Question

Air initially exists in an unconstrained piston-‐‐cylinder assembly at 200 kPa, 20°C, and 0.02 m3. The air is heated until the volume is doubled. Determine the following:
a. Mass of air in the piston-‐‐cylinder (kg).b. Pressure of the air at the end of the process (kPa).c. Temperature of the air at the end of the process (K).d. Boundary work (kJ).e. Heat transfer (kJ).

Answer 1

@phi

Answer 2

This is a closed system and I am assuming an ideal gas. Also T1 = T2 so this would be a isothermal process?

Answer 3

I'm an ee not a chem e

Answer 4

Oh thought you were ME. I am ME.

Answer 5

@Elsa213

Answer 6

@Photon336

Answer 7

I assume no air leaks in or out, so after heating the air, you still have the same mass ?

Answer 8

close system means no mass will cross the boundary.

Answer 9

PV=nRT

so you can find the number of moles of gas.

Answer 10

or PV = mRT. I used this to solve for mass m = R/M

Answer 11

I am thinking about part b now.

Answer 12

R being the gas constant and M is molecular mass

Answer 13

yes, which I assume we know for a standard atmosphere.

Answer 14

for part b could I use PV = mRT again instead P = (Pf - Pi), Pi = P_assembly?

Answer 15

Do I need to factor in P_surroundings? This is the confusing part. It doesn't say we have P_surrounding but do I just assume this?

Answer 16

wait piston is unconstrained would result in no atm pressure

Answer 17

we have to get rid of one of the variables. so it makes sense P stays the same

Answer 18

Pf = Pi? Because it's unconstrained? Heat is being applied to the piston cylinder thus expanding the gas and the gas is doing work (work being done by the system which results in positive work). If the gas is expanding than pressure will be the same at any point?

Answer 19

sounds right.

Answer 20

if P1 = P2 then this is an isboaric process. i think

Answer 21

if the pressure goes up, the piston will move until the pressure equalizes with the outside.

Answer 22

@Photon336

Answer 23

do I need to consider Psurr? The piston is unconstrained which makes me believe there is no Psurr acting on the piston.

Answer 24

But then I could be wrong...

Answer 25

I would think cylinder assembly at 200 kPa
means it is in equilibrium with the outside at that pressure.
and it will stay at that pressure. If the cylinder is free to move (and no friction)
then if the outside pressure was more, the cylinder would close. If the outside pressure was less, the cylinder would expand. But it is in equilibrium, so the outside is also 200 kPa