In some cases you can interchange their use, but only when talking about being free of restraints, restiction, or control. Otherwise the words are too different to interchange their use. The best way is to look up both words in a dictionary (they each have quite a few meanings), and then to look up each word in a thesaurus (they each have quite a few synonyms and antonyms). You'll get lots of results both times, and I'm sure you'll find it easy to judge the uses from there. I'm sorry, but I just can't think of any 'rules' to use to guide your choice in what word to use, except to follow what the thesaurus and dictionary suggests.