Thermodynamics/entropy question (1st yr chem)

[bentennason]

I pasted the question here: http://imgur.com/1Zts5Os

Could somebody explain what "U" and the naught on the U is? And is the question missing any data?

[LeviLamp]

At constant pressure, delta H (enthalpy change) is simply q (heat energy released/absorbed). I'm /guessing/ the little o denotes "standard" enthalpy or "standard internal energy", referring to enthalpy/internal energy of a substance in its standard state. Delta U is the internal energy change of a system during reaction. Since this is a gas in the standard state, the pressure will be 1 atm or 101.3 kPa

Delta U = q + w, where q is the heat of the reaction (and, with constant pressure, equals delta H so equals -1299.5 kJ) and work is the amount the volume of the system increases by.

Temperature and pressure are remaining constant, so delta H = delta U + P delta V.
-1299.5 = delta U + 101.3 x delta V

delta V= nRT/p (products) - nRT/p (reactants)
Assuming 1 mol of C2H2 is present at the beginning of the reaction;
           = 73.34 - 85.56
           = -12.22 <- need to divide by 1000 since I used mol, J K^-1 mol^-1 and K / kPa and we're treating the final part of this question in kJ; if you cancel the invisible units, you end up with J/kPa in the equation and want kJ/kPa for this question
           = -0.012
so delta U = -1299.5 - (-0.012 x 101.3)
                 = -1299.5 +1.24
                 = -1298.3 kJ

So I'm going to say C.
And no, I would NOT have managed that on the exam. Too many nerves