CHAPTER 11 - INTERMOLECULAR FORCES VERSION 5

The States of Matter

Figure c

Figure a

Figure d The state a substance is in at a particular temperature and pressure depends on two antagonistic entities: kinetic energy and intermolecular forces

Figure a

What are intermolecular forces? The short-range attractive forces operating between particles that make up the units of a liquid or solid substances. These same forces also cause gases to liquefy or solidify at low temperatures and high pressures.

Intermolecular versus Intramolecular Forces Compounds with covalent bonds Salt – ionic bonding Attractive forces between molecules Intermolecular Forces Intramolecular Forces

TypeExample Covalent chemical bondsC-Cl bond in CCl 4 Ionic chemical bondsBond between Na + and Cl- in NaCl Metallic chemical bondsBond between Na atoms in Na(s) Dipole-dipole (3) Force between 2 like or unlike polar molecules such as two acetone molecules or an acetone molecule and a chloroform molecule Ion-dipole (1) Force between a Na + ion and a water molecule Hydrogen bonds (special type of dipole-dipole) (2) Force between two water molecules Instantaneous dipole-induced dipole (London dispersion forces) (4) Force between two nonpolar molecules of I 2 The major types of forces that operate among atoms, molecules, and ions Intramolecular forces/bonds (blue) and Intermolecular forces (red) Dipole – dipole, ion – dipole and London dispersion forces are collectively referred to as Van der Waals forces

Ion-Dipole Attractive How do water molecules orient themselves around Na + and Cl - ions?

Ion-Dipole Attractive Forces Sodium ion, Na +, surrounded by H 2 O molecules

Dipole-Dipole Attractive Forces

Dipole-Dipole Attractive Forces - Acetone

Dipole – Dipole Attractive Forces between Acetone Molecules

Dipole – Dipole Attractive Forces

Dipole-Dipole Interactions The more polar the molecule, the higher is its boiling point.

Hydrogen Bonding Hydrogen bonding is a special type of dipole-dipole attractive force. Hydrogen bonding is an intermolecular attraction between the hydrogen atom in a polar bond (particularly H-F, H-O, or H-N bond) and a non-bonding electron pair on a nearby small electronegative ion or atom (usually an F, O, or N atom in another molecule). The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong.

Hydrogen Bonding Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine. Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.

Dipole-dipole force between oxygen atom and hydrogen atom results in a hydrogen bond

Which of the following correctly shows the hydrogen bonding between three methyl amine (CH 3 NH 2 ) molecules? Note: the hydrogen bonds are represented as dashed lines.

Practice Problem – Hydrogen Bonding?

Show hydrogen bonding between water and the organic molecule acetone

Boiling Pt. as a Function of Molecular Weight Group VIA hydrides Group IV hydrides

Titan or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid (ethane and methane) has been found. M.PB.P. Methane-182°C, 90.7 K-162°C, 110 K Ethane-183°C, 90.4 K-89°C, K The Liquid Phase is Rare in our Solar System

How do neutral molecules attract one another? BP = 77KBP = 90.2 K

Figure a

Figure c

Figure b

Figure a DipoleInduced dipole Because electrons repel one another, the motions of electrons on one atom influence the motions of electrons on its near neighbors.

Figure b A temporary dipole on one atom can induce a similar dipole on an adjacent atom, causing the atoms to be attracted to each other. This attractive interaction is called the London dispersion force (or merely the dispersion force). This force, like dipole-dipole forces, is significant only when molecules are very close together

Figure c

London Dispersion Forces London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole These forces are present in all molecules, whether they are polar or nonpolar. The tendency of an electron cloud to distort in this way is called polarizability.

Factors Affecting London Forces The strength of dispersion forces tends to increase with increased molecular weight. Larger atoms have larger electron clouds which are easier to polarize. Polarizability increases as you move down a column in the periodic table. Halogen M.W. (amu) B.P. (K) Total # of electron s Noble Gas M.W. (amu) B.P. (K) Total # of electron s F2F He Cl Ne Br Ar I2I Kr Xe

Factors Affecting London Forces Radon has more electrons than argon and these electrons occupy a larger volume and are more loosely held. The electron cloud surrounding radon may be distorted to a much greater extent than the electron cloud surrounding argon.

Factors Affecting London Forces – Molecular Shape The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n-pentane tend to have stronger dispersion forces than short, fat ones (like neopentane). This is due to the increased surface area in n-pentane.

IMF’s and Physical Properties IMF’s affect physical properties. In general the stronger the forces between the particles; The higher the melting point The higher the boiling point The lower the vapor pressure The higher the viscosity The higher the surface tension

Summary of “Inter” and “Intra” Molecular Forces

Homework Problem Dipole-Dipole Interactions

KCl = 1420 °C CH 2 O = °C CO 2 = -57 ° C (at pressures > 5 atm)

Practice Problem 1) What are the intermolecular forces present in each of these compounds: NH 3, CH 4, and NF 3. 2) Rank from highest to lowest boiling point.

Practice Problem 2) Of the following substances, only __________ has London dispersion forces as its only intermolecular force. The dipole moments for each molecule are provided below. CH 3 OH = 1.69 D, NH 3 = 1.42 D, H 2 S = 0.97 D, CH 4 = 0, HCl = 1.03 D

Practice Problem 5) Of the following substances, __________ has the highest boiling point. The dipole moments for each molecule are provided below. H 2 O = 1.85 D, CO 2 = 0, CH 4 = 0, Kr = 0, NH 3 = 1.42 D

Important Physical Properties Melting point Boiling point Vapor pressure

Vapor Pressure Vapor pressure is the pressure exerted by a vapor with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile.

Vapor Pressure No molecules in gas phase here, Zero vapor pressure Temperature is constant! Dynamic equilibrium

Vapor Pressure – Depends on T

Vapor Pressure Substances with high vapor pressures are called“volatile”

Evaporation A type of vaporization of a liquid that occurs only on the surface of a liquid Occurs in open container No dynamic equilibrium

Boiling Point A type of vaporization of a liquid that occurs only on the surface of a liquid A liquid boils when its vapor pressure equals the external pressure acting on the surface of the liquid. The boiling point of a liquid at 1 atm (or 760 torr) is called its normal boiling pt.

Boiling Point At 1 atm external pressure, water boils at 100°C or 212°F

Phase Changes (or Changes of State)

Names Associated With Phase Changes ∆H vaporization ∆H fusion ∆H sublimation

Phase Changes – A Comparison

Phase Changes

Clausius-Clapeyron Equation Relationship between vapor pressure and temperature: Two-Point Form P = vapor pressure of liquid, ∆H vap = heat of vaporization of liquid T = temperature (Kelvin), and R = gas constant, J/mol∙K

Clausius-Clapeyron Equation

Heating Curve

Use specific heat of ice here – C ice J/g-K * g * ∆K Use ∆H fusion here Use specific heat of water here – C water J/g-K * g * ∆K Use ∆H fusion here Use specific heat of water vapor here – C vapor J/g-K * g * ∆K ∆H total = Σ ∆H’s “C” is symbol for specific heat