Transcript 76 kJ/mole
Twisted - The Tail of Two Cities by Charles VB Tickens allene, C3H4 Twisted - The Tail of Two Cities by Charles VB Tickens Gaining a new perspective on the unexpected • Viewing, and then drawing a 3D perspective • wedge mark & hash mark allene, C3H4 Twisted - The Tail of Two Cities by Charles VB Tickens Gaining a new perspective on the unexpected • Viewing, and then drawing a 3D perspective • wedge mark & hash mark hash mark away wedge mark toward • Newman Projection angle of 90° Twisted - The Tail of Two Cities by Charles VB Tickens Molecular Orbitals • sigma and pi molecular orbitals • Regions of space of greatest probability of finding electrons begin shared between atoms in chemical bonding. • Know their shape and spatial orientation • Know their symbolism designation • Law of Conservation of Molecular Orbitals • Linear combination of Two AO and/LCAO’s in , Two MOs out pi bonding Twisted - The Tail of Two Cities by Charles VB Tickens Twisted - The Tail of Two Cities by Charles VB Tickens Molecular Orbitals • sigma and pi molecular orbitals • Regions of space of greatest probability of finding electrons begin shared between atoms in chemical bonding. • Know their shape and spatial orientation • Know their symbolism designation • Law of Conservation of Molecular Orbitals • Linear combination of Two AO and/LCAO’s in , Two MOs out sigma bonding H H C C H H Twisted - The Tail of Two Cities by Charles VB Tickens Liner Combinations of Atomic Orbitals • sp, sp2, sp3 LCAO’s • Regions of space of greatest probability of finding a electrons about an atom needed in chemical bonding • Know their shape and spatial orientation • Know their symbolism designation • Law of Conservation of LCAO Orbitals • Linear combination of Two AO’s in , Two LCAO’s out H H C C H H Wave Properites A. Wave funtions are mathematical quantum descriptions which can be used to describe atomic orbitals (AO) having specific 1) shape and 2) spatial orientation. B. Most importantly, AOs can interact, combine and overlap to give more complex wave having new shape and spatial orientation. C. These new wave functions are called linear combination of atomic orbitals (LCAOs) D. AOs, LCAOs and MOs are each different having their own specific shape and spatial 1) LCAO atomic hybrid orbitals occur when AOs on the same atom combine, or 2) MO molecular orbitals between atoms when orbitals (AO or LCAO) of different atoms Wavefuntion Wavefuntion or molecular orbitals (MOs) orientation: combine. Wavefuntion + x functions + - - y AO representation AO representation + node AO representation - + - Linear combination of atomic orbital (addition and subtraction of AOs on the same atom) + + + + s AO ++ - - + p AO - + + - + sp hybrid LCAO - - sp hybrid LCAO sp shape & spatial orientation node - + -- node - + Molecular Orbital (linear combination of AOs between atoms) node + + + + s AO added s AO sigma bonding + + s - + s substracted s AO AO + - sigma antibonding s* Electron configurations & AO electron filling 1 th "n " row n = 1 n = 2 n = 3 n = 4 2 H Li Be Na Mg K Ca 3 4 5 6 7 8 He B C N O F Ne Al Si P S Cl Ar Group Twisted - The Tail of Two Cities by Charles VB Tickens Prologue: • Isomers of C3H4 (there are three) • isomer - same chemical formula, different structures Twisted - The Tail of Two Cities by Charles VB Tickens Prologue: • Isomers of C3H4 (there are three) • Assign to each atom 1. angle 2. geometry 180°, linear, sp 3. VBT-LCAO, hybridization 120°, trigonal planar, sp skewed 120°, trigonal planar, sp2 109.5, tetarahedral, sp3 109.5°, tetarahedral, sp3 180°, linear, sp 180°, linear, sp Twisted - The Tail of Two Cities by Charles VB Tickens http://homework.sdmesa.edu/dgergens/chem100/vsepr/vsepr.html VBT- LCAO sp3 sp2 sp sp3 sp2 sp sp3 sp2 sp Twisted - The Tail of Two Cities by Charles VB Tickens Prologue: • Isotopes carbon-12 vs carbon-13 and their relative terrestrial abundance • Isotopes hydrogen-1 vs hydrogen-2 their relative terrestrial abundance • Watch http://homework.sdmesa.edu/dgergens/chem231/isotopes/isotope1_ipod.mov • Average atomic mass for naturally occurring terrestrial carbon 12 13 6 6 C x 99% + • C x 1% = 12.011 amu carbon-14 14 C 6 14 N 7 Twisted - The Tail of Two Cities by Charles VB Tickens Additional hybridization practice: • http://homework.sdmesa.edu/dgergens/chem231/hybridization/index.html Twisted - The Tail of Two Cities by Charles VB Tickens LCAO’s & Molecular Orbitals: • http://homework.sdmesa.edu/dgergens/chem231/VBT/index.html Twisted - The Tail of Two Cities by Charles VB Tickens Story Goals: • To briefly review heats of reactions for elemental carbon • Understand the purpose & function of thermodynamic reaction energy profiles • To gain additional appreciation as to why atoms of elements like to react chemically • Review the shapes and spatial orientation of probability maps for finding electrons. These are: • atomic orbitals, AO’s • hybrid orbitals, LCAO’s • molecular orbitals, MO • Identify hybridization about atoms chemically bonded to other atoms • Investigate bonding theories, • VBT, valance bond theory • MO, molecular orbital theory • Apply VBT and MO theories to understand why the end carbons in allene are allene, C3H4 • Apply VBT & MO theories to explain bonding in H2, O2, CO, CO2, H2O, CH4 Twisted - The Tail of Two Cities by Charles VB Tickens A. “It was the best of times, it was the worst of times" • Calculate the ethalpy ∆Hf (heat of formation) for methane in its standard state B. Carbon was imprisoned as methane gas 1 C(s) + 2 H2(g) ------> 1 CH4(g) ∆Hf = zero ∆Hf = zero • • • ∆Hf = -76 kJ/mole ZERO Test question, an enthalpy for any element in its standard state is “_______” Using Hess’ Law we can indirectly calculate the heats of formation ∆Hf ∆Hf for methane from its elements is -76 kcal/mol (GOOGLED Value) Twisted - The Tail of Two Cities by Charles VB Tickens What is the purpose for knowing the heat of formation of CH4 from C and H2???? C. Learning how to plot an energy profile for a chemical reaction 1 C(s) + 2 H2(g) ------> 1 CH4(g) • • ∆H = -76 kJ/mole high energy versus low energy over time D. In general • rocks roll downhill high ‡ Ea 1 C(s) + 2 H2(g) E. Overall reaction is exothermic • thermodynamics -76 kJ/mole F. Transition State, ‡ 1 CH4(g) low G. Ea, Energy of Activation • kinetics in the rate determining step rxn time Twisted - The Tail of Two Cities by Charles VB Tickens H. The “Revolution” is building and things are getting heated • Unlike incomplete combustion, less O2 , less heat & more soot (unburned carbon) 1 C (s) + 1/2 O2(g) ------> 1 CO (g) ∆H = -110.5 kJ/mole • complete combustion of methane in excess oxygen 1 C (s) + 1 O2(g) ------> 1 CO2 (g) ∆H = -393.5 kJ/mole 1 CH4(s) + 2 O2(g) ------> 1 CO2(g) + 2 H2O (l) ∆H = -890.4 kJ/mole high C CO 1 CH4(g) + 2 O2(g) CO2 -890 kJ/mole Do diamonds burn? 1 CO2(g) + 2 H2O(l) low rxn time Twisted - The Tail of Two Cities by Charles VB Tickens I. Why would carbon want to be imprisoned to four chemical bonds? BECAUSE (know your mantra): Atoms chemically like to 1 C(s) + 2 H2(g) ------> 1 CH4(g) • • • • J. ∆H = -76 kJ/mole lose, gain or share outermost valence electrons to get filled, half filled or empty outermost orbitals, and/or to become isoelectronic with their closest noble gas neighbor Along the high reaction pathway, rocks like to roll downhill 1 C(s) + 2 H2(g) -76 kJ/mole 1 CH4(g) low rxn time Twisted - The Tail of Two Cities by Charles VB Tickens K. Breaking down our mantra Mantra: Atoms chemically like to 1 C(s) + 2 H2(g) ------> 1 CH4(g) • • • • ∆H = -76 kJ/mole lose, gain or share outermost valence electrons to get filled, half filled or empty outermost orbitals, and/or like to become isoelectronic with their closest noble gas neighbor Twisted - The Tail of Two Cities by Charles VB Tickens K. Breaking down our mantra Mantra: Atoms chemically like to 1 C(s) + 2 H2(g) ------> 1 CH4(g) • lose, gain or share ∆H = -76 kJ/mole Twisted - The Tail of Two Cities by Charles VB Tickens K. Breaking down our mantra Mantra: Atoms chemically like to 1 C(s) + 2 H2(g) ------> 1 CH4(g) • outermost valence electrons ∆H = -76 kJ/mole Twisted - The Tail of Two Cities by Charles VB Tickens K. Breaking down our mantra Mantra: Atoms chemically like to 1 C(s) + 2 H2(g) ------> 1 CH4(g) • ∆H = -76 kJ/mole to get filled, half filled or empty outermost orbitals Twisted - The Tail of Two Cities by Charles VB Tickens K. Breaking down our mantra Mantra: Atoms chemically like to 1 C(s) + 2 H2(g) ------> 1 CH4(g) • ∆H = -76 kJ/mole like to become isoelectronic with their closest noble gas neighbor Twisted - The Tail of Two Cities by Charles VB Tickens M. Describe the carbon atom bonding dilemma in observed CH4 • Use VBT Theory & VBT diagrams to describe how the dilemma is resolved • Use MO theory to describe sigma bonding in CH4 • Study the materials at http://homework.sdmesa.edu/dgergens/chem231/VBT/index.html Electron Dot Structure = Group Number = Valence Electron (outermost) 1 Group 2 3 H Li Be Na Mg 4 5 6 7 8 He B C N O F Ne Al Si P S Cl Ar • • • Al • • • Si • • • •• • • P • O • • •• • • • N •• •• • • S •• F •• •• • • Cl •• Ne •• Ar •• • • • •• • • C •• Mg • •• •• •• • • B •• •• Na Be •• •• Li • •• He H •• • • • chemical reactivity - outermost valence electrons afford chemical reactivity