Intermolecular Forces

Intermolecular Forces By: Michael London Ellie Kaplan Mike Ghen

Intermolecular Forces • Strengths of intermolecular forces of difference substances vary over a wide range, but they are generally much weaker than ionic or covalent bonds • Many properties of liquids reflect the strengths of intermolecular forces • The stronger the attractive forces, the higher the boiling point • Similarly, melting points of solids increase as the strengths of the intermolecular forces increase

Intermolecular Forces Neutral Molecules Ion-Dipole Forces Dipole-Dipole Forces London Dispersion Forces Hydrogen- Bonding Forces Intermolecular Forces Con’t • Three types of intermolecular forces exist between neutral molecules (van der Waals forces) • Dipole-dipole forces • London dispersion forces • Hydrogen-bonding forces • Ion-Dipole forces also exist, and are important in solutions

Ion-Dipole Forces • Exists between an ion and the partial charge on the end of a polar molecule • Polar molecules have a positive and negative end (example: HCl) • Magnitude of the attraction increases as either the charge of the ion or the magnitude of the dipole moment increases • HINT: You are used to these

Dipole-Dipole Forces • Neutral polar molecules attract each other when the positive end of one molecule is near the negative end of another • Effective only when polar molecules are very close together • Weaker than ion-dipole forces • In Liquids: • For molecules of approximately equal mass and size, the strengths of intermolecular attractions increase with increasing polarity

London Dispersion Forces • German-American physicist Fritz London discovered that the motion of electrons in an atom or molecule can create an instantaneous dipole moment • Due to repulsion, the motions of electrons on one atom affect the motions of electrons on neighbors • The temporary dipole on one atom can induce a similar dipole on an adjacent atom (attraction) • Significant only when molecules are very close together

Polarizability • Polarizability: the ease with which the charge distribution in a molecule can be distorted by an external electric field • In other words: a measure of the “squashiness” of its electron cloud

SO… • The greater the polarizability of a molecule, the more easily its electron cloud can be distorted to give a momentary dipole • Therefore… • More polarizable molecules have stronger London dispersion forces • Strength of London dispersion forces increase with increasing molecular size • - Dispersion forces tend to increase in strength with increasing molecular weight

Dispersion forces operate between all molecules • Polar molecules will see dipole-dipole interactions as well as dispersion forces • When the molecules have comparable molecular weights and shapes, dispersion forces are approximately equal • When molecules differ widely in their molecular weights, dispersion forces tend to be decisive

Hydrogen Bonding • A special type of intermolecular attraction between the hydrogen atom in a polar bond and an unshared electron pair on a nearby small electronegative ion or atom (an F, O, or N atom) • EX: exists between the H atom in an HF molecule and the F atom of an adjacent HF molecule • Drawn out: F----H … F---H

Comparing Intermolecular Forces • Identifying the intermolecular forces in a substance can be done by considering its composition and structure • Dispersion forces are found in all substances • Strength increases with increased molecular weight and depend on the molecular shape • Dipole-Dipole forces are found in polar molecules • Hydrogen bonds require H atoms bonded to F, O, or N • Strongest type of intermolecular forces • None of these are as strong as ionic of covalent bonds

Questions • True or False • Molecules held together by London Dispersion Forces are most likely found in solid substances • Ionic bond are weaker than intermolecular forces • Strength of London dispersion forces decrease with decreasing molecular size • More polarizable molecules have stronger London dispersion forces • What is weaker Ion-Dipole or Dipole-Dipole • Can Non-polar molecules have Dipoles? • State the bond type in a substance containing the following molecules • a) NH3 b) He-He c) HCl-HCl • Explain why Fluoride has a lower boiling point than Chlorine

Na+ A O H B O H H C H Cl- Salt is dissolved in water. What are the intermolecular forces are present? A) B) C)

Sources • http://chemed.chem.purdue.edu/genchem/topicreview/bp/intermol/intermol.html • http://www.chemguide.co.uk/atoms/bonding/vdw.html • http://www.science.uwaterloo.ca/~cchieh/cact/c123/intermol.html • http://www.wwnorton.com/college/chemistry/gilbert/concepts/chapter9/ch9_1.htm • http://www.elmhurst.edu/~chm/vchembook/161Ahydrogenbond.html • http://itl.chem.ufl.edu/2041_f97/lectures/lec_g.html

CH4 CH4 CH4 CH4 What forces are attracting these molecules together? What forces are attracting the atoms of one CH4 molecular? How do there strengths compare?

A Substance contains 3 different molecules. Molecule C has a heavier weight, molecule A is much lighter. Molecule A Which molecule will attract molecule B more? If Molecule A is CO2 and molecule B is O2. What type of forces is between them? If molecule C is H2S and molecule B is Fluoride what will the attractive force be? Molecule B Molecule C

Intermolecular Forces