# Reversible adiabatic expansion of ideal gas, entropy change?

• krhisjun

## Homework Statement

One mole of an ideal gas at 0 celcius is subjected to changes below, calculate the change in entropy of the gas:

i) Gas is expanded reversibly and isothermally to twice its initial volume. DONE - 5.76 J/K
ii)A similar expansion to i. is performed reversibly and adiabatically [hint, for an ideeal gas undergoing reversible adiabatic expansion TV^(gamma - 1) = constant]

## Homework Equations

I know delta Q = -nRT Ln (Vf/Vi) for a reversible isothermal expansion

delta S = delta Q / absolute T

dE = dW + dQ

## The Attempt at a Solution

for a isolated system i know the entropy change would be zero, but as it doesn't state this i don't feel that i can just state "assuming system is isolated entropy change is zero"

Khris

## Homework Statement

One mole of an ideal gas at 0 celcius is subjected to changes below, calculate the change in entropy of the gas:

i) Gas is expanded reversibly and isothermally to twice its initial volume. DONE - 5.76 J/K
ii)A similar expansion to i. is performed reversibly and adiabatically [hint, for an ideeal gas undergoing reversible adiabatic expansion TV^(gamma - 1) = constant]

## Homework Equations

I know delta Q = -nRT Ln (Vf/Vi) for a reversible isothermal expansion

delta S = delta Q / absolute T

dE = dW + dQ

## The Attempt at a Solution

for a isolated system i know the entropy change would be zero, but as it doesn't state this i don't feel that i can just state "assuming system is isolated entropy change is zero"
The entropy change of the gas is not zero.

Work it out (Hint: you don't actually have to do any integration).

$$\Delta S = \int_1^2 dQ/T + \int_2^3 dQ/T$$

AM

i know
$$\Delta S = \int_1^2 dQ/T + \int_2^3 dQ/T$$
but dQ is zero for an adiabatic process isn't it?

Khris

The entropy change of the gas is not zero.

Work it out (Hint: you don't actually have to do any integration).

$$\Delta S = \int_1^2 dQ/T + \int_2^3 dQ/T$$

AM

Sorry to bring this up again, but doing a similar question...

Surely here dQ = 0 since the gas is expanded adiabatically?

The entropy change of the gas is not zero.

Work it out (Hint: you don't actually have to do any integration).

$$\Delta S = \int_1^2 dQ/T + \int_2^3 dQ/T$$

AM

heloooo? *bump*