Chem 105 Friday Bond length qualitative Infrared spectroscopy Bond and molecular polarity Polarity and geometry Bond energy

Transcription

1 11/7/ Chem 105 Friday Bond length qualitative Infrared spectroscopy Bond and molecular polarity Polarity and geometry Bond energy Hour Exam 3 on Chap 6, 7, and 8 is this Friday, 11/11/11.

2 11/7/ How can you distinguish single, double, & triple bonds? (1) Multiple bonds are shorter than single bonds. There are more electrons between the nuclei, which makes for a greater attractive force on the nuclei

3 11/7/ What are the local molecular geometries? Trigonal planar Trigonal planar O H C C C C N H H linear Do it in WebMO

4 11/7/ Bond lengths in Angstroms (1 Å = 100 pm)

5 11/7/ How can you distinguish single, double, & triple bonds? (2) Multiple bonds are stiffer than single bonds. They vibrate at a higher frequency in the infrared spectrum Visit the WebMO website of the UAF Chemistry Department ( Logon as Guest and open job #4212 C 4 H 3 ON by clicking the job name. Visualize distances and bond angles using the Select tool. Shift-click to select a distance or angle. Animate vibrations in the lower table by clicking the film icon.

6 11/7/2011 Computed infrared spectrum of 4-oxo-2-butenenitrile (B3LYP/6-31G(d,p) 6 Increasing frequency

7 11/7/ Bond and molecular polarity

8 11/7/ Polarity of Molecules Define electronegativity = the ability of an atom to pull bond electrons toward itself. Electrons in covalent bonds between identical atoms are equally shared. O O Electrons in covalent bonds between nonidentical atoms are not equally shared. The more electronegative atom pulls bond electrons toward itself it becomes partially negative O δ- C δ+

9 11/7/ F H O N C Cl S Br

10 11/7/ Increases (due to increasing effective nuclear charge) Increases

11 11/7/ Compare HF (left) and F 2 (right) δ+ δ H F H F Computed electrostatic potential mapped isosurfaces The arrow represents the electric field caused by charge separation. = dipole moment Shows direction of field (from PLUS to MINUS).

12 11/7/ Electric field = charge x separation 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 F 2 4.0, HF 2.5, , D

13 11/7/ Which has the largest dipole moment? 71% 1. F 2 2. Cl 2 3. Br 2 4. FCl F2 5% Cl2 2% Br2 1% FCl FBr 21% 5. FBr

14 11/7/ Which has the largest dipole moment? 1. F 2 2. Cl 2 3. Br 2 4. FCl 5. FBr Has the greatest difference in electronegativities.

15 11/7/ A polar molecule --> has a dipole moment > 0 A non-polar molecule --> has a dipole moment = 0 (Ions cannot have a dipole - they have just one overall charge.)

16 11/7/ In molecules with 3 or more atoms, the dipole moment depends on 1. the size of each bond dipole and 2. geometry of bonds Polar bonds pointing in opposite directions tend to cancel each other out. Example: CO 2 is non-polar δ δ+ δ O C O O C O bond angle = 180

17 11/7/ Molecular dipole = vector sum of bond dipoles H O H molecular dipole H H N H molecular dipole Bond dipoles

18 11/7/ You do not need to do exact geometric vector additions, however, you should be able to: 1. Identify situations where µ = 0.00 due to high symmetry 2. Guess which molecule (given 2 or 3) has the larger dipole moment.

19 11/7/ Which is more polar, HOH or FOF? Electronegativity values: H (2.2), O (3.4), F (4.0) Start with Lewis formulas. Both have the tetrahedral electron pair geometry around O, and the bent molecular geometry. H O H F O F Large bond dipoles due to large electronegativity difference between H and O. µ = 1.74 D Smaller bond dipoles due to smaller electronegativity difference between F and O. µ = 0.38 D

20 11/7/ There are huge practical consequences of polarity Polar liquids, especially water, which is very polar, dissolve ionic compounds Hydrogen bonding in water. Affects volatility of liquids polar molecules are sticky.

21 11/7/ 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) Molecules with one lone pair are polar (dipole moment > 0) SiF 4 SiF 3 I SCl 4 F F Si F F Si I F F F Cl Cl S Cl Cl

22 11/7/ Must have identical atoms! Must have identical atoms! 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) XeF 2 F Xe F XeF 4 F F Xe F F If the atoms are different, then they are POLAR molecules (dipole moment > 0).

23 11/7/ 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.

24 11/7/ Bond energies

25 11/7/ Bond energies Chemical reactions (of molecules) involve breaking bonds and making new bonds. This is the source of H of reaction. For a bond X - Y in molecule, we define the bond energy D X-Y as H 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 11/7/ Example: combustion of methane is exothermic: CH O 2 CO H 2 O Break 4 C-H Break 2 O=O Make 2 C=O Make 4 O-H

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29 11/7/ Break 4 C-H Break 2 O=O Make 2 C=O Make 4 O-H 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 H = ΣD bonds broken ΣD bonds made = 2648kJ kJ = -694 kj (experimental H = kj/mol)

30 11/7/ Hint for solving H, ΣD problems: Draw out the Lewis formulas of reactants and products, or sketch them, before you start punching the calculator keys.

31 11/7/ H=ΣD bondbrok -ΣD bondsmade H=2D O-F + 2D OH - D O=O - 2D HF D D D OF OF OF H 2D + OH DO= + O 2DHF = 2 318kJ 2(463)kJ+ 1(498)kJ+ 2(565)kJ = 2mol = 192kJ/mol

32 11/7/ The End