Question

A student wrote:

If I expand an ideal gas from V1 to V2 under constant external pressure and pex, then the work done

is w = -p_ex(V2 - V1). As a result, if the expansion is isothermal, then as delta U = q + w = 0, then

q = -w = p_ex(V2 - V1). Therefore, delta S =q_rev/T=(p/T)(V2 - V1).

Is this statement correct or incorrect? Explain your answer.

Answer 1

This statement is false or incorrect. This is because for the first case the student is expanding under constant external pressure, but along with increase in the Volume of the gas (change in volume) we will see an increase in temperature (change in temperature). This is because according to the ideal gas law, V = nRT/P. Hence, changing volume results in change in temperature. Since, we have a change in temperature, the experiment cannot be considered isothermal. Hence, the statement is false.