Homework. n = 3.39 mol of Hydrogen gas is initially at T = 322 K temperature and
ID: 1454495 • Letter: H
Question
Homework. n = 3.39 mol of Hydrogen gas is initially at T = 322 K temperature and pi = 2.57×105 Pa pressure. The gas is then reversibly and isothermally compressed until its pressure reaches pf = 9.46×105 Pa. What is the volume of the gas at the end of the compression process?
How much work did the external force perform?
How much heat did the gas emit?
How much entropy did the gas emit?
What would be the temperature of the gas, if the gas was allowed to adiabatically expand back to its original pressure?
Explanation / Answer
What is the volume of the gas at the end of the compression process?
Use ideal gas law:
pf Vf = n RTf
=>
Vf = n RTf / pf
= 3.39 mol 8.3145 Pam³K¹mol¹ 322 K / 9.46×10 Pa
= 0.959×10² m³
= 9.59 L ...........Ans.
> How much work did the external force perform
Reversible work done on the gas is given by the integral
....... Vf
W = - p dV
....... Vi
from ideal gas law follows for an isothermal process
pV = nRT = constant
=>
p = nRT/V
Hence,
................. Vf
W = -nRT (1/V) dV = - nRT(ln(Vf) - ln(Vi)) = nRTln(Vi/Vf)
................. Vi
The volume ratio can be converted to a pressure ratio:
pV = constant
=>
piVi = pfVf
=>
Vi/Vf = pf/pi
Therefore
W = nRTln(pf/pi)
= 3.39 mol 8.3145 Pam³K¹mol¹ 322 K ln( 9.46×10 Pa/ / 2.57×10 Pa)
= 11.827 kJ ............Ans.
> How much heat did the gas emit?
The change in internal energy of the gas equals the hat added to it plus the work done to it:
U = Q + W
The internal energy for an ideal gas is given by:
U = nCvT
So if the temperature is constant, the change in internal energy is zero.
Hence,
Q + W = 0
=>
Q = - W = - 11.827 kJ ............Ans.
That means 11.827 KJ of heat are emitted by the gas.
> How much entropy did the gas emit?
Change in entropy due a transfer of heat Q at constant thermodynamic temperature T is given by:
S = Q/T
For the gas in this process
S = -11.827×10³ J / 322 K = - 36.73 JK¹ ............Ans.
That means 36.73 JK¹ are emitted.
> What would be the temperature of the gas, if the gas was allowed to adiabatically expand back to its original pressure?
For an ideal gas undergoing a reversible and adiabatic process:
pV^ = constant
with = Cp/Cv
The heat capacity ration for a diatomic ideal gas like hydrogen (H) is:
= 7/5
Relation above can rewritten in terms of pressure and temperature using ideal gas law:
V = n RT/p
=>
p(n RT/p)^ = C
<=>
p^(1-) T^ = C /(n R)^ = constant
<=>
T / p^[( - 1)/)] = [C /(n R)^]^[1/] = constant
for = 7/5
T / p^(2/7) = constant
<=>
T' / pi^(2/7) = T / pf^(2/7)
=>
T' = T (pi/pf)^(2/7)
= 322 K (2.57×10 Pa / 9.46×10 Pa)^(2/7)
= 221.89 K.................Ans.
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