Implication of Intermolecular Forces

1. Implication of Intermolecular Forces

2. Here’s some… Physical properties that show “intermolecular forces”: • Boiling point • Melting point • Surface tension • Viscosity • Capillary action • Evaporation

3. Surface Tension You see it all the time – beading of water on a surface. Why are liquids liquid? Because the molecules like each other a little! (If they liked each other a LOT, they’d be solids. If they BARELY liked each other, they’d be gases.)

4. SURFACE tension What’s the difference between the surface of a liquid and the rest of the liquid? It’s lonely at the top!

5. It’s a lazy world… The world wants to be in the lowest energy state possible. Surface molecules are higher in energy than “bulk” molecules because their “friends” stabilize them. Liquids attempt to minimize the # of surface molecules. The lowest surface/volume solid object is a sphere!

6. Water bugs LOVE surface tension It’s how they walk on water! It’s also how a paperclip floats. Why do most things float rather than sink? DENSITY! But a paper clip(or water bug) have a higher density than water yet they float. Why? Surface Tension!

7. Surface Tension is Energy The “surface tension” of a liquid is the energy required to increase the surface area, so it has UNITS! UNITS! UNITS! of J/m2. In other words, water has a surface tension of 0.0728 J/m2 which means it takes 0.0728 J to increase the surface area by 1 square meter. If you put a paperclip (or a bug) onto the surface of the water, sinking increases the surface area!

8. Too much surface! The liquid resists the penetration because of the increase in surface. If this resistance is stronger than the down force – it floats!

9. Viscosity Another implication of intermolecular forces can be seen in the “viscosity” of a liquid. Viscosity is “pourability” of a liquid, termed “resistance to flow”. For example think of maple syrup vs. water. Or motor oil vs. water.

10. Viscosity is about the attractiveness of your neighbors If you are surrounded by VERY attractive neighbors, you don’t want to leave!

11. Capillary Action The source of your meniscus!! If you love the straw more than yourself!

12. Phase Changes When I want to talk to you in common terms, I very sloppily use the term “strong” or “weak”, “like a LOT”, “like a little”, etc. But what is “strong” attraction or “weak” attraction? The answer is: it DEPENDS!

13. It’s all relative. I’m at a party. I see a tall Brazilian lingerie model. Do I have a STRONG attraction for her? • Do I want to hook up for the night? • Take her on vacation for a week? • Marry her? Molecules feel the same way!

14. I made this drawing – we’re going to KEEP using it!  The force of attraction isn’t the only factor. The molecules are MOVING – or at least trying to!

15. Turn up the heat! Raising Temp, increases KE. Increasing KE, makes the molecules move faster. Their attraction to each other stays the same. At some point, flying away is preferable than staying put (including many Brazilian lingerie models!)

16. How do you know the phase has changed? We tend to think that we can “see” the difference. But there must be some “doorway” between “solid” and “liquid” – the exact location of that doorway is determined by the kinetic energy of the molecules compared to the force (energy) of attraction among the molecules.

17. 3 kinds of phase changes • Melting – solid becomes liquid “Fusion” is the reverse process. • Vaporization – liquid becomes gas “condensation” is the reverse process. • Sublimation – solid becomes gas (no middle man!) “deposition” is the reverse process.

18. How do you go from a liquid to a solid? Ice @ -10ºC How do you make water? Get to the melting point (0 C). How? Q=mcT Once you are at the melting point, does it just melt? Of course not – or I wouldn’t ask the question!

19. How do we start? With a balanced equation, of course! H2O (s) H2O (l) Barely a chemical reaction (no bonds get broken) but still a change…

20. At the melting point The molecules have enough energy to match the force of attraction between them. They are ready to go! But they still need to be pulled apart.

21. Organization The ice molecules are in a rigid lattice. Water molecules are loosely flowing around each other. It takes energy to set you free! Imagine if I handcuff you all together and put you in my dungeon. Even if I unlock all the handcuffs, you still need to get up and walk away!

