lec4 - Last Time Phase Changes Reaction Enthalpy Hf =nHf...

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1 Last Time c Phase Changes c Reaction Enthalpy Δ H f = Σ nH° f (products) - Σ nH° f (reactants) Δ H f positive Endothermic Δ H f negative Exothermic Hess’ Law A C B D ? Δ H f (AC) Δ H f (DB) Δ H f (CD) Reaction Cancellation A C Δ H f (AC) C D Δ H f (CD) D B Δ H f (DB) A B Δ H f (AB) Example - Hess’ Law Calculate Δ H f for NO 2 (g) from N 2 (g) and O 2 (g). N 2 (g) + O 2 (g) 2NO(g) Δ H f = 180 kJ 2NO(g) + O 2 (g) 2NO 2 (g) Δ H f = -112 kJ _________________________________ N 2 (g) + 2O 2 (g) 2NO 2 (g) Δ H f = 68 kJ
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2 Intermolecular Forces • Ionic Bonds • Dipole - Dipole • Hydrogen Bonding • London Forces Ionic Bonds Na + + Cl - NaCl Na + 0 valence electrons Cl - 8 valence electrons Equal and opposite charges attract !! Dipole - Dipole • Electronegativity based – electron/nucleus attraction • Polar molecules alignment C O H H δ - δ +
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3 Hydrogen Bonding • Electrostatic interaction between H and either O, N, or F. – Size and charge density
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lec4 - Last Time Phase Changes Reaction Enthalpy Hf =nHf...

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