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Oh boy, I’m so happy !!!! I’m back for more chemistry !!!!

Published byLawrence McCoy Modified over 8 years ago

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Presentation on theme: "Oh boy, I’m so happy !!!! I’m back for more chemistry !!!!"— Presentation transcript:

1 Oh boy, I’m so happy !!!! I’m back for more chemistry !!!!

2 U-Do-it Examples where we minimize formal charge & break octet rule- What is the octet rule prediction for SO 2 ? a) SO 2 SO 3 SO 4 2- (2+2+1+1)/4= 1.5 What will be the S-O bond order in SO 4 2- ?

3 Which is the best structure for PO 4 2- that satisfies the minimize formal charge rule ?

4 c) HClO 4 ClO 4 1- What will be the Cl-O bond order in ClO 4 - ? U-Do-it Examples where we minimize formal charge or simply break octet rule (continued) (2+2+2+1)/4 =7/4= 1.75

5 Excited states and formal charges The COCl 2 example No formal charges Can you think of another way we can satisfy octet rule for COCl 2 ?? ‘excited state’ of COCl 2

6 IN-CLASS BOARD OCTET RULE PRACTICE WITH:: Diatomics O 2  N 2  CO  tri,tetra-atomics CO 2  H 2 O  OF 2  COCl 2  oxyanions NO 3 -  CO 3 2-  SO 4 2- Non-Lewis formal charge rule H 2 SO 4  SO 2  SO 3  SO 4 2-  Yes ! Non-Lewis ”got no choice” and radicals PF 5 SF 6 NO NO 2 Any other problems/issues ? Excited states are predicted ! (a bonus )

7 PF 5 SF 6 Examples where we minimize formal charge or simply break octet rule because of the atom count on central atom.

8 Odd electron count compounds are radical compounds (=> they have an unpaired electron) Example 1) NO N has 5 valence e - O has 6 valence e- NO has 11 valence e... :N=O: Best we can do is minimize formal charge on N: U-Do-it Example 2) NO 2 N has 5 valence e - 2O has 12 valence e- NO 2 has 17 valence e Best we can do is minimize formal charge on N: U-Do-it..... :O=N=O:

9 Free radicals are very, very bad for you Prime sources of free radicals in our food??

10 `antioxidant’ is code word for free radical killer

11 IN-CLASS BOARD OCTET RULE PRACTICE WITH:: Diatomics O 2  N 2  CO  tri,tetra-atomics CO 2  H 2 O  OF 2  COCl 2  oxyanions NO 3 -  CO 3 2-  SO 4 2- Non-Lewis formal charge rule H 2 SO 4  SO 2  SO 3  SO 4 2-  Non-Lewis ”got no choice” and radicals PF 5  SF 6  NO  NO 2  Fer criss sakes, now are we okay ???

12 UGLY CHEMICAL FACT OF LIFE #2 For many compounds the Lewis octet prediction of bond lengths don’t match experiment even for elements < Si Expect O-O length = 15 pm Expect O=O length = 12 pm Example 1: Ozone O 3 Observe : Both bond lengths are identical=13.5 pm Lewis model prediction This sucks

13 Expect C-C lengths= 16 pm Expect C=C lengths = 13 pm Example 2: Benzene C 6 H 6 Observe: all C-C lengths are identical = 14.5 pm Lewis Model prediction

14 The common thread: if electrons can be moved to make the same molecule => `resonance’ Resonance structure All bonds are equivalent in resonance structures

15 ` RESONANCE’ circulates the electrons evenly between participating atoms so that the bond lengths between those atoms are identical and an average of the possible single/ double bond distributions. O-O length = 15 pm O=O length = 12 pm Observed O-O bonds all = 13.5 pm Average = 13.5 pm C-C lengths= 16 pm C=C lengths = 13 pm Average = 14.5 pm Observed C-C bonds all= 14.5 pm

16 U-Do-it: Oxyanion examples of resonance NO 3 - NO 2 - CO 3 -2 a)What is the static octet prediction for each ? b) What is the actual, expected bond order to O for each example ? 4/3=1.333 3/2=1.54/3= 1.333

17 The Lewis Model of Bonding Tells Chemists: 1)Bond order and electron ownership 2)Formal charge distributions 3)Excited state configurations (COCl 2 example) 4)Whether resonance exists (or not) The `big picture’ for the Lewis model, so far:

18 The Lewis model also provides: 1)Insight into chemical reactivity. 2)Predictions of Molecular structure (VSEPR theory pp. 193-217)

19 Lewis and chemical reactivity: some simple examples 1) CO and blood - vs. CO has 200-400X stronger electrostatic attraction to Fe 3+ from formal charge vs. O 2. Explains why CO so easily asphyxiates humans even at `low’ concentrations (400 ppm)…it never lets go of the Fe 3+ Fe 3+

20 Lewis and chemical reactivity: some simple examples (cont.) 2) complexation of metals Aquo complexes of metals’ color related to lone pair bonding to M z+ Cu(OH 2 ) 6 2 +

21 Change the source of the lone pair….. changes the color

22 Lewis and chemical reactivity: some simple examples (cont.) In biology we need bacteria and light to fix N 2 (convert to C- N single bonds). Chemically it is very hard to `fix’ N 2. Why ?? 3 bonds are tough to rupture When H 2 O and CO 2 mix to make carbonic acid (essence of Sprite), what goes where to make the acid ?? + +  - 2  -  +2 

23 Lewis Model and Molecular Structure predictions (VSEPR theory : electron clouds are balloons) :N  N : LINEAR TRIGON AL PLANAR TETRAHEDR ON

24 TRIGONAL BIPYRAMID OCTAHEDRON

25 What if lone pairs take up some of the balloon space ? No lone pairs: 4 bonds to atoms 1 lone pair + 3 bonds to atoms Pyramid or tetrahedron Trigonal pyramid

26 2 lone pairs + 2 bonds to atoms Chemical example Bent structure

27 Lewis structure formaldehyde Relevant shape? shape name? Trigonal plane carbon dioxide LINEAR

28 Lewis structure Relevant shape? shape name? Nitrite anion Bent structure Sulfuric acid TETRAHEDR ON

29 Lewis structure Relevant shape? shape name? Sulfur tetrafluoride Square pyramid Trigonal bipyramid 1 lone pair + 4 bonds

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