IM Forces Section 10.1

States of Matter

Forces Between Particles in Solids and Liquids Ionic compounds –Attractive forces between oppositely charged ions hold ionic compounds together. –Ionic bonds are the strongest interparticle force. –Smaller the ion and the larger the charge on the ion the stronger the attractive forces among the ions

Ionic Bonding

Forces Between Particles in Solids and Liquids Forces between molecular compounds –Intermolecular (IM) forces between molecules attract molecules to each other in the liquid and solid state. IM forces are very weak as compared to ionic or covalent bonds

IM Forces Three types of IM Forces 1.Dipole-dipole force 2.Hydrogen “bonding” 3.London dispersion forces See pages

Interparticle Forces and Physical Properties The stronger the attractive forces between particles in a liquid or solid, the –Higher the: Melting point Boiling point Surface tension Viscosity –Lower the: Vapor pressure

IM Forces Dipole-dipole forces –Attractive forces between oppositely charged dipoles. –Dipole-dipole forces are found between polar compounds. The more polar the compound the stronger the dipole-dipole force.

IM Forces Hydrogen “bonds” –Attractive force between a  + H bonded to an O, N, or F and a  - O, N, or F generally on another molecule. Really a relatively strong dipole-dipole force –Hydrogen bonding is the strongest of the IM forces. –H bonding is very important in water and in many biological molecules.

Hydrogen “bond” is a weak attractive force between a  + hydrogen and a  -  O, N, or F in a second polar bond

London Dispersion Forces London Dispersion force –Very weak and short-lasting attractive forces between temporary dipoles See figure 10.5 –Weakest of the IM forces

London Dispersion Forces London Dispersion forces –Found between all molecules in liquid/solid state. Of greatest significance in nonpolar molecules as it’s the only IM force between nonpolar molecules –The larger the molecule the stronger the dipersion forces.

Dispersion Forces Occur between every compound and arise from the net attractive forces amount molecules which is produced from induced charge imbalances The magnitude of the Dispersion Forces is dependent upon how easily it is to distort the electron cloud. The larger the molecule the greater it’s Dispersion Forces are.

Dispersion Forces and Molecular Shape Elongated molecules have higher dispersion forces than compact molecules Ringed structures have higher dispersion forces than straight chain molecules. –Consider: Hexane Cyclohexane 2,2 – dimethyl butane

Interparticle Forces Weakest to Strongest: Intermolecular forces – all relatively weak London dispersion forces Dipole-dipole force Hydrogen Bonding Ionic bond - BY FAR THE Strongest: - not an IM Force

Properties of Liquids Freezing and boiling point Surface tension Capillary action Viscosity  Which are directly related to the strength of the IM forces present between molecules?

Change of State Normal Freezing/Melting point –temperature at which the liquid and solid state co-exist at 1 atm pressure Normal boiling point –temperature at which the liquid and gaseous state co-exist at 1 atm pressure Predict the relative BP of: –Methane, acetone, methanol, ethanol, NaCl

Surface Tension Surface tension –Resistance of a liquid to increase its surface area –Measure of the energy needed to break the IM forces at the surface

Capillary Action Capillary action –Spontaneous rising of a liquid in a narrow tube Related terms: –Cohesive forces – attractive forces among like molecules –Adhesive forces – attractive forces among dislike molecules

See Figure 10.7, page 444 Concave meniscusConvex meniscus Adhesion > CohesionCohesion > adhesion

Viscosity Viscosity – resistance of a liquid to flow –Highly viscous liquids are thick (syrupy) –Consider relative viscosity of: Propanol - Glycerol

Solid State Amorphous solids – random arrangement of particles Crystalline solids – highly ordered arrangement of particles –Arrangement referred to as a lattice structure Unit cell – smallest repeating unit in the lattice –Use x-ray crystallography to determine the structure of crystalline solids

Common Unit Cells – All Cubic

Closest Packing and Unit Cells Often see a lattice structure based on the closest packing possible for the particles. Two common closest packing arrangements (pages 451/452) –Abab close packing – creates a hexagonal prism unit cell (hcp) –Abca close packing – creates a face-centered cubic unit cell

Types of Crystalline Solids Crystalline solids are defined by what’s in the lattice points The properties of crystalline solids depends upon the strength of the attractions between the particles –Stronger the attractive forces, the higher the mp, bp…..

Crystalline Solids Type of SolidIn Lattice PointsAttractive forces between particles Properties IonicIons -- NaCl -- CaO Ionic bondsHigh mp, poor conductors of heat and electricity in solid state, hard, brittle MolecularMolecules -- H 2 OH 2 O -- CO 2 -- H 2 IM Forces -- dispersion forces -- dipole-dipole -- H bonds Relatively low mp and bp (depends on force present), poor conductors of heat and electricity, soft

Crystalline Solids – Atomic Solids Type of ATOMIC Solid In Lattice PointsAttractive forces between particles Properties Group 8a – frozen noble gases Noble gas atoms -- He -- Ar Dispersion forcesVERY low mp, insulators MetallicMetal atoms -- Cu -- Fe Non-directional covalent bonds --sea of delocalized valence electrons Ductile, malleable, good conductors of heat and electricity Network – giant molecules, not individual atoms or molecules Nonmetal atoms -- C, diamond -- SiO 2 -- C, graphite (atypical network solid) Covalent bondsTypical – hard, high mp, poor conductors heat and electricity (insulators) Atypical – see graphite

Metallic Solids Metal atoms in lattice points –Variety of unit cells possible Electron sea model –delocalized valence electrons form strong nondirectional bonds Valence e are free to move – conduct electricty and heat Atoms remain bonded to each other as they bend/ move – malleable and ductile

Diamond Diamond – Network Solid A diamond is a gigantic molecule, each C atom is bonded to 4 other C atoms Each C is sp 3 hybridized

SiO 2 – Network Solid Why isn’t SiO 2 classified as a molecular solid?

Graphite Graphite – Atypical Network Solid Layers of ringed carbon structures –Each C is bonded to 3 other C –Each C is sp 2 hybridized

Change of State Terms Freezing –Normal freezing point Melting Vaporization –Normal boiling point Condensation Sublimation Deposition

Phase Diagram

Phase Diagram Terms Triple Point –temperature and pressure conditions when (any) 3 states co-exist Critical Point –Critical Temp – temp –above which a gas cannot be liquefied –Critical P – P required to create a liquid at critical T

Phase Diagram of Water 11.9

Phase Diagram for Sulfur