1. Non-ideal Gases Non-ideality naturally follows a consideration of intermolecular forces since these, in part, account for gas non-ideality. The next slide reviews the kinetic theory assumptions related to the ideal gas law. Let’s consider where these assumptions might break down.

2. Visualizing Molecular Motion Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6Slide 2 of 41 Kinetic Molecular Theory of Gases Particles are point masses in constant, random, straight line motion. Particles are separated by great distances. Collisions are rapid and elastic. No force between particles. Total energy remains constant.

3. Aside: rms velocities Ex. Calculate the root mean square velocity for gaseous CH 2 F 2 molecules at -15 o C. Would the rms velocity of CO 2 molecules be larger or smaller at the same temperature?

4. Non-ideal Gases: Treating gaseous molecules as point masses (zero molecular volume) is reasonable for small molecules and dilute gases. As molecular size increases the gas molecules less resemble point masses. As molar volume decreases (high P and low T) the volume occupied by molecules is no longer negligible compared to the overall gas volume.

5. Non-ideal Gases – cont’d: Intermolecular forces vary significantly from molecule to molecule – ranging from the very weak London dispersion forces (in He and CO 2 ) to the very strong hydrogen bonding interactions in molecules such as H 2 O, NH 3 and CH 3 OH. In some cases, hydrogen bonding can cause molecules to dimerize in the gas phase resulting in highly non-ideal behaviour.

6. Slide 6 of 61 An acetic acid dimer FIGURE 12-9 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 12 CH 3 -COOH

7. Non-ideality and the Physical World As mentioned (repeatedly?), intermolecular forces are responsible for molecules forming condensed phases – without which the universe would be a boring place! What intermolecular forces are important in the next couple of slides?

8. A Beautiful Picture – Thanks to Gas Non-ideality?

9. “Gotta love that hydrogen bonding!”

10. Class examples: 1. Which of the following gases would you expect to behave more ideally? Why? (a) Ne(g) or CO 2 (g) (b) CH 4 (g) or CH 2 Cl 2 (g) (c) CH 3 CH 2 F(g) or CH 3 CH 2 OH(g) Note that the molecules which behave least ideally also have the highest melting points and the highest boiling points.

11. Class examples: 2. Which of the following gas mixtures would you expect to behave more ideally? Why? (a) Ar(g) and CH 4 (g) or (b) CH 3 OH(g) and CH 3 NH 2 (g). Indicate what types of interactions are possible for both mixtures.

12. Manifestation of Nonideality Gas nonideality manifests itself as molar volumes decrease due to external P increase or a T decrease. Initially the ratio PV/nRT is smaller than the ideal value of 1 due to intermolecular forces. At very high pressures (and/or low T’s) the volume occupied by the gas molecules collectively is not negligible compared to the “empty space” in the gas and PV/nRT gets bigger than 1.

13. Intermolecular forces of attraction Figure 6-22 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6Slide 13 of 41 6-9 Nonideal (Real) Gases Compressibility factor PV/nRT = 1 for ideal gas. Deviations for real gases. PV/nRT > 1 - molecular volume is significant. PV/nRT < 1 – intermolecular forces of attraction.

14. van der Waals Equation Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6Slide 14 of 41 P + n2an2a V2V2 V – nb = nRT The van der Waals equation reproduces the observed behavior of gases with moderate accuracy. It is most accurate for gases comprising approximately spherical molecules that have small dipole moments.

15. Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 1Slide 15 of 41

16. Solutes, Solvents and Solutions: In Newfoundland and Labrador one can imagine that the following process might have been important at some time. NaCl(s) + Water →NaCl(aq) (Na + (aq) + Cl - (aq)) For this process to occur the ionic bonds in the NaCl(s) lattice must be broken, the solute particles (Na + and Cl - ions) must be separated, solvent molecules (water) must be separated and, finally, a solution is formed.

