INTERMOLECULAR FORCES Physical state Inter- vs Intra- Types of IM forces Dipole-dipole forces Hydrogen bonding Ion-ion forces Ion-dipole forces Van der Waal’s forces Liquid properties

Physical state Gas Liquid Solid Widely separated, disordered molecules in continuous motion Closely spaced, disordered molecules in continuous motion Ordered molecules in contact and in relatively fixed position Copyright © Houghton Mifflin Company.All rights reserved.

At room temperature (25 C) and pressure (1 atm), some substances are: Solid: Au, NaCl, Mg Liquids: H2O, Hg Gases: O2, N2, H2 Physical state depends on balance between kinetic energies of molecules and IM attractive forces.

Solids and Liquids Many properties of liquids & solids are similar Solids & liquids have much larger densities than gases 44 g solid CO2 or liquid CO2: volume ~30 mL 44 g gaseous CO2: volume ~ 25 000 mL Going from liquid to gaseous state involves much more energy: H2O(s)  H2O(l) energy required ~ 6 kJ/mol H2O(l)  H2O(g) energy required ~41 kJ/mol

Intra vs Inter CO2 Within (intra) or between (inter) molecules Intermolecular force weaker than Intramolecular force

Intermolecular forces Intramolecular forces Nonpolar covalent Polar covalent Ionic Intermolecular forces SOLID > LIQUID > GAS strong moderate weak

Types of Intermolecular (IM) forces The stronger the force, the more likely it is to play a dominating role in determining structure and properties of a substance. -Dipole-Dipole -Hydrogen Bonding -Ion-ion -Ion-dipole -London dispersion forces

Polarity If charge separation exists in molecule, it’s polar & the extent of polarity is measured by its dipole moment (μ) Influences melting/boiling point, solubility, & chemical reactivity

Polar Molecules Molecules can have polar bonds but not be polar molecules e.g. CCl4 To be a polar molecule it must: (1) contain at least one polar covalent bond (2) not be totally symmetrical

Dipole-dipole Present in polar molecules Molecules orientate themselves so +ve and-ve ends are close together Strength increases as polarity of molecule increases e.g. CH3Cl, HCl, CH3CN

Hydrogen bonding Present in molecules with -OH, -NH, -FH bonds Strong dipole attraction H is unique and sufficiently small that a nearby large electronegative atom’s lone pair of electrons can approach very close to it e.g. H2O, HF, NH3, CH3OH

Effects of Hydrogen Bonding Molecules with -OH, -NH, -HF have unusually high melting/boiling pts and high solubility in H2O

H2O -has a high boiling point - takes a lot of energy to break these attractions if no hydrogen bonding, water would boil at –75 °C -has a high specific heat i.e. 1.00 cal/g °C higher than most liquids (e.g. alcohol = 0.5 cal/g °C) -has a high heat of vaporization i.e. 41 kJ/mol (Hvap)

Ion-ion Strongest IM force Present in ionic compounds Stronger force if ion has a higher charge density e.g. NaCl, CuO

Ion-dipole Between an ion and the partial charge on the end of a polar molecule Strength increases as charge density of ionic compound increases Hydrated compounds -Water molecules coordinate around ion species by electrostatic attraction -Hydration of ion can be permanent e.g. CuSO45H2O

London dispersion forces Also known as van der Waals forces Present in all molecules, weak IM force Instantaneous dipole At some point in time electrons concentrate in one region of atom/molecule Atom/molecule which had symmetrical distribution of charge now becomes polar Induced dipole Instantaneous dipole induces a dipole on neighbouring atom/molecule

Higher molar mass means more electrons present  more polarizability In longer chain molecules, electrons more easily displaced than in compact molecules  more polarizability n-pentane Bp = 36.1 °C neopentane Bp = 9.5 °C

Summary Type of interaction Energy (kJ/mol) Interacting Species Ion-Ion 400 - 4000 Ions only Ion-Dipole 40 - 600 Ion and Polar molecule Hydrogen-bonding 10 - 40 H-X (X = N, O, F) Dipole-Dipole 5 - 25 Two polar molecules London 0.05 - 40 All types of molecules

Why does NH3 have a higher boiling point than CH4 ? Practice examples Why does NH3 have a higher boiling point than CH4 ?    Why does KCl have a higher melting point than I2 ?

Arrange the following in order of increasing boiling points and Practice example Arrange the following in order of increasing boiling points and identify the IM forces present in each compound: CH3OH, CO2, RbF, CH3Br

Liquid properties Viscosity Resistance to flow Measured by timing how long it takes a certain amount of liquid to flow through a thin tube under gravitational force. Strong IM forces impede movement Generally decreases as temperature increases

Surface tension Resistance to an increase in surface area Molecules within bulk of liquid have strong attractive forces versus those on the surface Gives surface properties of thin, invisible, elastic membrane

Capillary action Spontaneous rising of liquid in a narrow tube Due to adhesive forces between liquid and tube