Question

Assertion (A): $\Delta U=0$ for a reversible as well as irreversible expansion of an ideal gas under isothermal conditions, whereas $S_{total}\ne 0$ for an irreversible process.

Reason (R) : $\Delta U=0$ is independent of temperature whereas $S$ is proportional to temperature.

• A. The Assertion is correct but Reason is not correct
• B. The Assertion is not correct but Reason is correct
• C. Both Assertion and Reason are correct and Reason is the correct explanation of the Assertion
• D. Both Assertion and Reason are correct but the Reason is not the correct explanation of the Assertion

Answer 1

The correct option is A The Assertion is correct but Reason is not correct

The Assertion is correct, but the Reason is not correct.

An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange.

In contrast, an adiabatic process is where a system exchanges no heat with its surroundings (Q = 0). In other words, in an isothermal process, the value ΔT = 0 and therefore ΔU = 0 (only for an ideal gas) the change in entropy for a system and its surrounding is always positive for an irreversible process.

In general, the total entropy - and therefore the disorder - always increase in an irreversible process. Keeping these considerations in mind, we can state the second law of thermodynamics as follows:

The total entropy of an isolated system that undergoes a change can never decrease. Furthermore, if the process is irreversible, then the total entropy of an isolated system increases. In a reversible process, the total entropy of an isolated system remains constant.