Question

Which one of the following equations does not correctly represent the first law of thermodynamics for the given processes involving an ideal gas? (Assume non-expansion work is zero)

• A. Cyclic process: \(q = -w\)
• B. Adiabatic process: \(\Delta U = -w\)
• C. Isochoric process: \(\Delta U = q\)
• D. Isothermal process: \(q = -w\)

Hint:

In Chemistry competitive exams, always use the IUPAC sign convention (\(\Delta U = q + w\)) unless specified otherwise. In this convention, work done on the system is positive.

Answer 1

The Correct Option is B

To determine which equation does not correctly represent the first law of thermodynamics for the given processes involving an ideal gas, let us first understand the conditions under each type of process mentioned:

1. Cyclic Process: In a cyclic process, the system returns to its initial state. This implies that the change in internal energy (\(\Delta U\)) is zero. According to the first law of thermodynamics, which states that \(\Delta U = q + w\), the equation simplifies to \(q = -w\). Therefore, this representation is correct for cyclic processes.
2. Adiabatic Process: In an adiabatic process, there is no heat exchange with the surroundings (\(q = 0\)). For an adiabatic process, the first law of thermodynamics becomes \(\Delta U = w\). The given option states that \(\Delta U = -w\), which is incorrect. Thus, this equation does not represent the adiabatic process correctly.
3. Isochoric Process: In an isochoric process, the volume remains constant, which means no work is done (\(w = 0\)). The first law of thermodynamics is \(\Delta U = q\), which matches the given option. Therefore, this representation is correct.
4. Isothermal Process: An isothermal process occurs at constant temperature, meaning the change in internal energy (\(\Delta U\)) for an ideal gas is zero. According to the first law, \(q + w = 0\), which simplifies to \(q = -w\). This correctly represents an isothermal process.

Thus, the incorrect representation based on the options provided is the one for the adiabatic process: \(\Delta U = -w\).