Ok, here we go:
By consulting an English dictionary of definitions, I have found both terms:
Purpose: The quality of being determined to do or achieve something.
Reason: The capacity for rational thought or inference or discrimination.
By consulting an English-French dictionary, I could find the translation for both terms:
Purpose: But, résolution.
And finally, I give you an example of the use of both words in English:
- Her purpose is to help sick people.
- The reason for his behaviour was not clear.