Alkane halogenation is a two-step reaction, as shown in the image. Using the table of bond dissociation energies, calculate the enthalpy of cach step and the enthalpy of the overall reaction. Bond Dissociation Energies (for A-B A. ++B) Bond broken AH(kJ/mol) Bond broken AH(kJ/mol) Bond broken A H (kJ/mol) CH, CH, CH,-H CH,CH, CH, -Br H-H 436 423 H-Br 366 294 Br-Br 193 Step 1 Br: AH = kJ/mol CH,CH,CH,-H + CH,CH;CH, + HBr Step 2 AH = kJ/mol CH,CH,CH, + Br2 CH,CH,CH,-Br • Br: Overall reaction CH,CH;CH,-H + Br2 CH,CH,CH,-Br + HBr AH = kJ/mol
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
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