Idiomatic expressions (like "come in handy") have to be learned as complete pieces, because they're, well, idiomatic -- they can't be predicted from simple rules. I remember studying Spanish idiomatic expressions in Spanish classes I took years ago. Chunks, as described in this article, seem to be perhaps somehow more general than idioms; evidently, chunks seem to span across different languages and capture a broader range of constructs. However, most of the examples given could be considered examples of idioms. Thus it's not clear to me exactly how the two concepts differ in practice.