They are interchangeable.

Here are a couple examples of using these words in different ways: "That was a knee slapper" is a phrase which refers to the reaction a person has to a joke, they slap their knee with their hand. "She smacked her lips" is a phrase meaning a noisy way to close and open your lips. Slapping is typically an action performed with the hand or something that reminds you of that action. I could use both in a sentence like "the board slapped to the ground with the sound of a smack". Suppose my face was red in color and I was asked "What happened". If I respond with "I was slapped in the face" versus "I was smacked in the face" the difference is, in the case of "slapped" I would assume by someone's hand, but with "smacked" it could have been by hand but also a tree branch or something else that hit me in the face. In the first case I might ask "Who hit you" where in the second case I might ask "What hit you". In the case of hitting a person, as you said, slap is specifically an open hand. My mother-in-law has a funny phrase which sounds like it was used over 50 years ago (not something you'd hear today), it goes "I'll smack you so hard it'll jar your kinfolk". Which means the hit will be so hard that your relatives will feel it. "Wallop" is another fun one that means a hard hit though this is also an older word and I can't recall the last time I heard someone say it.

You could say " His Mum smacked him for his bad behaviour". "His Mum gave him a slap for his bad behaviour" but if you were to say "I smacked him", you had better be his Father or guardian... ;p A smack or a slap are synonyms of hit, so word usage would depend on the style of language you choose to use.

A slap would be usually on bare skin, like on the cheek. But a smack could be anywhere, like on the shoulder or stomach. Slap more refers to the sound that a slap makes when you hit an open hand on a flat surface. Just my interpretation as a native english speaker.

Yes, this is correct. "She smacked him on the head." "I'm going to smack you..." Interestingly, smack can also mean kiss. "She gave him a smack on the cheek." Smack can also mean to chew very loudly. "I can't stand when people smack their gum."