I've picked these as they're generally broad topics which are under-represented here on chem.SE at present, they're also both interesting topics that can be appreciated by beginners and chemists alike.
As previously, it would be great if people could get involved and suggest new topics by dropping a post on the original proposal.
In terms of what questions you can ask, anything goes as long as it's on-topic and broadly related to one of the chosen tags. Feel free to ask one/several questions on either/both, but remember that this is about encouraging quality questions, so lets not just add to the 'close' queue!
For any questions I think need or deserve it, I'll add (or try and coordinate) some bounties and in two weeks time I'll post an answer here with the stats on how many questions have been asked on these tags, how many answers those questions have had and anything else notable.
Remember to tag the question with the relevant tag (there is no specific topic challenge tag, and theres no need to try and make one).
If you have any questions, you can ask them here or bring it up in chat.