Unit 2: Nomenclature, Intermolecular Forces and Properties of Solutions

1. Unit 2: Nomenclature, Intermolecular Forces and Properties of Solutions • Nomenclature • Intermolecular Forces • Phase Changes and Phase Diagrams • Saturated Solutions • Solubility • Concentration Units • Colligative Properties

2. Nomenclature • You are responsible for reviewing the conventions for naming and writing formulas for ionic compounds, binary molecular compounds, and acids that were presented in Chem I. • This material will not be covered in lecture but will represent about 20% of the Unit 2 Exam. • You will be responsible for ionic compounds that contain any of the ions given in the Ion Chart handout.

3. Nomenclature • Resources for reviewing nomenclature: • Chapter 2 of your text • Nomenclature slides on KMB’s website under Unit 2 Powerpoint lectures. • Ion chart handout • General Chemistry Tutorial for naming and writing formulas for ionic compounds on KMB’s website.

4. Intermolecular Forces • The fundamental difference between states of matter is the distance between particles.

5. Intermolecular Forces • The solid and liquid states are referred to as condensedphases because the particles are closer together.

6. Intermolecular forces • The physical state of a substance at a particular temperature and pressure depends on two antagonistic entities: • The kinetic energy of the particles • The strength of theintermolecular forcesbetween the particles • the attractive forces between particles in a solid, liquid, or gas • Converting from one physical state to another requires the molecules to gain enough kinetic energy to overcome the intermolecular forces

7. Intermolecular Forces • Intermolecular forces are not nearly as strong as the intramolecular attractions (ionic or covalent bonds) that hold compounds together. • Intermolecular forces, however, have a significant impact on the physical properties of compounds: • boiling point • melting point • vapor pressure • solubility

8. Intermolecular Forces • There are four types of intermolecular forces: • Dipole-dipole interactions (forces) • polar molecules • London dispersion forces • all molecules • Hydrogen bonding • molecules with H-F, O-H, or N-H bonds • Ion-dipole forces • An ion and a polar molecule

9. d- d+ d- d+ d+ d- d+ d+ Intermolecular Forces • Polar Molecules • contain polar covalent bonds which are asymmetrically distributed within the molecule • contain a “positive” end and a “negative”end • Examples: • HCl • H2O • CH3OH

10. Intermolecular Forces • To determine if a molecule is polar, identify all polar covalent bonds: • 0 polar covalent bonds = nonpolar molecule • 1 polar covalent bond = polar molecule • >2 polar covalent bonds = polar or nonpolar • Must use the electron domain geometry to determine polarity

11. Intermolecular Forces • If a molecule has two or more polar covalent bonds, it may be either a polar or nonpolar molecule: • Draw the molecule in 3 dimensions using its electron domain geometry • Draw the bond dipole moments for the polar covalent bonds • Nonpolar if they are symmetrical or offset each other • Polar if they are asymmetrically arranged

12. Intermolecular Forces Example: Identify each of the following molecules as a polar or nonpolar molecule.

13. Intermolecular Forces Example: Which of the following molecules are polar: CHCl3, CO2, Br2, HF, CH3OH, CH3OCH3?

14. Intermolecular Forces • Polar molecules have large dipole moments • A measure of the separation between the positive and negative charges in polar molecules. d+ d- H – F d+ d-

15. Intermolecular Forces • Dipole-dipole interactions (forces) are found between polar molecules. • attractive intermolecular forces resulting from the attraction of the positive and negative ends of the dipole moments of polar molecules • The most stable arrangement of polar molecules is the one in which the positive end of one molecule is oriented toward the negative end of the other molecule.

16. Intermolecular Forces • For example, molecules of CH3Cl are held together by dipole-dipole interactions:

17. Intermolecular Forces • When a liquid vaporizes, the intermolecular forces must be overcome. • As polarity increases, the strength of the dipole-dipole interactions increase. • Large DHvap • High BP

18. Intermolecular Forces • London dispersion forces are present in ALL molecules. • Temporary dipole moments lasting for fractions of a second are induced in one molecule by other nearby molecules • electrons in molecules are displaced from symmetrical arrangement

19. Intermolecular Forces • London dispersion forces require close surface contact between two (or more) molecules. • The strength of the London dispersion forces is roughly proportional to surface area. • As surface area increases, LDF increase and BP increases

20. Intermolecular Forces • For molecules with similar molecular formulas: • Long, skinny (unbranched) molecules have greater surface area • higher BP • Shorter, branched molecules are more spherical and have less surface area • lower BP

21. Intermolecular Forces • The strength of dispersion forces tends to increase with increasing molecular weight. • Larger atoms have larger electron clouds, which are easier to polarize. • In general, as MW increases, BP increases.

22. Intermolecular Forces • Although BP tends to increase with increasing MW, there are some exceptions. • Why is the BP of water so much higher than expected?

23. Intermolecular Forces • Compounds containing H-F, O-H, and/or N-H bonds exhibit hydrogen bonding: • a strong dipole-dipole interaction between a hydrogen atom that is covalently bonded to either O, N, or F and a lone pair of electrons on a different O, N, or F atom

24. Intermolecular Forces • Due to the large difference in electronegativity, O-H, N-H, and F-H bonds are highly polar • H has a partial positive charge in such bonds • The H atom is strongly attracted to the nonbonding electrons on other N, O or F atoms. d+ d- d- d- d+ d+ O - H N - H F - H

25. Intermolecular Forces Example: Which of the following compounds can form hydrogen bonds with another molecule of the same substance?

26. Intermolecular Forces • Impact of hydrogen bonding on BP: • Hydrogen bonding leads to higher BP • H2O forms H-bonds • H2S, etc cannot form H-bonds • As the number of hydrogens capable of forming hydrogen bonds increases, the boiling point increases.

27. Intermolecular Forces • Impact of hydrogen bonding on BP: • Due to greater differences in electronegativity, OH forms stronger hydrogen bonds than NH • Compounds with OH have higher BP’s than similar compounds with NH

28. Intermolecular Forces Example:Consider each pair of compounds separately. Which compound in each pair has the higher BP? Explain why.

29. Intermolecular Forces and Solubility • Intermolecular forces also determine the solubility of organic compounds. • “Like dissolves like” • polar compounds dissolve in polar solvents • nonpolar compounds dissolve in nonpolar solvents • Ionic compounds generally dissolve readily in water because water hydrates or solvates the individual ions

30. Intermolecular Forces and Solubility • Ionic compounds tend to dissolve in polar solvents like water because of ion-dipole forces • attractive force between an ion and the partially “charged” end of a polar molecule.

31. + - Intermolecular Forces and Solubility • An aqueous solution of an ionic compound such as NaCl contains solvated cations and anions: Solvation of anion Solvation of cation Examples of ion-dipole forces

32. Intermolecular Forces and Solubility • Polar compounds dissolve in polar solvents due to: • dipole-dipole interactions • H-bonding Hydrogen bonding between methyl amine and water.

33. Intermolecular Forces and Solubility • Nonpolar compounds do not dissolve appreciably in water because they cannot break the hydrogen-bonding network that exists.

34. Intermolecular Forces and Solubility Example: Will each of the following vitamins be water soluble or fat soluble?