Question

During isothermal expansion of an ideal gas, its

• A. internal energy increases
• B. enthalpy increases
• C. enthalpy reduces to zero
• D. enthalpy remains unchanged.

Answer 1

Answer: (D)

enthalpy remains unchanged.

Answer 2

An isothermal process is a change of a system, in which the temperature remains constant. For an ideal gas during an isothermal expansion the enthalpy, as well as internal energy, remains constant.
During isothermal expansion of an ideal gas, $\Delta E =0, \Delta T =0$
From the definition of enthalpy,
$H = E + PV$
or $\Delta H =\Delta E +\Delta( PV )$
or $\Delta H =\Delta E +\Delta( nRT )$
Since, $PV=nRT$ for an ideal gas
or $\Delta H =\Delta E + nR \Delta T$
or $\Delta H =0$
Change in enthalpy is zero, means its enthalpy remains same or unchanged.