Final answer:
The overall order of the reaction, given by the rate law rate = k[Cl₂][NO]², is third order because the sum of the exponents of the concentration terms is 3.
Step-by-step explanation:
The rate law for the reaction of chlorine with nitric oxide is given by the expression rate = k[Cl₂][NO]². The orders of reaction are determined by the exponents on the concentration terms in the rate law. For Cl₂, the order is 1, since there is no exponent written, which means it's to the first power. For NO, the order is 2, as indicated by the squared term. To find the overall order of the reaction, we add the exponents: 1 (for Cl₂) + 2 (for NO), which gives us a total of 3. Therefore, the reaction is third order overall.