Answer:
The difference is that free goods have exactly zero opportunity cost, because they cost nothing, their price is zero, and their use is non rival, and non excludable, meaning that if a person uses a free good, that person does not prevent any other person from using the good.
Economic goods, on the other hand, are goods that have a price, and that have a cost of production. Besides they are rival and/or exclusionary. The opportunity cost of an economic good varies depending on the good, but it is never zero.
