Two moles of an ideal gas are taken from state A to state B by the following two processes.
• Process 1: A reversible isothermal expansion at 400 K.
• Process 2: An adiabatic expansion followed by heating at constant volume until state B is reached.
Explain why the entropy change of the gas has exactly the same value for Process 1 and Process 2, even though the quantities of heat transferred are different.