## Self Test 8.7A

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Nha Dang 2I
Posts: 53
Joined: Fri Sep 29, 2017 7:07 am

### Self Test 8.7A

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?

Hyein Cha 2I
Posts: 103
Joined: Fri Sep 29, 2017 7:05 am
Been upvoted: 1 time

### Re: Self Test 8.7A

At constant pressure, q = enthalpy. Thus, enthalpy only includes the "heat" part.

Pooja Nair 1C
Posts: 55
Joined: Thu Jul 13, 2017 3:00 am

### Re: Self Test 8.7A

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

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