Why Does Enthalpy Change Differ from Heat Change at Constant Pressure?

[yecko]

Homework Statement

Homework Equations

when constant pressure, h=w+u and so H=W+U=Q when U=E. However, in this question, when constant pressure, Q(=12750kJ)=W+U =/= H[=m(h2-h1)].

The Attempt at a Solution

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here's my calculation of enthalpy:

@75kPa,hf=384.44,hfg=2278,x=0.8==>h1=2206.4kJ

@225kPa, hf=520.71,hfg=2191,x=0.5614==>h2=1750.74kJ

total enthalpy change=m*(h2-h1)=11.22*(2206.4-1750.74)=5110.3kJwhich it is not equal to total heat change in constant pressure...why?

Thank you very much!

 

[yecko]

ok... somebody else answered me and solved