Chapter 8.1-8.3 – Intermolecular Forces CHM1111 Section 04 Instructor: Dr. Jules Carlson Class Time: M/W/F 1:30-2:20 Monday, November 13th

Intermolecular Forces in Different Phases As molecules are closer together (Distances in gases > liquids > solids), extent of intermolecular forces increase. Molecules go from being able to travel large distances without collisions (gases) to travelling short distances (liquids) to being locked in an array (solids).

Phases of Elements at Room Temperature At STP, only 11 elements are gases.

Phase Changes and Intermolecular Forces Melting points and boiling points can be used as indicators for intermolecular forces. For pressure of 1 bar, you have vapourization at the normal boiling point. Normal melting point is also at 1 bar pressure. Vapourization occurs when molecules escape the liquid phase more quickly then they are captured by the gas, reverse process is condensation. The stronger the intermolecular forces, the harder it is for the compound to melt or vapourize. Sublimation Liquefaction or Melting Evaporation or Volatilization Solid Liquid Gas Solidification or Freezing Condensation

Melting and Boiling Points Helium has the lowest melting and boiling points. Melting and boiling point increase with increasing charge separation in the molecule. For same charge separation, melting and boiling point increase with molecular size

Charge Separation in Molecules Electrons are always moving in orbitals. Charge can be produced temporarily from movement – occurs in diatomics, compounds with no dipole moment. Charge separation can be induced – induced dipoles. Charge can be separated in permanent dipole moments. Full charge separation occurs in ionic species.

Types of Intermolecular Forces Dispersion Forces – attraction between the negatively charged electron cloud of one molecule and the positively charged nuclei of a neighbor molecule. Dipolar forces – attraction between negatively charged end of one molecule with the positively charged end of another molecule. Induced Dipoles – attraction between an ion or a permanent dipole and a molecule which has had a dipole induced in it by the ion or permanent dipole. Hydrogen bonding forces – attraction between lone pair electrons on an O, N or F atom with a hydrogen atom.

Strength of Intermolecular Forces Larger charge separation means stronger coulombic attraction forces between oppositely charged regions of molecules.

Dispersion (London) Forces Dispersion forces are found in all molecular substances. Such forces are electrostatic in nature and arise from attractions involving induced dipoles. Dispersion forces help explain why such things as nonpolar compounds dissolve in water. The magnitude of dispersion forces depends on how easy it is to polarize the electron cloud of a molecule. A larger molecule has a larger polarizability. These are the weakest of intermolecular forces (0.5-5 kJ/mol)

Polarization Polarization: Process of inducing a dipole Larger molecules have larger electron clouds so are more polarizable. The more polarizable, the stronger the dispersion forces. Iodine is much larger than Fluorine and therefore more polarizable. More polarizable molecules have higher boiling points and melting points.

Boiling Point Versus Molecular Size Decane (C10H22) is much larger than pentane (C5H12) so is more polarizable and has a higher boiling point.

Induced Dipole Forces Stronger than Dispersion Forces, but one of the weaker intermolecular forces (2-10 kJ/mol) Occurs when a fully charged ion approaches a non-polar molecule with no dipole moment. I- interacts with I2 to form I3-, which has a higher water solubility.

Dipole-Dipole Forces Compounds with a permanent dipole moment are polar. Attraction between positive region of one molecule and negative region of another. The larger the electronegativity differences in the atoms, the larger the dipole moment, the stronger the dipole-dipole forces. Which of the following has the highest boiling point and why?

Hydrogen Bonding O H O H H H …… Hydrogen Bonding is a special type of dipole-dipole force that exists between H and highly electronegative N, O, or F. One molecule has a hydrogen atom attached by a covalent bond to an atom of oxygen, nitrogen, or fluorine. The other molecule has an oxygen, nitrogen, or fluorine atom. Strong intermolecular (sometimes intramolecular) forces (5-50 kJ/mol) Electron deficient - + Electron rich O H - …… O H H + + H + Hydrogen bond

Hydrogen Bonding in Biology Intramolecular H-bond Hydrogen bonding is important in many molecules important to biology.

I Clicker Question Which of the following statements are true: C5H12 has stronger intermolecular forces than C3H7 C5H12 has stronger intermolecular forces than C2H5OH F2 has stronger intermolecular forces than I2 Both (a) and (b) Both (a) and (c)

H-Bonding Problem Draw Lewis Structures and show the hydrogen bonding interaction for two methanol (CH3OH) molecules.

Boiling Point/Melting Point Example Arrange the following sets of compounds from lowest to highest boiling point or melting point. I2, F2, Cl2 C3H7, C12H26, C8H18 CH3OH, CH3CH2CH2OH, CH3CH2COCH3 NH3, NH4+, NH2NH2 H2O, CH3OH, NaCl