Q20 / 150 · MHT CET 2026 (20 April Shift 2) · Chemistry
Which of the following processes exhibits the decrease in internal energy of a system?
- AIsothermal expansion of an ideal gas.
- BIsothrmal compression of an ideal gas.
- Correct answerCAdiabatic expansion of an ideal gas.
- DAdiabatic compression of an ideal gas
Solution
According to the first law of thermodynamics, $\Delta U = q + w$.
For an isothermal process, the temperature remains constant ($\Delta T = 0$). Since the internal energy of an ideal gas depends only on temperature, $\Delta U = 0$ for both isothermal expansion and compression.
For an adiabatic process, there is no heat exchange with the surroundings, so $q = 0$. The first law becomes $\Delta U = w$.
During adiabatic expansion, work is done by the gas on the surroundings, meaning $w < 0$. Consequently, $\Delta U < 0$, which indicates a decrease in the internal energy of the system.
During adiabatic compression, work is done on the gas, meaning $w > 0$, leading to an increase in internal energy.
Answer: Adiabatic expansion of an ideal gas.
For an isothermal process, the temperature remains constant ($\Delta T = 0$). Since the internal energy of an ideal gas depends only on temperature, $\Delta U = 0$ for both isothermal expansion and compression.
For an adiabatic process, there is no heat exchange with the surroundings, so $q = 0$. The first law becomes $\Delta U = w$.
During adiabatic expansion, work is done by the gas on the surroundings, meaning $w < 0$. Consequently, $\Delta U < 0$, which indicates a decrease in the internal energy of the system.
During adiabatic compression, work is done on the gas, meaning $w > 0$, leading to an increase in internal energy.
Answer: Adiabatic expansion of an ideal gas.
Hence, the correct answer is option: 'C'.