chapter 11 liquids, solids, and intermolecular forces interparticle forces –what properties...

Chapter 11Liquids, Solids, and Intermolecular Forces

• Interparticle Forces– What properties correlate with overall

strength of– What different kinds are there?

• Different for ionic compounds, metals, covalent network solids (lab) and…

• Molecular Substances– London– Dipole-dipole– H-bonding

Reminder: Nanoscopic Representations of the Three States of Matter

Compressibility

3Tro: Chemistry: A Molecular Approach, 2/e

What kinds of forces hold particles together?

• It depends on the particles!– Ions attract ions differently than:– molecules attract molecules or– Atoms attract atoms (if atoms are

covalently bonded—covalent network solids)

The stronger the forces are between particles, the __________

• Higher the melting point (mp)

• Higher the boiling point (bp)

• Higher the Hmelting

• Higher the Hvaporization

• Greater the Surface tension

• Greater the Viscosity• Lower the vapor pressure (at a given T)

Copyright © Houghton Mifflin Company. All rights reserved. 10–6

Table 10.2 (Zumdahl) The Melting Points of the Group 8A Elements

Melting


Table 10.9 Melting Points and Enthalpies of Fusion for Several Representative Solids

Big Classifications First, then get into what attracts molecules to one another

• SEE EXP 17 GRID

Types of Solids (Tro) [only property noted is mp]


Table 10.7 (Zumdahl)

Types and Properties of Solids

Not soluble in any solvents

Most are soluble in

some solvent

Many are soluble in

water

Not soluble in any solvents

Three Possible Intermolecular Forces (IM forces) may act between molecules

(only if molecular!)

1. London forces act between ANY two molecules (polar or nonpolar)– Reason is not obvious (later)


→ N.B. Intermolecular forces are ALL much weaker than intramolecular forces (covalent bonds)!

2. Dipole-dipole forces act only between two POLAR molecules– Need to learn how to determine if a molecule is

polar or nonpolar (handout)

3. Hydrogen bonding occurs only in very special circumstances (handout)


Atomic Separations Within Molecules (Covalent Bonds)

vs. Between Molecules (Intermolecular Interactions)

Approach to Assessing Overall Strength of IM forces

• First estimate the overall strength of London Forces (next slide)

• Then consider if there are additional IM forces: Dipole-Dipole (if polar) H-bonding (special cases)


NOTE: No estimate of the strength of these two is made. Just “yes” or “no”.

London Forces depend on the number of total electrons per molecule

• More electrons (total), greater force of attraction

• MW also correlates– Greater MW more protons more electrons

• Correlation is only rough– Don’t overinterpret

• 20 vs 18 e-s about same

– Shape of molecule also plays a role


NOTE: Origin of London Forces will be discussed later

#e-’s per atom

2 (MM = 4)

8 (MM = 20)

18 (MM = 40)

36 (MM = 84)

54 (MM = 131)

Invoking London forces explains the observed bp trend of the noble gases (no dipole-dipole and no H-bonding)

IM forces(London)


Invoking London forces explains the observed bp trend of the simple alkanes

(no dipole-dipole and no H-bonding)

Must consider there may be two other types of IM forces!

1. London forces (always)

2. Dipole-dipole forces (only if polar)

3. Hydrogen bonding (special circumstance)



Dipole-Dipole Forces

Figure 10.2 (a) The Electrostatic Interaction of Two Polar Molecules (b) The Interaction of Many Dipoles in a Condensed State

**NOTE: All + and – signs should be + and - here!!

NOTE: How to determine if a

molecule is polar will be

discussed later

If other two types of forces are also present, that adds to the total IM forces

• Determine if molecules are polar– Br2 vs ICl (and next slide)

• similar London (70 e’s each)

• Only ICl has dipole-dipole forces Predict ICl has stronger IM forces higher bp (59 vs 97 C)


→ Must consider ALL possible forces to decide relative bp, mp, VP, etc.

