Chapter 24

1. Chapter 24 Animal, Vegetable or Mineral, It’s All Chemistry

2. Summary of bonding. Can you answer these?

3. Intermolecular Forces • “Between” different molecules • Caused by permanent or temporary (induced) dipoles on molecules • Wide range of strengths explains wide range of boiling, melting points of covalent materials Dispersion weakest -- but ever present found between all kinds of molecules and even between atoms of noble gases Hydrogen Bonds strongest but limited to molecules that contain H atoms attached to N, O, F. Dipole- Dipole Interactions occur between polar molecules

4. Relative Strengths of Intermolecular Interactions http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtml

5. Dispersion Forces Electron cloud can fluctuate Not perfectly symmetrical about the nucleus. Temporary dipole induced. Induces a new dipole in atoms or molecules that come close. Rn Xe Kr Ar Ne EVIDENCE FOR SUCH FORCES – NOBLE GASES CAN BE LIQUIFIED! Bigger electron cloud, the stronger the forces. Higher boiling temperature of gases.

6. Nitrogen • Properties • Chemically unreactive • Boiling point 77 K (-196 °C; -321 °F) • Colorless • Why? • Triple bond • What are strongest intermolecular forces between N2 molecules? • No low-lying molecular orbitals NN

7. Answer Now N2 is a non-polar molecule. Which interaction determines its boiling temperature? • Hydrogen bonding. • Dipole-Dipole Interactions • Dispersion forces

8. Dipole-Dipole Interactions Formed in molecules with bonds between atoms of different electronegativities. examples: C – Cl, C – O, C – F dispersion forces only Effect is to increase “stickiness” of molecules for each other. Displayed in differences of boiling temperatures. Also in solubilities dispersion + D-D forces

9. - - - + + + + + + Hydrogen Bonding • Limited to H bound to N, O, or F • Among strongest intermolecular interactions • Happens because H is small and has only 1 electron, and atom it is bound to is quite electronegative. H is essentially a “bare” proton • The most important H-bond: water

10. Water • Properties • Fairly reactive, “universal” solvent • Boils at 373 K (100 °C; 212 °F) • More dense as liquid than solid • Can react with itself: 2H2O = H3O+ + OH- • Water conducts electricity (but not well) • This reaction goes only to a very small extent (1 molecule in 10 million!) • Why? • Highly polar, so dissolves ionic materials • Extensive hydrogen bonding interactions dissolve nonionic materials like sugars, lead to high boiling point, unusual crystal structure

11. Glucose (a Kind of Sugar) • Properties • Crystalline, molecular solid • Melts, then decomposes on heating • Sticky • Why? • Many hydrogen bonds hold molecules together in crystal • Many H-bonds add up, so melting point is relatively high (compared to water, for example) • H-bonds make molecules “sticky”

12. Animals & Vegetables Fats (obtained from animals) & Oils (obtained from vegetables) Covalent Molecules Whose Melting Points Are Related To Behavior in Our Bodies Minerals Silicate Mineral Family Primarily Ionic Material Different Arrangements of SiO44- units lead to stringy, sheet-like or chunky minerals Atomic-Level Structure of Complex Materials Determines Properties

13. Fats and Oils differ in Their Physical State at Room Temperature

14. Heart Healthy Sequence (approximately) maybe the best for you Sequence of Melting Temperatures WHY THIS SEQUENCE? UNDERLYING MOLECULAR STRUCTURES

15. Did you read chapter 24 before coming to class? • Yes • No

16. Acetic Acid just a simple example; not a fatty acid. Components of Fats and Oils + Glycerol C3H8O3 Fatty Acids CO2H-(Hydrocarbon tail) Fats & Oils are Mono-, Di- and Tri-glycerides http://ps100.byu.edu/molecule_sg/fatty_acids.html

17. Fatty Acids – major component of fats and oils ▬Hydrocarbon tail CO2H ▬ Describe the structures: What molecular groupings do they have in common?

