Hour Exam 3 on Chap 6, 7, and 8 is this Friday 11/12. A practice exam is posted on the course website.

Transcription

1 11/8/ Chem 105 Monday 11 Nov 2010 Bond length qualitative Polarity Polarity and geometry Bond energy our Exam 3 on Chap 6, 7, and 8 is this riday 11/12. A practice exam is posted on the course website. Review for the exam will happen Wed 11/10.

2 11/8/ ow can you distinguish single, double, & triple bonds? (1) Multiple bonds are shorter than single bonds. More electrons between the nuclei makes for a greater attractive force on the nuclei (2) Single bonds are longer, floppier, and vibrate at a lower frequency.

3 11/8/ Trigonal planar O C Trigonal planar C C C N linear Do it in C

4 11/8/ Bond lengths in Angstroms (1 Å = 100 pm)

5 11/8/ or B, C, N, O,... Å X Y ~ X=Y ~ X Y ~ (Bonds are longer for the larger atoms in 3 rd, 4 th, and 5 th periods.)

6 11/8/ Polarity of Molecules Define electronegativity = the ability of an atom to pull bonding electrons toward itself. Electrons in covalent bonds between identical atoms are equally shared. Electrons in covalent bonds between non-identical atoms are not equally shared. O O δ- O C δ+ The more electronegative atom pulls bond electrons toward itself it becomes partially negative

7 11/8/ O N C Cl S Br

8 11/8/ Increases (due to increasing effective nuclear charge) Increases

9 11/8/ Compare 2 and Pauling Electronegativity Partial Charges Dipole moment 2 4.0, , , Debyes The arrow represents the electric field (dipole moment) caused by charge separation. Shows direction and magnitude of field.

10 11/8/ Electric field = charge x sep n distance Molecular dipoles are measured in Debyes ( D ) (after Peter Debye who first measured them). 1 Debye = 3.33 x coulomb - meter Pauling Electronegativity Partial Charges Dipole moment 2 4.0, , , D

11 11/8/ Which has the largest dipole moment? Cl 2 53% 3. Br 2 4. Cl 5. Br 6. Bi 14% 9% 11% 7% 7% 2 Cl2 Br2 Cl Br Bi

12 11/8/ Which has the largest dipole moment? Cl 2 3. Br 2 4. Cl 5. Br 6. Bi as the greatest difference in electronegativities.

13 11/8/ A polar molecule --> has a large dipole moment A non-polar molecule --> has a zero or small dipole moment (Ions cannot have a dipole - they have just one overall charge.)

14 11/8/ In molecules with 3 or more atoms, the dipole moment depends on 1. the size of dipole associated with EAC BOND and 2. geometry of bonds Polar bonds pointed in opposite directions can cancel each other out. Example: CO 2 is non-polar O C O O C O bond angle = 180 CO 2 is a liquid at high pressures, and is used for caffeine extraction from coffee beans and Dry Cleaning clothes.

15 11/8/ Molecular dipole = vector sum of bond dipoles O molecular dipole N molecular dipole Bond dipoles

16 11/8/ You need not do exact geometric vector additions, however, you should be able to: 1. Identify clear cases where μ = 0.0 due to high symmetry 2. Guess which molecule (given 2 or 3) has the greater dipole moment. S This is a correct Lewis formula for hydrogen sulfide. S 2 S has the tetrahedral electron pair geometry, and the bent molecular geometry. conclusion: μ > 0 cancel (But less than water because S is less electronegative than O).

17 11/8/ Which is more polar, O 2 or O 2? (The Pauling electronegativities of, O, and are 2.3, 3.5, and 4.0, respectively.) 1. O 2 2. O 2 3. Both = 0 4. Equal & > 0 62% 30% 4% 4% O2 O2 Both = 0 Equal & > 0

18 11/8/ O O Large bond dipoles due to large electronegativity difference between and O. = 1.74 D Smaller bond dipoles due to smaller electronegativity difference between and O. = 0.38 D

19 11/8/ Another example: Which is more polar: 1,1,2-trifluorocyclobutane or 1,1,3-? 2 C- bonds on same side tend to ADD dipoles. C C C C C C C C C-C and C- bond dipoles are small and so make a small contribution to the overall molecular dipole. 2 C- bonds on opposite side tend to SUBTRACT dipoles. Therefore, we would predict that the 1,1,2- molecule (left) is more polar.

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21 11/8/ There are huge practical consequences of polarity Polar liquids, especially water which is very polar, dissolve ionic compounds ydrogen bonding in water. Affects volatility of liquids polar molecules are sticky

22 11/8/ OWL examples: polarity X Identical atoms with no lone pairs are nonpolar (dipole moment = 0.00) Different atoms with no lone pairs are polar (dipole moment > 0) One lone pair are polar (dipole moment > 0) Si 4 Si 3 I SCl 4 Si Si I Cl Cl S Cl Cl

23 11/8/ Must have identical atoms too! Must have identical atoms too! Trigonal bipyramid with 3 lone pairs is nonpolar (dipole moment = 0.00) Octahedral with 2 or 4 lone pairs is nonpolar (dipole moment = 0.00) Xe 2 Xe Xe 4 Xe

24 11/8/ You should be able to... Draw Lewis structures for given formula (octet, sub-octet, and expanded-octet atoms) Predict and name the electron pair geometry and the molecular (or ionic) geometry. Assign nominal bond angle values (180, 120, 109, 90 ). Use a table of electronegativities to predict bond polarity. Use molecular geometry and bond polarity to predict relative polarity of molecules. Choose among several unequal resonance forms based on electronegativity considerations.

25 11/8/ Bond energies Chemical reactions (of molecules) involve breaking bonds and making new bonds. This is the source of of reaction. or a bond X - Y in molecule, define bond energy D X-Y as for the bond-breaking reaction: --X-Y --X + Y This is always an endothermic reaction, therefore D is always reported as a positive number (kj/mol).

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27 11/8/ Example: combustion of methane is exothermic: C O 2 CO O Break 4 C- Break 2 O=O Make 2 C=O Make 4 O-

28 11/8/ Break 4 C- Break 2 O=O Make 2 C=O Make 4 O- 498 kj 745 kj 498 kj 745 kj kj 413 kj 413 kj 413 kj 463 kj 463 kj kj 413 kj 463 kj 463 kj Estimated heat of reaction = ΣD bonds broken ΣD bonds made = 2648kJ kJ = -694 kj (experimental = kj/mol)

29 11/8/ int for, ΣD problems: Draw out the Lewis formulas of reactants and products, or sketch them, before you start punching the calculator.

30 11/8/ Δ=ΣD bondbrok - ΣD bondsmade Δ=2D O- + 2D O -D O=O -2D D D D O O O Δ 2D O DO O 2D 2 318kJ 2(463)kJ 1(498)kJ 2(565)kJ 2mol 192kJ/mol

31 11/8/ The End