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Topic 14 Bonding (HL)

Topic 14 Bonding (HL) . Shapes of molecules and ions Hybridisation Delocalisation of electrons. 14.1 Shapes of molecules and ions . V alence S hell E lectron P air R epulsion- VSEPR for 5- and 6-negatively charged centre

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Topic 14 Bonding (HL)

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  1. Topic 14 Bonding (HL) • Shapes of molecules and ions • Hybridisation • Delocalisation of electrons

  2. 14.1 Shapes of molecules and ions • Valence Shell Electron Pair Repulsion- VSEPR for 5- and 6-negatively charged centre => Shapes are based on trigonalbipyramid and octahedron

  3. Expanded valence shells • Sometimes the octet rule doesn’t hold • The atom have 8 or 10 electrons • The PCl5 molecule has 5 bonding electron pairs -a symmetrical trigonalbipyramidal shape. • 5 negative centres!

  4. Trigonalbipyramid- PCl5 • Two types of electron rich regions: • Equatorial: 3 bonds with 120obetween. • Axial: 2 bonds with 180o between • Equatorial to Axial: 90o. http://www.chem.ufl.edu/~myers/chm2045/shapes.htm

  5. Trigonalbipyramid- SF4 andClF3 • Non-bonding orbitals always occupy equatorial positions • SF4 Equatorial: 2 bonds with 104o (<120o), Axial: 2 bonds with 177o (<180o) • ClF3Equatorial: 1 bond Axial: 2 bonds with 87,5*2= 175o (<180o) http://www.chem.ufl.edu/~myers/chm2045/shapes.htm

  6. Octahedron • All positions are equal- 90o between all positions- SF6 • If two non-bonding orbitals: they take place opposite each other =>plan square shape- XeF4

  7. Many of the compounds thatare forming trigonalbipyramids and octahedrons are fluorides because only high electronegative ions can increase the number of valence electrons Fluoride is also quite small (bigger ions doesn’t have space enough).

  8. 14.2 Hybridisation • So far we have talked about s-p-d-f-orbitals. They only exist in single atoms in the gaseous state • When atom binds to each other the orbitals will change their shape; they will undergo a Hybridisation (mathematics: linear combination)

  9. s -bonds and p-bonds http://ibchem.com/IB/ibnotes/full/bon_htm/14.2.htm

  10. The bonds between carbon atoms http://www.chemguide.co.uk/basicorg/bonding/methane.html

  11. Single bonds in ethane C2H6 • Hybridisation: One s-orbital and three p-orbitals => Four sp3-orbitals (tetrahedral shape) • Two carbons with sp3-orbitals now bind 3 hydrogen s-orbitals, with s-bonds: The last orbital is used to s-bond to the next carbon:

  12. Single- and double bonds in ethene C2H4 • Hybridisation: One s-orbital and two p-orbitals => Three sp2-orbitals (trigonal planar shape). One p-orbital is left over (red) • Two carbons with sp2-orbitals now bind 4 hydrogen s-orbitals, with s-bonds: The green sp2-orbital is used to s –bond, and the red p-orbital is used to p -bond to the next carbon:

  13. Double bond, cont • Consist of ones -bond and one p -bond • The p -bonding to the next carbon is at a right angle, 90o, to the next carbon http://www.groveridgeconsulting.com/?page_id=546

  14. Single- and double bonds in ethyne C2H2 • Hybridisation: One s-orbital and one p-orbital=> Twoo sp-orbitals (trigonal planar shape). Two p-orbitals is left over (red) • Two carbons with sp-orbitals now bind 2 hydrogen s-orbitals, with s-bonds: The green sp-orbital is used to s –bond, and the red p-orbitals are used to p -bond to the next carbon:

  15. Triple bond • Consist of one s -bond and two p -bonds • The sp-orbitals give a linear shape • The two p -bonding to the next carbon is at a right angle to the next carbon and at right angle to each other

  16. Molecular shape and types of hybridisation • The shape of the hybrids corresponds to the structure given by VSEPR / Lewis structure. => Determine the hybridisation by studies of the shape of the molecule. • Ethane : Ethene : Ethyne sp3: sp2: sp • Ammonia: sp3 • Water: sp3

  17. 14.3 Delocalisation of electrons • Electrons that are not located at a certain atom (c.f. metallic bond) • Or in a certain bond between two atoms • Often gives rise to a stronger (shorter) bond • The delocalised electrons absorb light in the UV- or visible region

  18. Benzene • 6 sp2-hybridised carbons, 6 p-orbitals • The p-orbitalscanoverlapbothto the right and to the left- a system ofdelocalisedp-electronsareformed. The electrons are said to be delocalised, often shown as a circle • Single bond 154 pm, double 134 pm, benzene 140 pm

  19. Resonance • a and b are resonance structuresof benzene • c is a resonance hybrid-the most stable form. By delocalisation of the electrons the molecule gain resonance energy • d resonance in pyridine d

  20. Why has phenol acidic properties? LowpKa- strong acid (HCl -7) HighpKa (1-14)- weakacid pKa > 14 no acid PhenolpKa 8

  21. Why has acetic acid acidic properties but not ethanol? C2H5OH + H2O ↔ C2H5O- + H3O+ pKa= 16 CH3COOH + H2O ↔ CH3COO- + H3O+ pKa= 4.75 (Bond length C=O 124 pm, C-O 143 pm, but in acetate ion C´-O 127 pm)

  22. Draw the resonancestructures of :NO3-NO2-CO32-O3

  23. Draw Lewis/resonance structures of:NO3-, NO2-, CO32-, O3

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