It depends what subjects you really need for your future career.
To be honest, i really dislike Chemistry, mostly because i find the subject really dry and my teacher kinda isn't the enthusiastic type.
But Chemistry has it's upsides because there's a lot of Universities that make it a Prerequiste.
I reckon you should just pick Modern History rather wasting your 2 years of Senior School doing something you really hate.
Then you can do a bridging course at uni for Chemistry which only takes around 2 weeks of your life?