• Determine if molecules can H-bond with one another“self-self” H-bonding (see board & handout) if at least one N-H, O-H, or F-H bond

Example: Similar London, but Larger Dipole-Dipole Leads to Stronger (overall) IM forces and Higher bp, mp

Again, greater polarity (similar London) yields greater (overall) IM forces (and bp)


Figure 10.3 a-b (a) The Polar Water Molecule (b) Hydrogen Bonding Among

Water Molecules

Figure 11.13 Explain these trends by approximating the IM forces

What causes London (dispersion) forces?


Figure 10.5 Zumdahl

What causes LONDON FORCES?

(a) An Instantaneous Dipole can Occur on Atom A, inducing a dipole on nearby Atom B.

(b) The same thing can occur with nonpolar molecules

http://www.yteach.co.uk/page.php/resources/view_all?id=intermolecular_force_matter_dipol_polar_non_polar_dispersion_hyd

rogen_bonding_t_page_14


http://intro.chem.okstate.edu/1515SP01/Lecture/Chapter12/LondonDisp.html

More Examples (Explaining bp differences)

Explain the difference in boiling points: 1) n-pentane, C5H12 MW: 72 amu bp = 36.2 C vs. Neopentane, C5H12 MW: 72 amu bp = 9.5 C

2) dimethyl ether, CH3OCH3 MW: 46 amu bp = -25 C vs. ethanol, CH3CH2OH MW: 46 amu bp = 79 C

3) Naphthalene, C10H8 MW: 128 amu bp = 218 C vs. Acetic Acid, CH3CO2H MW: 60 amu bp =118 C

More Examples

What type of interparticle forces are present in these substances?Ar, HCl, CaCl2

What is the most important interparticle force in teflon, CF3(CF2CF2)nCF3?

Which substance has stronger intermolecular forces?SeO2 Vs. SO2

Which has the highest boiling point? NaCl or HCl

Which substance has the highest freezing point? H2O, NaCl, or HF

IM forces result in surface tension

• And why liquid droplets are spherical!



Figure 10.6 Zumdahl A Molecule in the Interior of a Liquid is Attracted by the Molecules

Surrounding It


Surface tension examples:

Liquid Mercury Forms a Convex Meniscus in a Glass Tube (unlike water)

Water beads on a wax surface

Back to molecular substances—How to tell if a molecule is polar

• Next slide (and handouts)


Polarity of Molecules (see handout #1 and Section 10.5 in Tro)

Having polar bonds in a molecule does not necessarily result in a polar molecule…

• ...if symmetry is very high, bond dipoles can cancel out– Recall VSEPR geometries! (see handout,

next slides)


Polarity of Molecules (see handout #2)

Polarity of Molecules (continued)

F

F FF

(Nonpolar) (Polar)

Back to non-molecular substances

• Metals

• Covalent network solids

• Ionic – won’t further address in this presentation—

ion-ion forces are generally strong and result from Coulombic forces between charged ions.



Figure 10.18 The Electron Sea Model for Metals Postulates a Regular Array of Cations

in a "Sea" of Valence Electrons

→ Ultra simplified, but does explain electrical conductivity, malleability, and ductility of metals


Figure 10.22 The Structures of Diamond and Graphite

→ Weak inter-layer bonding explains why graphite is used in pencils! Planes “shear off” on writing.


Figure 10.24. The p Orbitals (a) Perpendicular to the Plane of the Carbon Ring System in

Graphite can Combine to Form (b) an Extensive -Bonding Network

→ This explains directional electrical conductivity in a crystal of graphite.


Figure 10.26 The Structure of Quartz (Empirical Formula SiO2)

chapter 11 liquids, solids, and intermolecular forces interparticle forces –what properties...

Documents

e slide

dipoledipole forces

kinds of forces

mp slide

grid slide

origin of london forces

invoking london forces

additional im forces