Which process is acceptable in nature: a reversible or an irreversible process?

For a reversible process,the necessary condition is:

In a thermodynamic process of uncontrolled change satisfying the equation $Q=W=0$,where $Q$ is heat supplied and $W$ is work done,the process is:

Following are statements of a few processes taking place in nature:
$I$. Free expansion of a gas
$II$. The combustion of a mixture of petrol and air ignited by a spark
$III$. The leaking of gas from the kitchen cylinder
$IV$. The transfer of heat from one heated part of a liquid to the other colder part
Which amongst these processes are irreversible in nature?

$A$ process can be reversible if

If a Van der Waals gas expands freely,then the final temperature is: