Ch 6.2 Covalent Bonding

Ch 6.2 Covalent Bonding - Qualitative Description of...

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Chapter 5 16 Qualitative Description of Covalent Bonding Atoms share electrons to lower their energy. Atoms are shared by forming covalent bonds. A covalent bond is a molecular orbital with significant electron density between the nuclei of the bonded atoms. Electrons are shared in pairs – single bond – double bond – etc The electrons are shared in such a way that the atoms complete their valence shells.
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Chapter 5 17 The effect of an electric field on hydrogen fluoride molecules. Polar covalent bonds Dipole Moments Bonded atoms share electrons unequally, whenever they differ in Electronegativity E.g., HF The F atom carries a slightly negative electric charge and the H atom a slightly positive charge of equal magnitude. Aligns itself in an electric field Dipolar or polar molecules posses a dipole moment, μ
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Chapter 5 18 Electronegativity Electronegativity is the ability of an atom in a molecule to attract shared electrons to itself. – Greater electronegativity = greater ability to attract electrons. • Related to other concepts we’ve covered. – Electron Affinity – Ionization Energy • A number of different scales.
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Chapter 5 19 Electronegativity Trends
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Chapter 5 20 Classifying Bonds Covalent (H 2 , Cl 2 N 2 ) Polar Covalent (HF, H 2 O) Ionic Bond (LiF, NaCl)
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Chapter 5 21 Ionic Character of Covalent Bonds Ionic vs. Covalent bonding
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Chapter 5 22 Polar Molecules Dipole moment = μ = QR
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Chapter 5 23 Nonpolar Molecules with Polar Bonds Video and PRS
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Chapter 5 24 Covalent Bond Energies and Chemical Reactions Reaction Enthalpy ( Δ H) Bond Energy
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Chapter 5 25 Average Bond Energies C 2 H 6 ·C 2 H 5 + ·H Δ H°= + 410 kJmol -1 CHF 3 ·CF 3 + ·H Δ H°= + 429 kJmol -1 CHCl 3 ·CCl 3 + ·H Δ H°= + 380 kJmol -1 CHBr 3 ·CBr 3 + ·H Δ H°= + 377 kJmol -1 average Δ H°C-H = + 412 kJmol -1
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Chapter 5 26 Average Bond Energies
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Chapter 5 27 Example Estimate the Standard Enthalpy of Reaction for the gas-phase reaction that forms methanol from methane and water CH 4 (g) + H 2 O (g) CH 3 OH (g) + H 2 (g)
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H C H H H + O H H H H H + H H C O H -2496 kJmol -1 Exothermic +2586 kJmol -1 Endothermic 3 C-H or 3 x (-413) 1 O-H or 1 x (-467) 1 C-O or 1 x (-358) 1 H-H or 1 x (-432) 4 C-H or 4 x 413 2 O-H or 2 x 467 Formed Broken Δ H= Δ H bonds broken + Δ H bonds formed = +2586 +(-2496)= + 90 kJmol -1 (estimated) CH 4 (g) + H 2 O(g) CH 3 OH (g) + H 2 (g)
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Chapter 5 29 Average Bond Lengths
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Chapter 5 30 Models of Covalent Bonding Quantum Mechanics: Solve the Schrödinger Equation L ocalized E lectron Model (Valence Bond Theory) – A molecule is composed of atoms that are bound together by using atomic orbitals to share electron pairs .
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Ch 6.2 Covalent Bonding - Qualitative Description of...

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