Intermolecular Forces ©2011 University of Illinois Board of Trustees

Like dissolves like. Property of Solubility Since you now know how to address polarity, Which of the following compounds will be most soluble in carbon tetrachloride? a. CH 2 O b. H 2 O c. IF d. CF 4 e. SO 2 ©2011 University of Illinois Board of Trustees

Like dissolves like. Property of Solubility Since you now know how to address polarity, Which of the following compounds will be most soluble in carbon tetrachloride? a. CH 2 O b. H 2 O c. IF d. CF 4 e. SO 2 ©2011 University of Illinois Board of Trustees

Boiling Points and Melting Points To make predictions about melting and boiling points, you must have some knowledge of how strongly molecules are held to each other in their solid or liquid state. This requires that we look at the strength of the forces holding the molecules or particles together. ©2011 University of Illinois Board of Trustees

Intermolecular Forces (IMF) Intermolecular forces are forces (pulls) that exist between separate molecules. Intramolecular forces (AKA bonds) are forces that hold an individual molecule together. All forces that are due to oppositely charged particles are described by Coulomb’s Law, including IMF. ©2011 University of Illinois Board of Trustees

Hmmmm …. The more polar a substance the greater the strength of attraction between the molecules. Which of the following substances would have the greatest IMF? a. CO 2 b. NH 3 c. CCl 4 d. H 2 O ©2011 University of Illinois Board of Trustees

Hmmmm …. The more polar a substance the greater the strength of attraction between the molecules. Which of the following substances would have the greatest IMF? a. CO 2 b. NH 3 c. CCl 4 d. H 2 O ©2011 University of Illinois Board of Trustees

Types of IMF There are different types of intermolecular forces that attract molecules to each other. 1.Dipole-dipole forces 2.Hydrogen bonding 3.London dispersion forces These forces are prevalent when the particles of a substance are close together. ©2011 University of Illinois Board of Trustees

Comparison of Forces Type of Interaction Approx.Energy (kJ/mol) Intermolecular London, dipole-dipole Hydrogen bonding 10 – 40 Intramolecular (bonding) Covalent Ionic ©2011 University of Illinois Board of Trustees

Hmmmmm…. Which of the following will have the weakest IMF? a. Solid b. Liquid d. Gases ©2011 University of Illinois Board of Trustees

Hmmmmm…. Which of the following will have the weakest IMF? a.Solid b. Liquid d. Gases mistry/liquidsolid.html mistry/gasliquid.html ©2011 University of Illinois Board of Trustees

London Dispersion Forces Weakest type of IMF in molecules of similar size. Present between all molecules. The only type of IMF between nonpolar molecules. Results from the instantaneous uneven distribution of electrons in the electron cloud, which induces a dipole in nearby molecules. This is called polarizability. ©2011 University of Illinois Board of Trustees

London Dispersion Forces Cont’d. Larger molecules have more electrons and so their electron clouds are more polarizable. Consider the halogens and the noble gases. HalogenMW (amu) Boiling point (K) Noble Gas MW (amu) Boiling Point (K) F2F He Cl He Br Ar I2I Kr Xe ©2011 University of Illinois Board of Trustees

London Dispersion Forces Cont’d. Neopentane C 5 H 12 n-Pentane C 5 H 12 B.P. = KB.P. = K LDF also depend upon the shape of the molecule. ©2011 University of Illinois Board of Trustees

Hmmmm….. The greater the intermolecular forces the higher the boiling point. Which of the following substances have the highest boiling point? a. C 2 H 6 b. CH 4 c. C 2 H 4 Cl 2 d. C 2 H 4 ©2011 University of Illinois Board of Trustees

Dipole-Dipole Forces Occur between polar molecules. The positive pole of one polar molecule is attracted to the negative end of another polar molecule. The more polar the substance, the stronger the dipole-dipole force. ©2011 University of Illinois Board of Trustees

Hydrogen Bonding Strongest type of intermolecular force. Must have hydrogen bonded to a small very electronegative atom (i.e., F, N or O) in the molecule. The positive hydrogen pole of one molecule is attracted to the negative pole (F, N or O) of a different molecule. ©2011 University of Illinois Board of Trustees

Hmmmmm…. Which of the following molecules would show the greatest amount of IMF? a)methanol, CH 3 OH b.) water c.) formaldehyde d.) glycerin ©2011 University of Illinois Board of Trustees

Intermolecular Forces: Summary London Dispersion Forces – Always present – Strength is proportional to molecular weight Dipole-Dipole – Only present in polar molecules – Strength increases with dipole moment for molecules of approximately equal size and mass Hydrogen Bonding – Only present when a highly polar N-H, O-H, or F-H bond is present in a molecule ©2011 University of Illinois Board of Trustees

We’re Off to the Races…. The stronger the intermolecular forces between molecules, the slower a substance will evaporate. If all substances are at the same temperature, which of the following substances which will evaporate first? a.methanol b. pentane c. acetone Explain the results. ©2011 University of Illinois Board of Trustees

We’re Off to the Races….. Of the following substances which will evaporate first? a.pentane b. hexane c. heptane Explain the results. ©2011 University of Illinois Board of Trustees

We’re Off to the Races….. Of the following substances which will evaporate first? a.water b. propanol c. glycerine Explain the results. ©2011 University of Illinois Board of Trustees