Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: cofi on April 19, 2006, 07:18:25 AM
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1. A sample of carbon dioxide of mass 2,45g at 27°C is allowed to expand reversibly and adiabatically from 500 cm3 to 3.00 dm3. What is the work done by the gas?
2. A sample of 4.0 mol O2 is originally confined in 20 dm3 at 270 K and then undergoes adiabatic expansion against a constant pressure of 600 Torr until the volume has increased by a factor of 3.0. Calculate q, w, deltaT, deltaU and deltaH. (The final pressure of the gas is not necessarily 600 Torr.)
I've been trying to solve these problems for about 2 hours... I just can't find the right formula to start with...
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In this case dU = -dW
Remember that for an adiabatic process,? T1V1(gamma - 1) = T2V2(gamma - 1)
You can determine the final temp from here ( I think gamma for CO2 is given in your thermodynamics book ), and then find dU.
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In this case dU = -dW
I believe this should be dU = dW. Then, a decrease in temperature would result in a negative sign for work, indicating work being done by the system through expansion.
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dU = -dW is correct taking into consideration the work done by the gas on the surr.
dWint = - dWext
dWext = - PdV , dWint = PdV
( this can be proven using vectors )
dU = -dWint + dq ---> dU = - dWint = - PdV
dU = dWext +dq ---> dU = dWext = - PdV
The expression of dU depends on how u define the work ... but it's the same value.
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cofi: what data are you provided?