Intermolecular Forces

1. Intermolecular Forces

2. Chemistry Jokes of the Day • Why did the white bear dissolve in water?   Because it was polar. •  What do dipoles say in passing?  "Have you got a moment?“ • Why are chemists great for solving problems?   They have all the solutions.

3. States of Matter • Solid, Liquid, and Gas • Which of the 3 states do you think would have similar properties? • Solid & Liquid

4. They are similar • incompressible; constant density • These similarities are due • to the molecules being close together in solids and liquids • but far apart in gases • What holds them close together?

5. Intermolecular Forces • Intermolecular Forces: forces exist between molecules. • Intramolecular Forces: forces formed within the molecule. Bonding between atoms. Why do we need to know about intermolecular forces? - Because these intermolecular forces are responsible for the existence of the liquid state and explained some unusual properties of liquids: i.e. Water - high boiling point

6. Types of Intermolecular Forces • There are 3 types of Intermolecular Forces • Dipole-Dipole (DDF) • London Dispersion (LDF) • Hydrogen Bonding (H-bond) • These intermolecular forces (DDF & LDF) are VERY WEAK, 1% as strong as a covalent bond • Then what are the strong forces of attraction? • Covalent Bonding & Ionic Bonding

7. In terms of the Strength of Attractions Strong: Covalent Bonding & Ionic Bonding Medium Strength: Hydrogen Bond Weak: Dipole-Dipole & London Dispersion

8. Dipole-Dipole Forces • Exist between Polar Molecules • The partial negative ends is attracted to the partial positive ends of the molecules. • Weak Forces • Example: • H-Cl

10. What about non-polar molecules? • Does that mean no forces of attraction exist? • Then, how does solid or liquid state exist?

11. London Dispersion Forces • Exist in ALL molecules (regardless of polarity) • In any molecule, even non-polar, there will be an instantaneous dipole moment due to random electrons movement. • This instantaneous dipole moment induces near by molecules to have a partial negative and partial positive. • These dipole-induced molecules orient themselves to attract one another. (very weak attraction) • Attractive Forces are said to be induced. • Also known as: induced-dipole-dipole interactions

12. London Dispersion Forces For Example: N2, H2, CH4

13. London Dispersion Forces • Very Weak Forces • Strength of LDF increase as the # of electrons in a molecule increases. • Similarly, strength of LDF increase with the size of the molecules. • The larger the molecule the more polarizable it is. (more easily distorted to give instantaneous dipoles because electrons are farther from the nuclei) • Also, the longer the chain, the easier it is to polarize the molecule. • Therefore, higher Melting Point & Boiling Point.

14. London Dispersion Forces • Try to predict the boiling point for SiH4 , GeH4, and SnH4 • Higher or Lower?

15. SnH4 GeH4 SiH4 CH4 100 Boiling Points 0ºC -100 200

16. What about isoelectric molecules of same molecular weight? • Like pentane, isopentane, and neopentane? • All have the same molecular formula, C5H12 • Which one has a higher Boiling Point? • Pentane • Longer Chain

17. Empirical Data Revealed to Researchers More Than What They Were Looking For! • Let’s take a look at some of their findings • Results are consistent with Intermolecular Forces suggested: Dipole-Dipole Forces, and London Dispersion Forces. • But something else interesting was revealed to them • They found that H2O, HF, and NH3 repeatedly yield higher boiling point than predicted. • How do we explain such high boiling points for these 3 molecules? • We are onto something here!!! Let’s take a look

18. H2O HF H2Te H2Se NH3 SbH3 H2S HI AsH3 HCl HBr PH3 100 Boiling Points 0ºC -100 200

19. Hydrogen Bonding • Very strong dipole-dipole forces exist when H is attached to F, O, or N • Strictly between a molecule with Hydrogen and another molecule with F, O, or N in it. • Use dotted line to indicate hydrogen bonding. • These three because- • They have high electronegativity. • They are small enough to get close. • Affect certain physical properties like boiling point. • Explained the empirical data obtained.

20. Water d+ d- d+

22. Properties of Liquids • Aside from Melting & Boiling Points, • Surface Tension • Viscosity • Capillary Action • Beading All of these properties are greatly depended on the strength of the intermolecular forces: DDF, LDF, H-bond.

23. Surface Tension • Intermolecular Forces are responsible for the ability (tendency) of a liquid to reduce its surface area. • This is why raindrops are nearly spherical (sphere has the smallest surface area for a given volume of any geometrical shape)

24. Surface Tension • Molecules at the top are only pulled inside. • Molecules in the middle are attracted in all directions. • Minimizes surface area.

25. Surface Tension • Surface Tension: the energy required to increase the surface area of a liquid by a unit amount. • Because of surface tension, a liquid behaves as though it had as “skin”.

26. Mini-Investigation • The effect that soap has on the surface tension of water. • What did you observe? • Explain, in terms of surface tension & surface area, what you have seen?

27. Viscosity • Viscosity: the resistance to flow. • Greater Intermolecular Forces = More Viscous • Larger Molecule = Greater LDF = More Viscous • Cyclohexane vs. Hexane : which one is more Viscous ?

28. Capillary Action • Liquid slowly rise in a narrow tube. • Intermolecular Forces responsible: H-Bond, DDF, and LDF • Cohesion – between water molecules • Adhesion – between water + something else Like glass, or cellulose of cell wall. • Glass is polar, therefore, attracts water molecules up the column.

29. Capillary Action

30. Beading • If a polar substance is placed on a non-polar surface. • There are cohesive, • But no adhesive forces. • Same with non-polar substances on a polar surface

31. Homework • P.260 # 1-4 • P. 264 # 9 • P. 266 # 1-5 • P. 282 # 18, 28