Liquids and Solids

Chapter 10

Intramolecular Forces

In chapters 8 and 9 we looked closely at the formation of molecules

The forces that held them together were...

But now...

We look at the forces that cause the aggregation of the components of a substance to form a liquid or solid (condensed states)

The forces?

Could be…covalent or ionicor weaker interactions loosely categorized as “intermolecular forces”

So, remember...

Intra- within the molecule

Inter- between molecules

Changing states

Molecules stay together!Changes are due to forces among the molecules, not within the molecules

How much energy?

H2O(s) --> H2O(l) 6.02 kJ

H2O(l) --> H2O(g) 40.7 kJO-H bond break 934 kJ!It takes a lot of energy to break up molecules!

Polar Bonds

Have center of positive charge and negative charge

exhibit a dipole moment

Dipole-dipole attraction

Molecules with dipoles will attract each other and line up to maximize attraction and minimize repulsion

Dipole-dipole forces

Only have the strength of 1% of a covalent or ionic bond

become weaker as further apart

Strong dipole forces

Hydrogen + highly electronegative atom

N, O, F

Hydrogen bonding

Strength accounted for by:

1. Great polarity of bond2. The close approach of dipoles (H so small)

Molecules without dipoles

Even they can exert forces on one another- but they’re weak!

We know they do b/c they exist in liq/sol form

The weak forces

We assume that the e-’s of an atom are uniformly distributed at every instant

This is not true!

The weak forces

Atoms can develop a momentary nonsymmetrical e- distribution that produces a temporary charge

This instant dipole can give the next atom a dipole

Called….

London Dispersion Forces

These forces have to be strong enough to produce a solid out of noble gases!!

Periodic Trend

Freezing point rises going down a group

WHY???

Two factors

1. As atomic mass increases, the average velocity of an atom at a given temp decreases, allowing it to “lock into” the solid pattern easier

Two factors

2. As atomic number increases, the number of e-’s increases and the chance of a momentary dipole increases.

So...

We say that large atoms are more “polarizable” than small atoms!

Liquids...

Some qualities…low compressibilitylack of rigidityhigh density (compared to gases)

Pouring liquids...

Beads as dropletsb/c of intermolecular forces!

Surface molecules only attracted from sides & below

Beads because

Minimal surface area is:Sphere!In order to increase a liquid’s surface area, you must add energy

Surface Tension

The resistance of a liquid to an increase in its surface area

Capillary action

The spontaneous rising of a liquid in a narrow tube

Capillary action

Cohesive forces- intermolecular forces among the molecules

Adhesive forces- forces b/w liquid molecules and container

Viscosity

Measure of a liquid’s resistance to flow

the stronger the intermolecular forces, the higher the viscosity

Molecular model?

Easy for:GasSolidWHY???

Liquid State

Has strong intermolecular forces AND significant molecular motions

Liquid model

Large areas similar to solids, but with more disorder, large holes, and rapid fluctuations