Intermolecular Attractions -- Liquids and Solids

L. Scheffler

IB Chemistry 1-2

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Intermolecular Forces

Intermolecular forces are the forces of attractions that exist between molecules

The strength of these forces determine:• The state of matter: solid, liquid, or gas • The melting and boiling points of

compounds• The solubilities of one substance in

another.

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Types of Intermolecular Forces

Intermolecular forces include:

Hydrogen bonding

Dipole to dipole interactions

van der Waals forces or dispersion forces

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Hydrogen BondingHydrogen bonding occurs between polar covalent molecules that possess a hydrogen atom that is bonded to an extremely electronegative element; specifically - N, O, and F.

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Weak attractions occur between the hydrogen atoms of one molecule and the oxygen atom of another.

Hydrogen Bonding

The weak attractions that result form hydrogen bonding cause molecules to stick together.

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As a result molecules with significant hydrogen bonding have higher melting points and boiling points than they would otherwise have.

Hydrogen BondingHydrogen bonds are the strongest of all of the intermolecular forces. They are about one-tenth the strength of a covalent bond .

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Because hydrogen bonds must be overcome for a substance to melt or evaporate, substances that have significant hydrogen bonding have higher than normal melting and boiling temperatures

Dipole-Dipole Attractionso If the permanent net dipole within the polar

molecules results from a covalent bond between a hydrogen atom and either fluorine, oxygen or nitrogen, the resulting intermolecular force is referred to as a hydrogen bond

o If this attraction occurs between other polar molecules it is referred to as a dipole to dipole interaction

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Dipole-Dipole Attractionso Dipole-Dipole attractions occur between

molecules that have permanent net dipoles. (polar molecules),

o The partial positive charge on one molecule is electrostatically attracted to the partial negative charge on a neighboring molecule.

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Dipole-Dipole Attractionso Some examples of molecules with dipole-

dipole interactions include:• SCl2

• PCl3

• CH3Cl

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van der Waals Forces

van der Waals or dispersion forces are very weak forces of attraction between molecules

They result from:1. momentary dipoles occurring due to uneven electron

distributions in neighboring molecules as they approach one another

2. the weak residual attraction of the nuclei in one molecule for the electrons in a neighboring molecule.

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Dispersion Forces

..

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van der Waal's Forces are named after the person who contributed to our understanding of non-ideal gas behavior). They are also as known dispersion forces or as London Forces (named after Fritz London who first described these forces theoretically in 1930)

Dispersion Forces The more electrons that are present in the molecule,

the stronger the dispersion forces will be. Dispersion forces are the only type of intermolecular

force that operates between non-polar molecules Dispersion forces exist between non-polar

molecules such as hydrogen (H2)

chlorine (Cl2)

carbon dioxide (CO2)

methane (CH4)

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Dispersion Forces

van der Waals or dispersion forces are the weakest of the intermolecular forces

They are typically only 0.1% to 1% as strong as covalent bonds between atoms in a molecule

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London Dispersion Forces

The van der Waals or London dispersion force is a temporary attractive force that occurs when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles.

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This force is sometimes called an induced dipole-induced dipole attraction.

Effects of London Dispersion Forces

London forces are the attractive forces that cause non-polar substances to condense to liquids and to freeze into solids when the temperature is lowered sufficiently.

Phase changes occur when molecules are sufficiently close and dispersion forces are sufficiently strong to hold molecules together

The Liquid StateThe liquid state of a material has a definite volume, but it does not have a definite shape and takes the shape of the container, unlike that of the solid state. Unlike the gas state, a liquid does not occupy the entire volume of the container if the container volume is larger than the volume of the liquid.  

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The Liquid State

At the molecular level, the arrangement of the molecules is random, unlike that of the solid state in which the molecules are regular and periodic. Molecules are still closely packed but they can slip past each other and move around the body of the liquid.There may be some short order intermolecular ordering or structure, however.  

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Solids, Liquids and Gases

The intermolecular forces between particles become stronger as particles are packed closer together and move less rapidly

Energy is required to convert from solid to liquid to gas

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Vapor Pressure and Boiling

o Energy is required for a liquid to evaporate

o The vapor pressure of a liquid depends on the degree to which it will evaporate at a given temperature

o Liquids evaporate at the surface as long as the vapor pressure of the liquid is less than the pressure of the atmosphere above the liquid

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In order to evaporate, a water molecule must have enough energyTo overcome the hydrogen bonds that hold it in place

Vapor Pressure curves for various liquids

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v it in place

Pressure in torr

Vapor Pressure and Boiling

o The weaker the intermolecular forces in a substance, the higher its vapor pressure will be at any given temperature

o Volatile liquids have relatively high vapor pressures and hence they also have low boiling temperatures

o If the vapor pressure of a liquid is equal to the atmospheric pressure the substance will boil.

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Boiling

The boiling temperature depends on the pressure above the liquid.

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Boiling point of water and elevation

The boiling temperature depends on the pressure above the liquid.Atmospheric pressure decreases with increasing elevation.

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Hydrogen bonding & the Hydrogen bonding & the boiling pointboiling point

Molecules that undergo significant hydrogen bonding tend to have much higher boiling points than they would otherwise have.

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Characteristics of the Liquid State

The most familiar liquid states at room temperature are water, alcohol, benzene, carbon tetrachloride, corn oil, and gasoline.

Two elements, Bromine and Mercury are liquids at room temperature. A third element Gallium has a melting point slightly above room temperature

Glasses, although solids, are often called frozen liquids, because the arrangements of molecules in glasses are very similar to those in liquid states.

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Br

Ga

Properties of liquids- Viscosity

Viscosity of a liquid is a measure of the resistance of a liquid to flow,

Viscosity is measured in N s m-2 (SI Units) or poise (P) or centipoise (cP).      1 P = 0.1 N s m-2

      1 cP = 0.001 N s m-2

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Surface Tension

Surface tension is the energy required to stretch a unit change of the surface area. Thus its units are N * m m-2 = N/m.

There is no direct correlation between viscosity and surface tension. These two properties are independent of each other.

The surface tension is due to the unbalanced force experience by molecules at the surface of a liquid.

As a result of surface tension, a drop of liquid tends to form a sphere, because a sphere offers the smallest area for a definite volume.

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Surface Tension

Substances with low surface tension have a tendency to form thin films.

When detergent is added to water, it lowers the surface tension.

Blowing soap water with a straw forms bubbles, due to the low surface tension.

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Cohesion and Adhesion

Cohesion is the intermolecular attraction between like molecules,

Adhesion is the intermolecular attraction between unlike molecules.

Liquids with high surface tensions have strong cohesion forces, and they are poor wetting liquid due to low adhesion forces.

A detergent or wetting agent is a substance that increases the adhesion force between two different materials.

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Soaps and Detergents

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Molecules of soaps and detergents have both a polar and an non-polar portion.

Soaps and Detergents

For this reason soaps and detergents are referred to as wetting agents.

The wetting agent increases the wetting action of water with the non-polar material.

By this action, dirt is removed when washed with water.

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Capillary Action When a small tube is dipped into a liquid,

the level in the tube is usually higher or lower than that of the bulk liquid.

If adhesion force between the tube material and the liquid is stronger than the cohesion force, the level is higher. Otherwise, the level is lower.

Such phenomena are called capillary action. Capillary action is one of the factors

responsible for transport of liquid and nutrients in plants, and sometimes in animals.

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