A 80.0 cm3 box contains helium at a pressure of 1.90 atm and a temperature of 90

Question

A 80.0 cm3 box contains helium at a pressure of 1.90 atm and a temperature of 90.0 C. It is placed in thermal contact with a 200 cm3 box containing argon at a pressure of 4.00 atm and a temperature of 420 C.

What is the initial thermal energy of each gas?

Enter your answers numerically separated by a comma.

What is the final thermal energy of each gas?

Enter your answers numerically separated by a comma.

How much heat energy is transferred, and in which direction?

What is the final temperature?

What is the final pressure in each box?

Enter your answers numerically separated by a comma.

Explanation / Answer

a.
Thermal energy of an ideal gas is given by:
U = nCvT
(n number of moles, Cv molar heat capacity at constant volume , T absolute temperature)

The molar heat capacity of monatomic ideal gases like helium and argon is:
Cv = (3/2)R

The number of moles of each gas can be found from ideal gas law:
pV = nRT => n = pV/(RT)

-helium
n = pV/(RT)
= 1.9*1.01*105*80*10-6/8.314*(90+273)
= 5*10-3mol
-argon
n = pV/(RT)
= 4*1.01*105*200*10-6/8.314(420+273)
= 1.4*10-2mol

So the initial energies are:
-helium
U = (3/2)nRT
= (3/2) *5*10-3*8.314*363
= 22.63J
-argon
U = (3/2)nRT
= (3/2) *1.4*10-2*8.314*693
= 121J


b.
Assuming threre is only energy exchanged between the boxes, the total internal energy is conserved. So the sum of the final thermal energies equals the sum of the initial energies:
U' + U' = U + U

On the other hand both boxes have same final temperature final T' at equilibrium:
U' = (3/2)nRT'
U' = (3/2)nRT'
hence:
U'/n = U'/n
<=>
U' = (n/n) U'

substitute to energy balance
U' + (n/n) U' = U + U
=>
U' = (U + U) / (1 + (n/n))
= (22.63J + 121J) / (1 + (1.4*10-2mol/5×10-3mol) )
= 37.8J
=>
U' = U + U - U'
= 22.63J + 121J - 37.8J
= 105.83J


c.
The heat energy transferred is equal to the absolute value of thermal energy change in each box:
U = U' - U = 37.8J - 22.63 = 15.17J

U > 0 , so the thermal energy of the helium has risen, while argon's thermal energy has dropped by the same value. That means heat is transferred from argon to helium.


d.
U' + U' = U + U
<=>
(3/2)nRT' + (3/2)nRT' = U + U
=>
T' = (U + U) / ((3/2)R(n + n))
= (22.63J + 121J) / ((3/2)8.314(5*10-3mol + 14×10-3mol))
= 606K
= 333°C


e.
since n and V is constant in each box
p/T = nR/V = constant
<=>
p' = p(T'/T)

=>
p' = p(T'/T)
= 1.9atm (606K/ (90+273)K)
= 3.172atm
p' = p(T'/T)
= 4.0atm (606K/ (420+273)K)
= 3.497atm

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