CHEMISTRY COMMUNITY

For practice example 8.5 on page 274, why did we assume that delta U was 0 in the second step of part B?

Delta U=0 for the isothermal expansion of an ideal gas.

Path A is a reversible expansion and Part B is an irreversible expansion but both still take place isothermally.

Even as an ideal gas expands isothermally, its molecules continue to move at the same speed, so the kinetic energy doesn't change. Also the total potential energy of these molecules is the same because there aren't any intramolecular forces. Since neither the total kinetic energy nor the total potential energy changes, the internal energy of the gas doesn't change, making delta U = 0 for isothermal expansions of ideal gas.