22. Our old pal, H! H2O (s) H2O (l) It’s a reaction, there’s an enthalpy change!!! Hfus = (+) This is the amount of energy it takes to separate the molecules once they have enough energy.

23. Works for vaporization also H2O (l) H2O (g) It’s a reaction, there’s an enthalpy change!!! Hvap = (+)

24. Melting curve! This is why your entire driveway doesn’t melt at 0! steam Temp 100 C water Phase change Phase change 0 C ice Heat added (J)

25. Clicker question I have 10 g of ice at -10 C. If ice has a specific heat of 2.09 J/gC and a Hfus = 6.02 kJ/mol, how much heat must be added to completely melt the ice? A. 209 J B. 3552 J C. 6020 J D. 60,229 J E. I love to hate you.

26. Clicker question I have 10 g of ice at -10 C. If ice has a specific heat of 2.09 J/gC and a Hfus = 6.02 kJ/mol, how much heat must be added ? 1st, I heat up the ice: Q = mcT = 10 g (2.09 J/gC)(10C) = 209 J Then I melt it: Q = n Hfus 10 g H2O*(1 mol/18.01 g) =0.5552 mol Q = 0.5552 mol * 6020 J/mol = 3343 J Total heat required: 209 J + 3343 J = 3552 J

27. Vaporization works the same Once you are at the boiling point, you need to add heat to make the phase change. When you are going the opposite direction (freezing or condensing) the H is just the opposite sign: Hfus = - Hfreeze Hvap = - Hcondense

28. Vaporization is more interesting! There’s a gas! Gases have pressure! Gases are more interesting!

29. What is the boiling point? We could start with the melting point: what is it? It’s the temperature at which the solid will spontaneously turn to liquid. It’s the temperature at which the solid and liquid are at equilibrium.

30. What is the boiling point? Could we say the same thing? It’s the temperature at which the liquid will spontaneously turn to gas. It’s the temperature at which the gas and liquid are at equilibrium. Well…sort of…

31. Vapors…what about ‘em?!?! Give her a squeeze: Liquid  gas Changing the pressure should move the boiling point!

32. Why does water evaporate at room temperature? Liquid  gas This equilibrium actually exists at multiple temperatures. And some molecules have enough energy to escape! # mol Kinetic Energy

33. Vapor Pressure Liquid  gas Since this equilibrium exists at multiple temperatures, there is always a little gas. If there is a little gas, there is a little pressure due to the gas. Vapor pressure is the pressure exerted by the gas molecules in equilibrium with the liquid!

34. A whole new definition of boiling! Boiling point can now be defined in terms of the vapor pressure: Boiling is the point at which the vapor pressure EQUALS atmospheric pressure

35. Vapor Pressure is predictable Clausius-Clapeyron equation Pvap = e-Hvap/RT Look familiar! k = Ae-Ea/RT - Arrhenius equation – it’s general for all activated processes!

36. What’s old C-C good for? Hvap(water) = 40.7 kJmol • (water) = who cares!!! ln P2= - Hvap(1 – 1) P1 R T2 T1 What does this tell us!!! (Boiling point of water at any pressure!)

37. Clicker Question What’s the boiling point of water of water at the top of Mt. Everest where the atmospheric pressure is only 0.32 atm? A. 70 C B. 130 C C. 110 C D. 90 C E. Loving the weather.

38. What’s old C-C good for? Hvap(water) = 40.7 kJmol • (water) = who cares!!! P2 = 0.32 atm T2 = ? R = 8.314 J/mol K ln P2= - Hvap(1 – 1) P1 R T2 T1 Do I know anything else! (Water boils at 100 C at 1 atm pressure)

39. What’s old C-C good for? Hvap(water) = 40.7 kJmol P2 = 0.32 atm T2 = ? R = 8.314 J/mol K ln 0.32 = - 40,700 J/mol(1 – 1 ) 1 8.314 J/mol K T2 373 K -1.139 = -4895 (1/T2 – 2.681x10-3) 2.3269x10-4 = 1/T2 – 2.681x10-3 2.914x10-3 = 1/T2 343 K = T2 (or 70 C)