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Self Test 8.7A

Posted: Thu Jan 25, 2018 3:23 am
by Nha Dang 2I
In an exothermic reaction at constant pressure, 50 kJ of energy left the system as heat and 20 kJ of energy left the system as expansion work. What are the values of (a) deltaH and (b) delta U for this process?

Answer: (a) 50 kJ (b) 70 kJ

Why don't you include the energy that left the system as expansion work in delta H?

Re: Self Test 8.7A

Posted: Thu Jan 25, 2018 9:18 am
by Hyein Cha 2I
At constant pressure, q = enthalpy. Thus, enthalpy only includes the "heat" part.

Re: Self Test 8.7A

Posted: Thu Jan 25, 2018 6:44 pm
by Pooja Nair 1C
The formula for deltaH is deltaU + P*deltaV or work + heat + P*deltaV. At constant pressure, pressure internal and pressure external are the same so work (aka -Pex*deltaV) = P*deltaV. Thus the two values cancel out, and deltaH = heat