Intermolecular Forces, Liquids, and Solids

Intermolecular Forces, Liquids, and Solids

Chapter 11 Brown-LeMay

I. Kinetic Molecular Description

Intermolecular forces (I.F.) – attractive electrostatic interactions that occur between molecules, atoms, or ions of a substance.

Liquids – I.F. are strong enough to hold sub together but weak enough to allow movement

Solids – I.F. are strong so no movement of molecules occurs (they vibrate)

Types of solids

Crystalline solids – molecules and ions arranged in repeating patterns

Amorphous solids – molecules and ions arranges in random fashion

II. Intermolecular Forces tend to be weak Type of Interaction Aprox. E kg/mol

Van der Waals (dd,ld) I.F.

0.1 - 10

Hydrogen Bonding I.F.

10 - 40

Ionic Chem Bond 100 - 1000

Covalent Chem Bond 100 - 1000

A. Importance of I.F.

The stronger the I.F. the greater the boiling and melting point and the lower the vapor pressure – the weaker the I.F. the lower the melting and boiling point and the higher the vapor pressure

Woops Debbie Deeeeeeeee’sB. Van der Walls forces –

are all electrostatic-2-types

1. Dipole-Dipole (D.D.) Debbie De – result from the tendency of polar molecules that position themselves so that the pos. and neg. ends of different molecules are near to each other

Debbieeeeeeeee D

For molecules with the same approximate mass and size I.F. increases with increasing polarity

2. London forces (dispersion forces-L.D.) in non-polar molecules no D.D. forces exist – small dipoles exist because of electron movement – position of electrons in clouds- all molecules and compounds have these forces.

London Dispersion Forces

London Dispersion Forces

L.D. forces ten to increase with increasing molecular weight

Polarizable- larger atoms have electron clouds that are easily distorted

3. Hydrogen Bonding (x – oxygen very negative atom – y- hydrogen) Exists between a

hydrogen atom covalently bonded to a very electronegative atom X, and a lone pair of electrons on another small, electronegative atom Y.

Hydrogen Bonding

The hydrogen bond in water may be explained in part on the basis of the dipole moment of the –O—H

when the electrons in the hydrogen atom are furthest away from the oxygen atom

X & Y atoms are usually F, N, or O

Strength of Hydrogen Bonding

H---N < H---O < H---F

4. Ion-dipole forces – attraction between an ion and an opposing charge pole of a molecule

III. Properties of Liquids

A. Viscosity – resistance of a liquid to flow Ex. Motor oil – SAE (society of automotive

engineers) viscosity of motor oil increases as temperature increases – SAE ratings are for 0oF or -18oC increased ratings indicate greater viscosity for a given tempt.

The greater the rating the thicker the oil. SAE 10w/40 behaves like a 10 in the winter and

a 40 in the summer. The thicker grade is needed in the summer because the oil is heated and becomes thinner.

B. Surface Tension

The energy required to increase the surface area of a liquid by a unit amount (1m2)

Why do liquids have this property?

molecules at the surface experience a net force towards the middle

Molecules at the center experience no net force

Surface Tension Forces

Cohesive - bind like molecules together

Adhesive – bind a substance to a surface

Meniscus-curved surface of a liquid caused by the combination of cohesive forces between the liquid and container

Surface Tension Forces

Convex Meniscus ex mercury cohesive forces are greater

Concave Meniscus ex water adhesive forces greater

IV. Changes of State

A. Phase changes or transitions – substance structure is altered

solid liquidgas

More ordered to less ordered state energy is supplied to overcome IF (endothermic)

Less ordered to more ordered state energy is released (exothermic)

B. Energy Changes and Changes of State

liquid-gas- vaporizationsolid- liquid- meltingsolid- gas- sublimationendothermic processes gas- Liquid condensationliquid-solid- freezinggas – solid depositionexothermic processes

GasGas

liquid

Solid

Transitions between solid, liquid, and gaseous phases typically involve large amounts of energy compared to the specific heat. If heat were added at a constant rate to a mass of ice to take it through its phase changes to liquid water and then to steam, the energies required to accomplish the phase changes (called the latent heat of fusion and latent heat of vaporization ) would lead to plateaus in the temperature vs. time graph. The graph below presumes that the pressure is one standard atmosphere.

B-C melting, D-E boiling, heat is added but no change in temperature occurs because all the energy is used to break IF. Less energy is required to freeze water (delta H fusion) than to vaporize it (delta H vaporization)

The energy required by the 5 processes warming the solid(1), melting(2), warming the liquid(3), boiling(4), and warming the gas(5) is determined by the identity of the substance ant the amount of the sample present.

Warming processes q=s x m x delt t s= specific heat- different for each

substancePhase transitions q = (number of moles) x

delta H

Heating curve for water