Question

1. A gas is confined to a cylinder which is attached to a second, evacuated, container of the same size. When the valve is opened between the containers, the gas makes a free expansion. The entire system is well insulated so that no thermal exchange with the surroundings rakes place.

   If the gas is a real gas, its temperature during the free expansion would be expected to _________. If the gas is an ideal gas, its temperature during the free expansion would be expected to _______.

   	a.	rise; drop
   	b.	drop, remain the same.
   	c.	remain the same; remain the same
   	d.	rise; remain the same

Answer 1

Answer is option (b).drop,remain the same.

For Ideal gas

In free expansion a gas is kept in one side of the vessel and other side is evacuated.On removing the valve the gas is allowed to expand against vacuum and fills whole vessel.So no external work is done by the gas.The vessel is isolated from the surrounding.so, no exchange of heat occurs.So, dQ=0.And workdone also dW=0.

From the first law of thermodynamics,

dQ=dU+dW ................(1)

So Now ,dU=0.

Since Internal enegy is function of temperature only.so if internal energy U remains constant,then temperature T also remains constant.

For Real gas

Free expansion of real gas causes cooling. Here Work done in free expansion of real gases is not zero as there is a force of attraction between molecules of real gas.So free expansion of real gas reduces temperature.