[dancinginthesky]

Hi, my AP Chemistry class is learning how to calculate the heat of formation and bond energies. I'm stuck on part 2 of this problem:

Consider the combustion of ethane:
C2H6(g) + 7/2O2(g) -> 2CO2(g)+3H2O(l)    change in heat at standard conditions = -1559 KJ/mol
I used Hess' law to find the heat of formation of ethane, and got -85.51 KJ/mol (which according to my teacher, is the answer).

The next part confuses me: For the above reaction, how much energy is released if 1Kg of water is formed? (ans: -28870.5KJ)

Should I convert 1Kg to moles of water? 1Kg*(1000g/1Kg)(1mol/18g) = 55.56mol. Then recalculate heat of formation using Hess's Law? The numbers get so strange, I don't think I'm using the right method.

Thank you!

[Dan]

1. Can you show exactly what you've done? You say the numbers seem strange, can you post them?

2. Yes, convert 1 kg of water to moles. Use your answer to calculate how many moles of ethane were burnt to produce 1 kg of water, let's call that number x mol. You know that burning ethane releases 1559 kJ/mol - that means that if you burn 1 mol of ethane, 1559 kJ of energy are released. So how much energy is released when you burn x mol of ethane?

[Vidya]

yes you are right that you convert 1Kg of water into moles
now calculate the heat of reaction for new moles of water
so it should be calculated in this way

-1559 KJ/mol   X 55.55 moles of H2O / 3 moles of H2O