As a Units 3/4 chem student (now graduate), knowing the content from Units 1/2 is very crucial. There are several key concepts that don't get retaught in 3/4, and if you're coming into them without the prior knowledge it is going to cause a lot of stress.
Then again I believe the study design is changing so it might be changing in terms of the content crossover.
However based on my experiences I can only blame myself with how trash I did this year in chem and probably bombed the exam, because I didn't revisit concepts and felt so lost throughout this year. Alongside having to learn new formulas and concepts without having the prior knowledge led me to essentially give up not even a term in (horrible mindset I know).
It was only the support of my teacher and my friend in the class who basically got me my S's. When I saw I managed to get a 96 on an assessment it really brought up my spirits and had given me short-lived confidence in the subject. This part isn't really relevant but I think it helps contextualise the impact not knowing the content had for me in my graduating year.
Also for my fellow units 3/4 chem students...electrolysis can go die in a ditch.
