Condensed Phases and Intermolecular Forces

1. Condensed Phases and Intermolecular Forces

2. Fundamentals • How do particle diagrams of liquids & solids compare to those of gases?

3. Describe the relative positions and motions of particles in each of the 3 phases.

4. The Question • Why do some substances exist as gases at room temperature, some as liquids, and some as solids?

5. 2 kinds of forces • Part of the answer has to do with the forces between separate molecules. • There are 2 broad categories of forces you need to be aware of.

6. Intramolecular Forces • Intramolecular forces = attractive forces that hold particles together in ionic, covalent, or metallic bonds. • Intra means “within.” • Intramolecular forces = bonding forces.

7. Intermolecular Forces • Inter means “between” or “among.” • Intermolecular forces are forces between molecules. • Intermolecular forces are weaker than intramolecular forces. • There are 3 types of intermolecular forces.

8. Intermolecular forces determine phase. • “Competition” between strength of intermolecular forces and kinetic energy determines phase. • If intermolecular forces are strong, substance will be a solid or liquid at room temperature. Particles want to clump together. • If intermolecular forces are weak, substance will be a gas at room temperature. Particles spread apart.

9. It’s a balancing act! Kinetic Energy Intermolecular Forces This substance = a gas at room temperature.

10. Intermolecular Forces vs. Kinetic Energy This substance = a condensed phase. Intermolecular Forces Kinetic Energy

11. Why Temperature Changes Affect Phase • Since temperature is a measure of avg kinetic energy, changing temperature changes phase. • Changing temperature changes the average kinetic energy

12. Changing the temperature Intermolecular Forces Kinetic Energy

13. Intermolecular Forces • A few % the strength of the intramolecular or bonding forces • Account for phase at room temperature. • Strong intermolecular forces  condensed phase. • Weak intermolecular forces  gas phase

14. Intermolecular Forces & Phase • Strong Intermolecular Forces = Solids • Solids have high melting points, high boiling points. • Weak Intermolecular Forces = Gases • Gases have low melting points, low boiling points.

15. 3 Types of intermolecular forces • Dispersion Forces: These are the weakest. Occur between all molecules but are most important for nonpolar molecules. • Dipole-dipole forces: These are intermediate. Occur between polar molecules. • Hydrogen bonds: These are the strongest. Occur between molecules containing an H-F, H-O, or H-N bond.

16. Dispersion Forces Instantaneous and momentary; fluctuating. Results from motion of electrons. Avg = fig. a. If the charge cloud is not symmetrical it will induce an asymmetry in its neighbor’s charge cloud!

17. Nonpolar molecules Nonpolar means no poles. Can’t tell one end of the molecule from the other. Symmetrical.

18. Examples of Nonpolar Molecules • Monatomic gas molecules: He, Ne, Ar, Kr, Xe, Rn • Diatomics where both atoms are the same element: H2, N2, O2, Cl2, F2, I2, Br2 • Larger molecules if they are very symmetrical molecules: CH4, C2H6, C3H8

19. Dispersion Forces and Size • Dispersion forces increase with the size of the molecule. • The larger the electron cloud, the greater the fluctuations in charge can be. • Rn > Xe > Kr > Ar > Ne > He • I2 > Br2 > Cl2 > F2 • C8H18 > C5H12 > C3H8 > CH4

20. Boiling point of N2 is 77 K. Intermolec-ular forces are very weak dispersion forces.

21. Dipole-dipole Forces & Polar Molecules Molecule shows permanent separation of charge. It has poles – one end is somewhat negative & one end is somewhat positive.

22. Polar Molecules Source: http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter10/Text_Images/FG10_01-01UN.JPG Polar means the molecule has poles, +’ve & -’ve. The geometry and the charge distribution are not symmetrical.

23. What do you know about charge? • Opposites Attract! • This time, the situation is permanent! • Examples: HI, CH3Cl

24. Hydrogen Bonding Occurs between molecules containing an H-F, H-O, or H-N bond. (FON!!!)

25. Hydrogen Bonding • Hydrogen bonding is the extreme case of dipole-dipole bonding. • F, O, and N are all small and electronegative. They really attract electrons. H has only 1 electron, so if it’s spending time somewhere else, the proton is almost “naked.” • The H end is always positive and the F, O, or N is always negative.

26. The bonding electrons spend more time by the oxygen atom than by the hydrogen atoms. Oxygen end – bit negative Hydrogens – bit positive Source: http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/notes/chpt2/water.gif

27. Hydrogen Bonding in Water Source: http://images.google.com/imgres?imgurl=http://food.oregonstate.edu/images/learni/w6.jpg&imgrefurl=http://food.oregonstate.edu/learn/water.html&h=231&w=216&sz=19&tbnid=niAcgd-QCTMJ:&tbnh=103&tbnw=96&hl=en&start=7&prev=/images?q=hydrogen+bonding+in% Source: http://images.google.com/imgres?imgurl=http://food.oregonstate.edu/images/learni/w6.jpg&imgrefurl=http://food.oregonstate.edu/learn/water.html&h=231&w=216&sz=19&tbnid=niAcgd-QCTMJ:&tbnh=103&tbnw=96&hl=en&start=7&prev=/images?q=hydrogen+bonding+in%

28. Hydrogen bonding is the strongest intermolecular force and influences the physical properties of the substance a great deal.

30. Result of Hydrogen Bonding • Increased Boiling Point over predicted value based on molecular size & comparison with similar compounds.

31. Strength of Hydrogen Bonding • Fluorine is the most electronegative element, so H-F bonds are the most polar and exhibit the strongest hydrogen bonding. • H-F > H-O > H-N

32. Intermolecular Forces vs. Physical Properties • If intermolecular forces increase, • Boiling point  • Melting point  • Heat of Fusion  • Heat of Vaporization  • Evaporation Rate  • Vapor Pressure 

33. Intermolecular Forces vs. Temperature • Intermolecular forces become more important as the temperature is lowered. • EX: • Low temperature – low evaporation rate • High temperature – high evaporation rate

34. NH3 Ar N2 HCl HF Ne O2 HBr CH3NH2 Hydrogen bonding Dispersion forces Dispersion forces Dipole-dipole forces Hydrogen bonding Dispersion Dispersion Dipole-dipole Hydrogen bonding Indicate the kind of intermolecular forces for each molecule below.

35. Forces