Chem 105 Friday Review Lewis formulas and geometry Bond length qualitative Polarity Polarity and geometry 11/20/2009 1

Transcription

1 hem 105 riday Review Lewis formulas and geometry Bond length qualitative Polarity Polarity and geometry 11/20/2009 1

2 Shown below is a partial Lewis formula for hydrogen carbonate ion ( 3- ). What are the best estimates of the actual charges on the indicated atoms? int: draw the correct Lewis structure including resonance. 41% 1. -1, , , , , , 0 4% 13% 0, -1-1, -1 21% 21% 0, 0-0.5, -0.5?? 11/20/2009 2

3 (N.B. Both of these together is the correct Lewis formula for 3-. Do provide such a formula on exam questions requesting a written Lewis formula for a resonance stabilized molecule or ion. ne of these by itself, is nearly correct, but is incomplete.) ~ -1/2 ~ -1/2 (N.B. This is an incorrect Lewis structure. Do not offer such a formula in 11/20/2009 answer to exam questions that request a written Lewis formula. It is shown merely to give a qualitative understanding of resonance bonding.) 3

4 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 attraction for the + nuclei (2) Single bonds are longer, floppier, and vibrate at a lower frequency. 11/20/2009 4

5 Trigonal planar N Trigonal planar linear Bond lengths in Angstroms (1 Å = 100 pm) 11/20/2009 5

6 or B,, N,,... pm Å X Y ~ ~ X=Y ~ ~ X Y ~ ~ (Bonds are longer for the larger atoms in 3 rd, 4 th, and 5 th periods.) 11/20/2009 6

7 Polarity of Molecules Electrons in covalent bonds between identical atoms are equally shared. Electrons in covalent bonds between nonidentical atoms are not equally shared. The more electronegative atom pulls bond electrons becomes partially negative δ- δ+ Define electronegativity = the ability of an atom to pull the bond electrons toward itself. 11/20/2009 7

8 N l S Br 11/20/2009 8

9 Increases (due to increasing effective nuclear charge) Increases 11/20/2009 9

10 ompare and 2 Pauling Electronegativity Partial harges Dipole moment 2 4.0, , , Debyes δ+ δ The arrow represents the electric field (dipole moment) caused by charge separation. Shows direction and magnitude of field. 11/20/

11 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 harges Dipole moment 2 4.0, , , D 11/20/

12 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.) 11/20/

13 In molecules with 3 or more atoms, the dipole moment depends on 1. the size of dipole associated with EA BND and 2. geometry of bonds Symmetric polar bonds cancel each other. Example: 2 is non-polar δ δ+ δ bond angle = is a liquid at high pressures, and is used for caffeine extraction from coffee beans and Dry leaning clothes. 11/20/

14 Molecular dipole = vector sum of bond dipoles molecular dipole N molecular dipole Bond dipoles 11/20/

15 No requirement for exact vector additions by students, however, students should be able to: 1. Identify clear cases where µ = 0.0 due to high symmetry 2. Guess which molecule 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 µ is less than water because S is less electronegative than ). 11/20/

16 Which is more polar, 2 or 2? (The Pauling electronegativities of,, and are 2.3, 3.5, and 4.0, respectively.) % Both = 0 4. Equal & > 0 16% 0% 0% 2 2 Both = 0 Equal & > 0 11/20/

17 Large bond dipoles due to large electronegativity difference between and. µ = 1.74 D Smaller bond dipoles due to smaller electronegativity difference between and. µ = 0.38 D 11/20/

18 Another example: Which is more polar: 1,1,2-trifluorocyclobutane or 1,1,3-? The two - bonds on same side tend to REINRE dipoles. - and - bond dipoles are small and so make a small contribution to the overall molecular dipole. The two - bonds on opposite side tend to ANEL dipoles. Therefore, we would predict that the 1,1,2- molecule (left) is more polar. 11/20/

19 11/20/

20 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 11/20/

21 WL 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) ne lone pair are polar (dipole moment > 0) Si 4 Si 3 I Sl 4 Si Si I 11/20/ l l l S l

22 Must have identical atoms too! Trig bipyramid with 3 lone pairs is nonpolar (dipole moment = 0.00) Must have identical atoms too! ctahedral with 2 or 4 lone pairs is nonpolar (dipole moment = 0.00) Xe 2 Xe Xe 4 Xe 11/20/

23 You should be able to... Draw Lewis structures for given formula (octet, sub-octet, and expanded-octet atoms) hoose among several unequal resonance forms based on electronegativity considerations. Predict and name the electron pair geometry and the molecular (or ionic) geometry. Assign nominal bond angle values (180, 120, 109, 90 ). Use electronegativity data to predict bond polarity. Use molecular geometry and bond polarity to predict relative polarity of molecules. 11/20/