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### Topic: Thermo- First Law Question: Finding energy change in Kcal.  (Read 1917 times)

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#### zerothedestroyer

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• Mole Snacks: +0/-0 ##### Thermo- First Law Question: Finding energy change in Kcal.
« on: June 04, 2010, 10:21:47 AM »
3. Thermo-Firstlaw a (Points: 1)
Gas in a piston-cylinder apparatus is heated with 1.500 kcal while simultaneously increasing its volume from 0.500 L to 1.500 L at a constant pressure of 1.000 atm. What is the total energy change for this process? Pay attention to units.

1. +1.524 kcal

2. 0 (the work and heat contributions cancel out)

3. +1.476 kcal

4. -1.476 kcal

5. -1.524 kcal

Step 1. I know that I have to find work done on system and work done to system. Is that where I start off?

It is asking What is the total energy change for this process? So that's like

ΔU = Q - W,

where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done by the system. of course, when work is done to the system (like your problem), W is negative, so that -W is positive, resulting in an increase in internal energy (like your problem):

How would I convert 0.500 L to 1.500 L at a constant pressure of 1.000 atm to Kcal and then use the Delta U=Q-W equation to solve the change?

#### tamim83 ##### Re: Thermo- First Law Question: Finding energy change in Kcal.
« Reply #1 on: June 05, 2010, 10:47:13 AM »
You need an equation for work done by an expanding gas.  This will allow you to calculate the work done.

Also, here are some conversions that may help

1 L-atm = 101.325J
1cal = 4.184J