Honestly, there really isn't anything to worry about. I was stressing out so much for the whole day but they put you so at ease right before and during the interview its fine. It was my first proper interview too so I know exactly how you feel right now.
I spoke a lot about organic chemistry and drug synthesis in my personal statement (I mentioned aspirin) so my interviewer asked me to draw the molecule. He then asked me to circle and name the functional groups in aspirin.
Continuing on the theme of functional groups he drew out a compound that I had never seen before using skeletal formula and asked me to name the functional groups. I couldn't name one of them but he was like don't worry, I'll let you think about it for a bit and ask you something else in the meantime (couldn't figure it out in the end but it was fine, he was really cool about it). The compound he had drawn was superglue and he asked me if I knew what made superglue superglue. Then asked me what allowed a monomer to become a polymer.
The last thing he asked me was why I wanted to do chemistry and why Leeds. I kind of when on a really weird tangent and he was like honestly I know exactly what you mean, you are speaking to someone who decided to do chemsitry for many of the same reasons. So basically whatever your reason for doing chemistry they will love it. But mentioning something about practices is really good.
My dad came with me on the day and when we (the prospective students) left to go to lunch he talked with the admissions tutors and they explained that normally all the people who have been asked there for an interview had a place, unless they see that that person completely lied on their personal statement, and that this day is more for them to convince you to go than the other way around. It will not affect the offer they will give you but can affect whether or not they will give you a spot if you miss the bsc offer by a grade or something like that as they will have seen whether you seem to know your stuff or not and if you just kinda messed up in the exam even if you understand.
So good luck with your interview and I'm sure you're going to do well. Don't over think it and really just be yourself.