Question

Ethane, a minority component of natural gas, burns to form carbon dioxide and water according to this reaction: 2H3C−CH3(g)+7O2(g)→4CO2(g)+6H2O(g) Note that the average bond energy for the breaking of a bond in CO2 is 799 kJ/mol. Use average bond energies to calculate ΔHrxn for this reaction.

Answer 1

Answer:

The answer is -2812 kJ.

Explanation:

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The following values will vary depending on the source;

C-C  347 kJ/mole;  C-H 414 kJ/mole;  O-H 464 kJ/mole ;  O=O  498 kJ/mole

It takes energy to break bonds and energy is released when bonds are formed.

Ethane has 1 C-C bond  ;  1(347 )  347 kJ/mole

Ethane has 6-C-H bonds;  6(414) = 2484 kJ/mole

Break the bonds of 1 mole of C2H6  =  2831 kJ

2 moles of ethane in balanced equation:  2(2831 kJ) =  5662 kJ

Break 7 O=O bonds  =  7(498 ) = 3486

Total energy to break all bonds: 5662 +   2486 = +9148 kJ

2C=O bond formed in CO2  = 2(799) =  1598 kJ/mole

4 moles of CO2 = 4(1598) =  6392

2(O-H) bonds in H2O =  2(464)  =  928 kJ/mole

6 moles of H2O  =  6(928 kJ) = 5568

Total energy released =  6392 + 5568 = 11,960 kJ  

The sign of this energy is -11,960 since it is exothermic.

Energy change for 2 moles of ethane = -11,960 + 9148 = -2812 kJ

ΔHrxn = -2812 kJ