substance condenses intermolecular forces are formed.

Intermolecular Forces in Solutions

Intermolecular Forces

Intermolecular ForcesIon-Dipole ForcesInteraction between an ion (e.g. Na+) and a dipole (e.g. water).Strongest of all intermolecular forces:

Since Q1 is a full charge and Q2 is a partial charge, F is comparatively large.F increases as Q increases and as d decreases:the larger the charge and smaller the ion, the larger the ion-dipole attraction.

Intermolecular Forces

Intermolecular ForcesDipole-Dipole ForcesDipole-dipole forces exist between neutral polar molecules.Polar molecules need to be close together.Weaker than ion-dipole forces:

Q1 and Q2 are partial charges.

Intermolecular ForcesDipole-Dipole ForcesThere is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble.If two molecules have about the same mass and size, then dipole-dipole forces increase with increasing polarity.

Intermolecular ForcesLondon Dispersion ForcesWeakest of all intermolecular forces.It is possible for two adjacent neutral molecules to affect each other.The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom).For an instant, the electron clouds become distorted.In that instant a dipole is formed (called an instantaneous dipole).

Intermolecular ForcesIon-Induced Dipole:An ion induces a dipole moment in an adjacent molecule or atom.Interaction between an ion (e.g. Na+) and a dipole (e.g. water).

Intermolecular ForcesLondon Dispersion Forces

Intermolecular ForcesLondon Dispersion ForcesOne instantaneous dipole can induce another instantaneous dipole in an adjacent molecule (or atom).The forces between instantaneous dipoles are called London dispersion forces.Polarizability is the ease with which an electron cloud can be deformed.The larger the molecule (the greater the number of electrons) the more polarizable.

Intermolecular ForcesPolarizability & Periodic TablePolarizability increases down a group of atoms or ions because size increases & larger electron clouds are more easily distortedPolarizability decreases from left to right across a period because the effective nuclear charge holds the electrons more tightlyCations are less polarizable than parent atom because they are smaller, whereas anions are more polarizable because they are larger

Intermolecular ForcesLondon Dispersion Forces

Intermolecular ForcesLondon Dispersion ForcesLondon dispersion forces increase as molecular weight increases.London dispersion forces exist between all molecules.London dispersion forces depend on the shape of the molecule.The greater the surface area available for contact, the greater the dispersion forces.London dispersion forces between spherical molecules are lower than between sausage-like molecules.

Intermolecular ForcesHydrogen BondingSpecial case of dipole-dipole forces.By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high.Intermolecular forces are abnormally strong.H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N).Electrons in the H-X (X = electronegative element) lie much closer to X than H.H has only one electron, so in the H-X bond, the + H presents an almost bare proton to the - X.Therefore, H-bonds are strong.

Intermolecular ForcesHydrogen Bonding

Intermolecular ForcesHydrogen BondingHydrogen bonds are responsible for:Ice FloatingSolids are usually more closely packed than liquids;therefore, solids are more dense than liquids.Ice is ordered with an open structure to optimize H-bonding.Therefore, ice is less dense than water.In water the H-O bond length is 1.0 .The OH hydrogen bond length is 1.8 .Ice has waters arranged in an open, regular hexagon.Each + H points towards a lone pair on O.Ice floats, so it forms an insulating layer on top of lakes, rivers, etc. Therefore, aquatic life can survive in winter.

Intermolecular ForcesHydrogen BondingHydrogen bonds are responsible for:Protein StructureProtein folding is a consequence of H-bonding.DNA Transport of Genetic Information

Intermolecular ForcesComparing Intermolecular Forces

Intermolecular Forces in Solutions Ion-Dipole (40-600 kJ/mol)

H bond (10-40 kJ/mol)

Dipole-Dipole (5-25 kJ/mol)

Ion-Induced dipole (3-15 kJ/mol)

Dipole-Induced Dipole (2-10 kJ/mol)

Dispersion (0.05-40 kJ/mol)Strongest

to

Weakest

The Uniqueness of WaterWater has many unusual properties when compared with properties that periodic trends would otherwise predict:Higher boiling pointHigher specific heat capacityHigher surface tension, capillarityHigher heat of vaporizationLower vapor pressureHigher viscosityDissolves many substancesLiquid state at room T & PSolid form floats on liquid less dense

The electrons forming each bond betweenhydrogen and oxygen are drawn strongly towardthe oxygen atom

This results in two very polar bonds

The 104.5 bond angle makes a strong dipole

Water molecules also form hydrogen bonds

Comparison of Ice and WaterIssues: H-bonds and MotionIce: 4 H-bonds per water moleculeWater: 2.3 H-bonds per water moleculeIce: H-bond lifetime - about 10 microsecWater: H-bond lifetime - about 10 psec(10 psec = 0.00000000001 sec)Thats "one times ten to the minus eleven second"!

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