- #1
Silvius
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Homework Statement
What is the ΔU° at 25°C for the following reaction at constant pressure:
C[itex]_{2}[/itex]H[itex]_{2}[/itex] (g) + 5/2O[itex]_{2}[/itex] (g) [itex]\rightarrow[/itex] 2CO[itex]_{2}[/itex] (g) + H[itex]_{2}[/itex]O (g) ΔH° = -1299.5kJ
Homework Equations
ΔU = Q ± W
PV = nRT
W = PΔV
The Attempt at a Solution
Since we have constant pressure, ΔH° = Q, and since we are at standard conditions P = 1atm = 101.3kPa. Additionally T = 298K (which is given anyway). I try and find the work done on the system by finding the reduction in volume;
ΔV = ΔnRT/P
ΔV = (0.5 x 8.31 x 298)/101.3 = 12.2L
W[itex]_{on system}[/itex] = 101.3kPa * 12.2L = 1238.2kJ
ΔU = -1299.5kJ + 1238.2kJ = -61.3kJ
However, the answer is -1298.3kJ.
Interestingly, this answer is what I would get if I had accidently calculated the W[itex]_{on system}[/itex] was in joules and forgotten to convert - but I don't see how this can be the case - I'm dealing with kPa not Pa as my unit for pressure. Any help would be much appreciated!