(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

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

2. Relevant equations

ΔU = Q ± W

PV = nRT

W = PΔV

3. 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!

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# Homework Help: Basic Thermodynamics; Change in U at Constant Pressure

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