Page 1 of 1

Midterm Question 5 (Step 2)

Posted: Tue Mar 06, 2018 3:05 pm
Why is the internal energy positive if when we calculated it in step 1 it was negative?

Re: Midterm Question 5 (Step 2)

Posted: Tue Mar 06, 2018 3:39 pm
I'm not really certain what you mean by internal energy being positive, but here's how I solved Q5:

Because step one concerns an adiabatic process, q = 0, and consequently internal energy in step one = work. But the second step returns the internal energy to its initial state before step one happened, and since internal energy is a state function, overall(both step one and step two) deltaU = 0. The overall change of internal energy of the system = internal energy change in step one + internal energy in step two.

0 = work in reaction one + heat gain/loss in reaction two + work in reaction two.

From there you just plug in the information given to you to calculate the volume.

Re: Midterm Question 5 (Step 2)

Posted: Sun Mar 11, 2018 10:42 pm
The question states "as it returns to the original internal energy". It was calculated, in step 1, that the change in internal energy was -158 so to return to the original internal energy, step 2 internal energy would have to be the opposite, +158J.