General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake 5.9 Attractive Forces in Compounds Chapter 5 Compounds and Their Bonds © 2013 Pearson Education, Inc. Lectures

© 2013 Pearson Education, Inc. Chapter 5, Section 9 2 Ionic Compounds In ionic compounds, ionic bonds  require large amounts of energy to break.  hold positive and negative ions together.  explain their high melting points.

© 2013 Pearson Education, Inc. Chapter 5, Section 9 3 Covalent Compounds In covalent compounds, the attractive forces between solid and liquid molecules  are weaker than ionic bonds.  require less energy to break.  explain why their melting points are lower than ionic compounds. These attractive forces include  dipole-dipole attractions,  dispersion forces, and  hydrogen bonding.

© 2013 Pearson Education, Inc. Chapter 5, Section 9 4 Dipole-Dipole Attractions In covalent compounds, polar molecules exert attractive forces between molecules called dipole-dipole attractions.

© 2013 Pearson Education, Inc. Chapter 5, Section 9 5 Dipole-Dipole Attractions, Hydrogen Bonds In covalent compounds, some polar molecules form strong dipole attractions called hydrogen bonds, which occur between the partially positive hydrogen atom of one molecule and a lone pair of electrons on a nitrogen, oxygen, or fluorine atom in another molecule.

© 2013 Pearson Education, Inc. Chapter 5, Section 9 6 Dispersion Forces Dispersion forces are  weak attractions between nonpolar molecules.  caused by temporary dipoles that develop when electrons are not distributed equally. Nonpolar molecules form attractions when they form temporary dipoles.

© 2013 Pearson Education, Inc. Chapter 5, Section 9 7 Comparison of Bonding and Attractive Forces

© 2013 Pearson Education, Inc. Chapter 5, Section 9 8 Melting Points and Attractive Forces The stronger the attractive force between ions or molecules, the higher the melting points. Ionic compounds, have the strongest attractive force and, therefore the highest melting points. Covalent molecules have less attractive forces than ionic compounds and, therefore lower melting points.

© 2013 Pearson Education, Inc. Chapter 5, Section 9 9 Melting Points and Attractive Forces The attractive forces between covalent molecules vary in magnitude; the stronger the attractive force, the higher its melting point.  Hydrogen bonds are the strongest type of dipole– dipole attractions, requiring the most energy to break, followed by dipole–dipole forces.  Dispersion forces are the weakest, requiring even less energy to break them, and therefore have lower melting points than hydrogen bonds and dipole– dipole forces.

© 2013 Pearson Education, Inc. Chapter 5, Section 9 10 Melting Points of Selected Substances

© 2013 Pearson Education, Inc. Chapter 5, Section 9 11 Learning Check Identify the main type of attractive forces for each of the following compounds: ionic bonds, dipole–dipole, hydrogen bonds or dispersion. 1. NCl 3 2. H 2 O 3. Br 2 4. KCl 5. NH 3

© 2013 Pearson Education, Inc. Chapter 5, Section 9 12 Solution Identify the main type of attractive forces for each of the following compounds: ionic bonds, dipole–dipole, hydrogen bonds or dispersion. 1. NCl 3 dipole–dipole 2. H 2 Ohydrogen bonds 3. Br 2 dispersion 4. KClionic bonds 5. NH 3 hydrogen bonds