One kilogram of air is heated reversibly at constant pressure from an initial state of 300 K and 1 bar until its volume triples. Calculate W, Q, ?U, and ?H for the process. Assume for air that PV / T = 
and 
What will be an ideal response?
If PV/T is constant then at constant pressure, the volume will triple when the temperature triples, so the final state will be 900 K and 1 bar. The enthalpy change for this constant volume process with constant heat capacity is given by 
where we've used the molecular weight of air of about 
to convert from molar enthalpy change to specific enthalpy change.
By the definition of U, we have
The work is given by –P?V = –?PV at constant P, which we just computed to be
. For a constant pressure process, Q is equal to the enthalpy change, so Q = 600 kJ/kg
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