18. Fatty Acids – major component of fats and oils Describe the structures: n=10 How does one differ from another? n=14 n=16 n=18 Length of the tail ▬(CH2)nCH3

19. Room temperature What difference does a tail make? Fats containing these fatty acids are solids at room temperature

20. Its members have the CO2H ▬ group How do we get oils?(lower melting temperatures)A new family of fatty acids

21. How do we get oils?(lower melting temperatures)A new family of fatty acids They have the hydrocarbon tail. So What’s Different? THE TAILS HAVE KINKS!

22. Straight chain vs Kinky chain

23. Fats containing these fatty acids are solids at room temperature 1 kink 2 kinks 3 kinks Room temperature 4 kinks What difference does a kink make? Oils containing these fatty acids are liquid at room temperature If you were a fish, swimming in the cold North Atlantic, what would you want flowing through your veins?

24. Saturated fatty acids No kinks Unsaturated fatty acids One kink (mono) More than 1 kink (poly) What causes the kinks?Differences in the Tails each C has 2 H atoms C▬ C ▬ C ▬ C (carbon-carbon single bonds) Some C have only 1 H atom C▬ C = C ▬ C (carbon-carbon double bonds) Kinks occur at double bonds. (True of Unsaturated Fatty Acids that are found in Nature.)

25. Why do kinks make a difference? Molecules without kinks can snuggle closer together. RESULT: more & stronger dispersion forces between tails stronger hydrogen bonding between CO2H groups on different molecules Strong forces mean high melting temperatures

26. Trans-Fatty Acids – doing away with the kinks Saturated Fatty Acid No Kinks Unsaturated Fatty Acid Kink at Double Bond Unsaturated Fatty Acid But no Kink at Double Bond

27. Cis Double Bond Gives Kink Trans Double Bond Has No Kink Cis vs Trans Double Bonds – Where are the H atoms? Not Naturally Occurring in Foods occurs during processing of saturated fats

28. Where do trans fats come from?

30. Good Fats kinky unsaturated fats low melting points don’t clog your arteries good for your brain Bad Fats unkinky fats saturated & trans-fats high melting points Good Fats vs Bad Fats lard shortening prime rib olive oil cold water fish Wendy’s announced in August that it had switched to a new cooking oil that contains no trans fatty acids. Crisco now sells a shortening that contains zero trans fats. Frito-Lay removed trans fats from its Doritos and Cheetos. Kraft’s took trans fats out of Oreos. Doughnuts in danger? NYC may ban trans fats Health officials unveil proposal to bar substance in restaurants MSNBC Sept 27, 2006

31. Silicate Minerals Atomic Arrangements Give Rise to Structural Features Irregular Chunks Strings or Fibers Sheets or Plates

32. Molecular Ions • Covalent bonding within the ion • Stronger covalent bonds if number of electrons doesn’t match total nuclear charge  resulting molecule is charged • These charged molecules assemble together in crystal lattice like ionic materials • Examples: nitrate, silicate, sulfate, chlorate

33. The basis of Silicate Minerals: The Silicate ion, SiO44- Tetrahedral arrangement of oxygen atoms around a central silicon atom

34. Two tetrahedra share an oxygen atom at the connection as isolated units Two tetrahedra share an oxygen atom at the connection as chains or double chains or as flat sheets of connected chains SiO44- - a versatile connector It is found in minerals

35. Wait, there’s still more…. Oxygen Silicon or as networks connected equally in all directions

36. Weaker interactions between units Strongly-bound unit: “Submarine sandwich” Formation of Fibers Positively Charged Ions (Ca2+, Mg2+) Double Chain You can pull apart fibers with your fingers

37. silicate sheets with negative charges Small Al3+ Ions Strongly-bound unit You can pull apart sheets with your fingers Positively Charged Ions, K+ and/or Na+ Formation of Sheets

38. Formation of Irregular Chunks You need a hammer to break crystal into smaller chunks Tetrahedron-tetrahedron connections extend in 3 directions. Interactions are equally strong in all directions.