Slap and smack are the same and can be used for anything .Slap is probably most most common for hitting the face , for example " I slapped him" they most likely got hit in the face. While to say " I smacked him" isn't as specific and could be anywhere . But anyways , the difference between them is so small, it doesn't matter which one you use اتمنى افدتك بأجابة وبالتوفيق