1. Chapter 10 Properties of Solidsand Liquids 10.4 Attractive Forces in Compounds Learning Goal Describe the attractive forces between ions, polar covalent molecules, and nonpolar covalent molecules.

2. Attractive Forces in Compounds Attractive forces between molecules and ions hold them close together in liquids and solids. Solids melt, and liquids boil when the attractive forces between molecules are broken. Attractive forces between molecules can be • dipole−dipole attractions • hydrogen bonding • dispersion forces

3. Dipole-Dipole Attractions Polar molecules are attracted to each other by dipole-dipole attractions • when the positive end of one dipole is attracted to the negative end of a second dipole • such as the attractive forces between two molecules of H—Cl

4. Hydrogen Bonds Hydrogen bonds, especially strong dipole-dipole attractions, occur between • polar molecules containing hydrogen atoms bonded to very electronegative atoms such as fluorine (F), nitrogen (N), and oxygen (O) • a hydrogen atom with a partial positive charge attached to N, O, or F and a partial negative charge on N, O, or F

5. Dispersion Forces Dispersion forces, very weak attractive forces that occur between nonpolar molecules, • occur when movement induces a temporary distortion of the electrons in a molecule, creating a temporary dipole • make it possible for nonpolar molecules to exist as liquids and solids

6. Bonding and Attractive Forces

7. Learning Check Indicate the major type of molecular interaction such as dipole-dipole attractions, hydrogen bonds, or dispersion forces expected between each of the following: A. NF3 B. Cl2 C. HF

8. Solution Indicate the major type of molecular interaction such as dipole-dipole attractions, hydrogen bonds, or dispersion forces expected between molecules of: A. NF3 dipole-dipole attractions B. Cl2 dispersion forces • HF hydrogen bonds

9. Size, Mass, and Boiling Points As the size and mass of similar types of molecules increase, • the attractive forces between the molecules also increase • there are more electrons available to produce stronger temporary dipoles These increased attractive forces increase the boiling points of these molecules.

10. Molar Mass and Boiling Points, Alkanes

11. Attractive Forces and Melting Points The melting point of a substance is related to the strength of the attractive forces between its particles. Molecules with • weaker dispersion forces have lower melting points; it takes less energy to break the dispersion forces • stronger dipole-dipole forces, or hydrogen bonds, require more energy to break the forces between them

12. Highest Melting Points The highest melting points occur in ionic compounds that have very strong attractive forces between positive and negative ions. Ionic compounds require large amounts of energy to break these forces and melt the substance.

13. Melting Points of Selected Substances

14. Chemistry Link to Health Biological molecules such as proteins have many different functions. They are needed for • structural components such as cartilage, muscles, hair, and nails • the formation of enzymes to regulate biological reactions • transport of oxygen in blood and muscles

15. Chemistry Link to Health Proteins are composed of building blocks called amino acids. Amino acids have a central carbon atom bonded to • an –NH3+ from an amine and a –COO− from a carboxylic acid • an H atom and a side chain called an R group

16. Chemistry Link to Health Different amino acids have different R groups. The R group on alanine is a –CH3 group. (a) The ionized form of alanine contains —NH3+, —COO–, H, and a —CH3 group. (b) The structure of alanine represented as a ball-and-stick model.

17. Chemistry Link to Health Examples of other amino acids with their side chains include:

18. Chemistry Link to Health In the primary structure of proteins, amino acids are linked by peptide bonds between the amine and the carboxylic acid groups.

19. Chemistry Link to Health In a higher level structure ofproteins, hydrogen bonds areformed to fold the proteinsinto different shapes, such asan alpha helix.

20. Chemistry Link to Health Hydrogen bonding inproteins also occursbetween the polarside chains on aminoacids on the outersurface of the protein,and the –OH and –Hof polar watermolecules.

21. Learning Check Identify the compound in each pair that has the higher melting point. Explain. A. NCl3 or NH3 B. HBr or Br2 C. KCl or HCl

22. Solution Identify the compound in each pair that has the higher melting point. Explain. A. NCl3 or NH3 NH3 has a higher melting point than NCl3 because the hydrogen bonds in NH3 are stronger than the dipole-dipole attractions in NCl3.

23. Solution Identify the compound in each pair that has the higher melting point. Explain. B. HBr or Br2 HBrwould have a higher melting point than Br2 because the dipole-dipole attractions in HBr are stronger than the dispersion forces in Br2.

24. Solution Identify the compound in each pair that has the higher melting point. Explain. C. KCl or HCl KClwould have a higher melting point than HCl because the ionic bonds in KCl are stronger than the dipole-dipole attractions in HCl.