# How to identify if a chemical equation has a 'fractional order'?

Consider the following reaction: $$\ce{CHCl3 + Cl2 -> CCl4 + HCl}$$ The rate law expression is: $$\text{Rate} = [\ce{CHCl3}]\cdot[\ce{Cl2}]^\frac{1}{2}$$ with an order of $1.5$. But 'how can we determine' this fact? And why is it $1.5$ and not $2$?