Chapter 11 Liquids, 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 Chapter 11, Unnumbered Figure 1, Page 458

Compressibility Tro: 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 DHmelting Higher the DHvaporization Greater the Surface tension Greater the Viscosity Lower the vapor pressure (at a given T)

Table 10.2 (Zumdahl) The Melting Points of the Group 8A Elements Copyright © Houghton Mifflin Company. All rights reserved. 10–6

Table 10.9 Melting Points and Enthalpies of Fusion for Several Representative Solids Copyright © Houghton Mifflin Company. All rights reserved. 10–7

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] Chapter 11, Figure 11.50 Types of Crystalline Solids

Table 10.7 (Zumdahl) Types and Properties of Solids Most are soluble in some solvent Many are soluble in water Not soluble in any solvents Not soluble in any solvents Copyright © Houghton Mifflin Company. All rights reserved. 10–10

Three Possible Intermolecular Forces (IM forces) may act between molecules (only if molecular!) London forces act between ANY two molecules (polar or nonpolar) Reason is not obvious (later) Dipole-dipole forces act only between two POLAR molecules Need to learn how to determine if a molecule is polar or nonpolar (handout) Hydrogen bonding occurs only in very special circumstances (handout) → N.B. Intermolecular forces are ALL much weaker than intramolecular forces (covalent bonds)! Copyright © Houghton Mifflin Company. All rights reserved.

Atomic Separations Within Molecules (Covalent Bonds) vs Atomic Separations Within Molecules (Covalent Bonds) vs. Between Molecules (Intermolecular Interactions) Copyright © Houghton Mifflin Company. All rights reserved. 10–12

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”. Copyright © Houghton Mifflin Company. All rights reserved.

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 Copyright © Houghton Mifflin Company. All rights reserved.

Invoking London forces explains the observed bp trend of the noble gases (no dipole-dipole and no H-bonding) #e-’s per atom 2 (MM = 4) 8 (MM = 20) 18 (MM = 40) 36 (MM = 84) 54 (MM = 131) IM forces (London) Click to add notes

Invoking London forces explains the observed bp trend of the simple alkanes (no dipole-dipole and no H-bonding) Copyright © Houghton Mifflin Company. All rights reserved.

Must consider there may be two other types of IM forces! London forces (always) Dipole-dipole forces (only if polar) Hydrogen bonding (special circumstance) Copyright © Houghton Mifflin Company. All rights reserved.

Dipole-Dipole Forces Figure 10 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 d+ and d- here!! Copyright © Houghton Mifflin Company. All rights reserved.

NOTE: How to determine if a molecule is polar will be discussed later Chapter 11, Figure 11.7 Dipole–Dipole Interaction

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) 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 → Must consider ALL possible forces to decide relative bp, mp, VP, etc. Copyright © Houghton Mifflin Company. All rights reserved.

Example: Similar London, but Larger Dipole-Dipole Leads to Stronger (overall) IM forces and Higher bp, mp Chapter 11, Unnumbered Table, Page 462

Again, greater polarity (similar London) yields greater (overall) IM forces (and bp) Chapter 11, Figure 11.8 Dipole Moment and Boiling Point

Figure 10.3 a-b (a) The Polar Water Molecule (b) Hydrogen Bonding Among Water Molecules Copyright © Houghton Mifflin Company. All rights reserved.

Figure 11.13 Explain these trends by approximating the IM forces Chapter 11, Figure 11.13 Boiling Points of Group 4A and 6A Compounds

What causes London (dispersion) forces? Chapter 11, Figure 11.4 Dispersion Interactions

Figure 10. 5 Zumdahl What causes LONDON 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 Copyright © Houghton Mifflin Company. All rights reserved.

http://www. yteach. co. uk/page. php/resources/view_all http://www.yteach.co.uk/page.php/resources/view_all?id=intermolecular_force_matter_dipol_polar_non_polar_dispersion_hydrogen_bonding_t_page_14 http://intro.chem.okstate.edu/1515SP01/Lecture/Chapter12/LondonDisp.html Copyright © Houghton Mifflin Company. All rights reserved.

More Examples (Explaining bp differences) Explain the difference in boiling points: n-pentane, C5H12 MW: 72 amu bp = 36.2 C vs. Neopentane, C5H12 MW: 72 amu bp = 9.5 C dimethyl ether, CH3OCH3 MW: 46 amu bp = -25 C vs. ethanol, CH3CH2OH MW: 46 amu bp = 79 C 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! Copyright © Houghton Mifflin Company. All rights reserved.

Figure 10.6 Zumdahl A Molecule in the Interior of a Liquid is Attracted by the Molecules Surrounding It Copyright © Houghton Mifflin Company. All rights reserved.

Surface tension examples: Liquid Mercury Forms a Convex Meniscus in a Glass Tube (unlike water) Water beads on a wax surface Copyright © Houghton Mifflin Company. All rights reserved.

Back to molecular substances— How to tell if a molecule is polar Next slide (and handouts) Copyright © Houghton Mifflin Company. All rights reserved.

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

...if symmetry is very high, bond dipoles can cancel out 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) Copyright © Houghton Mifflin Company. All rights reserved.

Polarity of Molecules (see handout #2)

Polarity of Molecules (continued)

(Nonpolar) (Polar) F Chapter 10, Unnumbered Figure 2, Page 415

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. Copyright © Houghton Mifflin Company. All rights reserved.

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 Copyright © Houghton Mifflin Company. All rights reserved.

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. Copyright © Houghton Mifflin Company. All rights reserved.

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 p-Bonding Network → This explains directional electrical conductivity in a crystal of graphite. Copyright © Houghton Mifflin Company. All rights reserved.

Figure 10.26 The Structure of Quartz (Empirical Formula SiO2) Copyright © Houghton Mifflin Company. All rights reserved.