Zero, First, and Second Order reactions

[Maayan Epstein 14B]

I still a bit confused on what exactly order of reaction means and how this interacts with different species. For example, in the reaction:
H2 + I2 ----> 2HI. Why is this a second order reaction? Is it because each reactant has a coefficient of 1 and 1+1 equals 2, or because the product has a coefficient of 2? In other words, do you determine order based on reactants or on products or on both?
Thanks!

[Alexa Tabakian 1A]

Yes, it is a second order reaction because the exponents that you multiply by in the rate equation add up to 2.

[Matthew Choi 2H]

Instead of looking at the molar coefficients in the reaction, you would only use the exponents of the reactants to determine the overall order of the reaction. So, when you write the rate law of the reaction, H2 and I2 will each have an order of 1 which you add up to get an overall order of 2.