# Homework Help: Thermochemistry probelm

1. Sep 17, 2009

### winterwind

1. The problem statement, all variables and given/known data
Calculate $$\Delta$$rH0 and $$\Delta$$rU0 at 348 K for the hydrogenation of ethyne (acetylene) to ethene (ethylene) from the enthalpy of combustion and heat capacity data in Tables 2.5 and 2.7. Assume the heat capacaties to be constant over the temperature range involved.

$$\Delta$$fH0 of ethyne = +226.73 kJ/mol
$$\Delta$$p,mC0 of ethyne = 43.93

$$\Delta$$fH0 of ethene = +52.26 kJ/mol
$$\Delta$$p,mC0 of ethene = 43.56

Other relevant data in the tables are also given, such as Enthalpy of fusion, enthaply of combustion, molar heat capacity, Benson thermochemical groups, of various molecules, including water, oxidation, hydrogen gas, carbon dioxide gas, ethane, ethene, and ethyne.
I only included the above because I think I would need to use those for sure. The other data can be easily found.

2. Relevant equations
Hess's Law
Kirchhoff's Law

3. The attempt at a solution
I might try combining equations to arrive at the right equation (Hess's Law). Maybe Kirchoff's Law to find the values at 348 K? What is meant by $$\Delta$$rU0? Is this the change in internal energy? How does it relate to enthalpy (fusion, heat capacity, etc.)?

Thanks!

EDIT: I only need help with question in post #2 now. I figured out the other parts already.

Last edited: Sep 17, 2009
2. Sep 17, 2009

### winterwind

I just need help with this part of the probelem now:

Calculate $$\Delta$$rU0 at 298 K for the hydrogenation of ethyne (acetylene) to ethene (ethylene) from the enthalpy of combustion and heat capacity data in Tables 2.5 and 2.7. Assume the heat capacaties to be constant over the temperature range involved.

I figured out the other parts of the problem. I am still not sure of the relationship between $$\Delta$$rU0 and enthalpy values.

3. Sep 19, 2009

### limonysal

Hi winterwind,
I'm also working on this problem for a homework due this week...which makes me wonder...

Anyway, I found in my notes that $$\Delta$$H=$$\Delta$$U+$$\Delta$$ngasRT.

I hope this helps :)