CHEMICAL THERMODYNAMICS 1 chem 16 4 slides per page

Common bond energies d and bond lengths r bond h h h

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Unformatted text preview: r: heat is “negative” if it is ADDED to the calorimeter from a source heat is “positive” if it is REMOVED from the calorimeter by a source • When 3.425 kJ of heat is added to a calorimeter containing 50.00 g of water the temperature rises from 24.00oC to 36.54oC. Calculate the heat capacity of the calorimeter in J/oC. The specific heat of water is 4.184 J/g oC. 10 1/7/2014 Bond Energies Bond Energy • Bond energy is the amount of energy required to break the bond and separate the atoms in the gas phase. – To break a bond always requires an absorption of energy! Common Bond Energies (D) and Bond Lengths (r) Bond H-H H-B H-C H-Si H-Ge H-Sn H-N H-P H-As H-O H-S H-Se H-Te H-F H-Cl H-Br H-I D (kJ/mol) 432 389 411 318 288 251 386 322 247 459 363 276 238 565 428 362 295 r (pm) 74 119 109 148 153 170 101 144 152 96 134 146 170 92 127 141 161 Bond C-C C=C C≡C C-Si C-Ge C-Sn C-Pb C-N C=N C≡N D (kJ/mol) 346 602 835 318 238 192 130 305 615 887 r (pm) 154 134 120 185 195 216 230 147 129 116 Bond C-P C-O C=O C≡O C-B C-S C=S C-F C-Cl C-Br C-I D (kJ/mol) 264 358 799 1072 356 272 573 485 327 285 213 Table of average bond energies Molecule Bond Energy (kJ/mol) F2 (F-F) 159 243 Cl2 (Cl-Cl) Br2 (Br-Br) 192 O2 (O=O) 498 N2 (N≡N) 946 Bond Energies r (pm) 184 143 120 113 182 160 135 177 194 214 In gas phase reactions, ∆Ho values may be related to bond energies of all species in the reaction. • Example: Use the bond energies to estimate the heat of reaction at 25oC for the reaction below. CH 4(g) + 2 O 2(g) → CO 2(g) + 2 H 2 O (g) You should know how to draw the structure ∆H o = [4 BE C-H + 2 BE O =O ] - [2 BE C=O + 4 BE O -H ] 298 ∆H o = {[4 (414) + 2 (498)] - [2 (741) + 4 (464)]} kJ/mole 298 ∆H o = - 686 kJ/mole 298 11 1/7/2014 Bond Energies • Calculate the average N-H bond energy in ammonia, NH3. NH3(g ) → N(g ) + 3 H(g ) ∆Ho = 3 BEN-H 298 ∆H o = [ ∆H fo N ( g ) + 3 ∆H fo H ( g ) ]...
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This document was uploaded on 03/26/2014.

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