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.

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