17. Formation of Ionic Solutions Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 13 Slide 17 of 46 FIGURE 13-6 An ionic crystal dissolving in water

18. Enthalpies involved in the solution process: Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 13 Slide 18 of 46 NaCl(s) → Na + (g) + Cl - (g)ΔH lattice > 0 Na + (g) + xs H 2 O(l) → Na + (aq)ΔH hydration < 0 Cl - (g) + xs H 2 O(l) → Cl - (aq)ΔH hydration < 0 ΔH soln > 0 (~ 5 kcal/mol) but ΔG solution < 0

19. Water of Hydration: When ions move through water they are commonly surrounded by several water molecules. This is true, in particular, of the smaller and higher charged metal ions. The combination of transition metal ions and water molecules gives us beautiful colours (blue for hydrated Cu 2+ ). Water molecules commonly remain when ionic salts are crystallized – eg. CuSO 4 ∙5H 2 O.

20. Solution Process – Energetics: The energetics of the solution process are discussed on the next slide. The overall solution process/change can be exothermic or endothermic. It’s easy to rationalize solutions forming where the intermolecular forces in the solute molecules (before mixing with solvent) are similar in type and magnitude to the intermolecular forces between solvent molecules.

21. Intermolecular Forces and the Solution Process Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 13 Slide 21 of 46 FIGURE 13-2 Enthalpy diagram for solution formation

22. Intermolecular Forces in Mixtures Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 13 Slide 22 of 46 FIGURE 13-3 Intermolecular forces in a solution ΔH soln = 0 Magnitude of ΔH a, ΔH b, and ΔH c depend on intermolecular forces. Ideal solution Forces are similar between all combinations of components.

23. Hydrocarbons – C X H Y : Most hydrocarbon molecules are nonpolar or very weakly polar. Dispersion forces and very weak dipole-dipole forces hold hydrocarbons together in condensed phases. It’s not surprising that different hydrocarbon molecules easily mix to form nearly “ideal” solutions (ΔH Solution approximately zero).

24. Two components of a nearly ideal solution FIGURE 13-4 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 13 Slide 24 of 46

25. “Like Dissolves Like”: It seems reasonable that highly polar molecules – such as CH 2 Cl 2 and CH 2 Br 2 – might happily mix together (form a solution). This is true. One might expect that methanol (CH 3 OH) and methyl amine (CH 3 NH 2 ) might also form a solution since both molecules can hydrogen bond. This is also true.

26. More Complex Examples: Water also “happily” forms solutions with CH 3 OH and CH 3 CH 2 OH but, more complex alcohols, such as CH 3 (CH 2 ) 6 OH are only sparingly soluble in water. Why?

27. Solubilities of Alcohols and Alkanes AlcoholAlcohol Solubility (g/L) Alkane (Hydrocarbon) Alkane Solubility (g/L) CH 3 OH MiscibleCH 3 CH 3 CH 2 OH MiscibleCH 3 CH 2 CH 3 CH 3 CH 2 CH 2 OH MiscibleCH 3 CH 2 CH 2 CH 3 0.061 CH 3 (CH 2 ) 3 OH 73CH 3 (CH 2 ) 3 CH 3 0.040 CH 3 (CH 2 ) 4 OH 22CH 3 (CH 2 ) 4 CH 3 0.0095 CH 3 (CH 2 ) 5 OH 5.9 CH 3 (CH 2 ) 6 OH

28. Boiling Points of Alcohols and Alkanes AlcoholAlcohol Boiling Point Alkane (Hydrocarbon) Alkane Boiling Point Boiling Point Difference CH 3 OH 66 o CCH 3 -88 o C 154 o C CH 3 CH 2 OH 78 o CCH 3 CH 2 CH 3 -43 o C 121 o C CH 3 CH 2 CH 2 OH 97 o CCH 3 CH 2 CH 2 CH 3 -1 o C 98 o C CH 3 (CH 2 ) 3 OH 118 o CCH 3 (CH 2 ) 3 CH 3 36 o C 82 o C CH 3 (CH 2 ) 4 OH 137 o CCH 3 (CH 2 ) 4 CH 3 69 o C 68 o C

29. Copyright  2011 Pearson Canada Inc. 13 - 29

30. Gas Law Review Ex. A sample of O 2 gas initially present in a 2.00 L container at a pressure of 2.40 atm and 22 o C is moved to a 5.00 L container at 15 O C. A sample of Ne gas initially held at a pressure of 4.00 atm and -16 O C in a 4.00 L container is added to the oxygen gas in the 5.00 L container (still at 15 O C). What is the total pressure in the 5.00 L container?