Question

Use bond energies to estimate ΔH for the combustion of 1 mole of acetylene:

C2H2(g) + (5/2)O2(G) --> 2CO2(g) + H2O(g)

I wrote out the Lewis structure for C2H2 and got 2 C-H bonds and a triple bond between C and C. So my equation looked like this:

ΔH = ΣD(bonds broken) - ΣD(bonds formed)
= [2DH-C + D(triple bond C-C) + (5/2)DO-O] - [2DC-O + 2DH-O]
= [2(413) + 839 + (5/2)(146)] - [2(358) + 2(467)]

I got ΔH = 380 kJ, but the textbook says it's -1228 kJ. What am I doing wrong?

Answer 1

just noticed that the bond energy for CO2 is actually 799 kJ; but my answer is still not right?

Answer 2

Did you use OO double bond for O2, CO DOuble bonds for CO2?

Answer 3

Let me look them up.

Answer 4

O=O should be 498 kJ/mol, C=O is 805 and HO is right. But you have another mistake, you should multiply the C=O number by 4, not 2. Each mole of CO2 has 2 CO bonds, you have 2 molecules of CO2, so you need to break 4 CO bonds. Try that