In a free expansion of an ideal gas, the gas is initially in one side of the container, and when the stopcock is opened, the gas expands into the previously evacuated half which as shown in below figure. No weights can be raised in this process results no work is done. The process is adiabatic since the container is insulated. Thus W = 0 and Q = 0. So, according to first law of thermodynamics the internal energy will be zero.
The internal energy of an ideal gas undergoing a free expansion remains constant. This signifies that the temperature of the ideal gas does not change in a free expansion. But the internal energy of a real gas undergoing a free expansion does not remain constant results a change in temperature will occur in the expansion of real gas. That is why real gases always cool when making a free expansion, whereas an ideal gas does not.