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) 2 H 3 C − C H 3 ( g ) + 7 O 2 ( g ) → 4 C O 2 ( g ) + 6 H 2 O ( g ) Note that the average bond energy for the breaking of a bond in CO2 C O 2 is 799 kJ/mol k J / m o l .

Use average bond energies to calculate ΔHrxnΔHrxn for this reaction.

Answer 1

Balanced chemical reaction is

2CH3 - CH3 + 7O2    $1585524726726_blob.png$ 4CO2 + 6H2O

type of bond	bond dissociation enthalpies	no.of bond in reactant	no. of bond in product
C - H	413 KJ/mol	12	0
C - C	347 KJ/mol	2	0
O = O	498 KJ/mol	7	0
C = O	799 KJ/mol	0	8

O - H

464 KJ /mol

0

12

we know the formula

∆Hrxn = ∑ ∆H(reactant bond ) -   ∑∆H(product bond)

∆Hrxn = [ 12(∆H C -H) + ​​​2(​​​​∆H C - C ) + 7(​​​​∆H O = O ) ] - [ 8( ​​​​​​​∆H C = O) + 12(​​​​​​​∆H O - H )  ]

substitute the value

∆Hrxn = [ ( 12 X 413 KJ / mol ) + ( 2 X 347 KJ / mol )+(7 X 498) ] - [ (8 X 799  KJ / mol ) + ( 12 X 464 KJ / mol) ]

∆Hrxn = [ 9136 KJ] - [ 11960 KJ]

∆Hrxn = -2824 KJ