Question

5. Heptane is the zero of the octane rating of motor fuel. It burns according to the following: CH16+ 1102 à 7CO2 + 8H20 Estimate the enthalpy change in the reaction by using bond dissociation energies, and classify the reaction as exothermic or endothermic overall. (Average BDEs in kcal/mol: C-H, 99, 0-0, 119; C=0, 191; H-O 112]. (Hint: C-H16 has 16 C-H bonds) 6. Noting that all the reagents and products in problem 5 are gases under the conditions of combustion in an engine, what is the sign of AS for the combustion?

Answer 1

5. Ghio + 110, +7O2 + 8 H₂O c-c=83 kcal/mol Energy of product = 7x2 (c=0) + 8x2 (0-1) fenergy involved) 5 14 x 191 + 16 X 112 release] :2674 + 1792 Inegative signl .=-44 66 k callmol Energy of reactant =) 16 (C-H) +7 (6-() +11 (0=0) (energy required) 8 16 x 99 +7x83 + 11 x 119 (positive sign) B = 3474 Kcal/mol - 6. y change = - 44 66 + 3474 1. [-992 k call moll Overall & Exothermic (negative sign) AS = (t ve positive] Number of mole of gaseous product is greater than a reactant.