Ch9_BE - 1 2 3 Using Bond Energies Estimate the energy of...

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Unformatted text preview: 1 2 3 Using Bond Energies Estimate the energy of the reaction H—H + Cl—Cl ----> 2 H—Cl Cl— H—Cl Net energy = Hrxn = Net = energy required to break bonds energy - energy evolved when bonds are made Using Bond Energies Estimate the energy of the reaction H—H + Cl—Cl ----> 2 H—Cl H—H ==436 kJ/mol Cl— H—Cl H—H 436kJ/mol kJ/mol Cl— Cl—Cl = 242 kJ/mol Cl—Cl = 242kJ/mol kJ/mol H—Cl = 432kJ/mol H—Cl = 432 kJ/mol H—Cl kJ/mol Using Bond Energies Estimate the energy of the reaction 2 H—O—O—H ----> O=O + 2 H—O—H Is the reaction exo- or endothermic? exo- Sum of H-H + Cl-Cl bond energies = Cl436 kJ + 242 kJ = +678 kJ 2 mol H-Cl bond energies = 864 kJ mol H-Cl Net = H = +678 kJ - 864 kJ = -186 kJ Which is larger: A) B) energy req’d to break bonds or energy evolved on making bonds? H—H = 436 kJ/mol H—H = 436 kJ/mol kJ/mol Cl—Cl = 242 kJ/mol Cl— Cl—Cl = 242 kJ/mol kJ/mol H—Cl = 432 kJ/mol H—Cl = 432 kJ/mol H—Cl kJ/mol 4 5 Using Bond Energies 2 H—O—O—H ----> O=O + 2 H—O—H Energy required to break bonds: break 4 mol of O—H bonds = 4 (463 kJ) break 2 mol O—O bonds = 2 (146 kJ) TOTAL ENERGY to break bonds = 2144 kJ TOTAL ENERGY to break bonds = 2144 kJ TOTAL ENERGY evolved on making O=O TOTAL ENERGY evolved on making O=O bonds and 4 O-H bonds bonds = 2350 kJ bonds and 4 O-H bonds bonds = 2350 kJ Using Bond Energies 2 H—O—O—H ----> O=O + 2 H—O—H Net energy = +2144 kJ - 2350 kJ = - 206 kJ Net energy = +2144 kJ - 2350 kJ = - 206 kJ The reaction is exothermic! The reaction is exothermic! More energy is evolved on More energy is evolved on making bonds than is making bonds than is expended in breaking expended in breaking bonds. bonds. Page 1 ...
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