Question

5. (40 pts) At 298 K, the standard enthalpies of formation for ethene C2H4 (8) and hexene C6H12 (1) are 227 kJ/mol and - 73 kJ/mol, respectively. 1) Calculate 4Hxn for CH 2 (1) 3 C2H4 (8) 2) Given C-H bond energy is 414 kJ/mol and C-C bond energy is 347 kJ/mol. Use the above equation and your calculated Arxº to estimate the C=C bond energy. 3) Both ethene and hexene can be used as fuels. Which compound would liberate more energy per gram when completely combusted in air? Explain the reasons of your predictions.

Answer 1

1) Griven reaction C6H12 (1) 3 K₂ Hugo AHFCC Hus = 227 kJ/mol DHA (Colie) = -73 kJ/mol A Hrxn - EA Hf (products - Z A Hf (reactents = 3x227 KJ/mol - (-73 kJ/mol ) . 681 kJ/mol + 73 k 5 /mol = 754 kJ/mol

- 2) bond energy of C-H = 414 kJ/mol bond energy of c-c = 347 kJ/mol H Hi :c-c-C-(=C-t - 3 1 c =¢ H H H H 11 H H (9) H H H H H H l 1 ) H - C - C - I Admite bond energy of reactant - Ebond energy of product =) Ahron (12x bond energy of C-H 7 ur ux bond energy of C-H I + 4 x bond energy of C-C1 - 3* + 1x bond energy of C=C + 1x bond energy cec - 754 kJ/mol = 12 x 414 KJ /ml + 4x 347 kJ/mol - 12 x 414 kJ/mol + bond energy of C=C - 3 x bond energy of 2 754 kJ /mol = 1388 kJ/mol - 2 x bond energy of c=0 C=0

=) 2x bond energy of c=1 = 1388kJ/mol - 754 kJ/mol bond energy of c= c = 634 kJ/mol = 317 kJ/mol 3) total bond energy 0 CHI 12 x bond energy of C-H + 4x hond enersy of c- c t bond energyof c=0 = (12x414 + 4X347 +317) kJ/mol 6673 kJ/mol .:Mw of Co H12849. So, '84g of ethi has bond energy of 6673 kJ 1 g of Co Hiz has bond energy of 66 73 kJ 24 so, bond energy per gram of C6H12 79.44 kJ 7 total bond energy of C2 Hy = 4x bond energy of c-H+ band energy of c=c = 148414 +317) KJ/mol = 1973 kJ/mol Mw of Ca Hu= 289 28g of Calty has bond energy 1973 kJ --Ig of Ca Hu has bond energy 1973 kJ so, bond energy ber gram of a Hu = 70.46 kJ 28 required to break the bonds so so In CHu it will release less energy is more energy.