Question

If an ideal gas, at constant temperature and pressure, expands, then its

• A. entropy increases and then decrease
• B. internal energy increases
• C. internal energy remains the same
• D. internal energy decreases

Answer 1

Answer: (B)

internal energy increases

Internal energy at any state is a function of the number of moles n and the temperature. If the volume expansion takes place at constant temperature and pressure we see that using the gas equation $PV=nRT$, we get $V\propto n$ and thus the number of moles of gas is increasing(maybe the gas is being introduced by an external agent), thus internal energy is increasing.