This will depend on your presentation style and the specifics of the assessment task, always try to answer the question/aims of the assessment task required and ask for a marking criteria if one is not given. -presentation tips would depend on whether you use things like Prezi or you want to go with the more traditional power point presentation,regardless, my top tips are
1)make sure you have a consistent background or formatting-don't try to use different backgrounds for each slide and don't try to have different fonts for each slide-you can have variation on your transitions, but don't make it too diverse-otherwise people will get dizzy watching your presentation
2)make sure you have less text on the screen than you talk about in your speech-your screen should be an outline of your points-don't read off the screen.
3)remember to have pictures whenever possible-keep the audience engaged-whether it is clip art or actual pictures related to the chemistry topic you are presenting.
4)make sure there is a strong contrast between your text colour and background so it is easy to read-also make sure you don't have curly text-far too difficult to read
For some more general tips about how to ace oral presentations, feel free to check out my guide
http://community.boredofstudies.org/4/english/318818/how-ace-oral-presentations.html
Hope this helps