What is the overall order of the reaction for rate = k [H2] [NO]^2?

• A. 3
• B. 2
• C. 4
• D. 1

Answer: (A)

Overall order equals the sum of the exponents in the rate law. Here the rate depends on H2 to the first power and NO to the second power, so it is first order in H2 and second order in NO. Adding those gives a total order of 3 (third order). You can check the idea by seeing how the rate changes with concentration: doubling H2 alone doubles the rate, doubling NO alone quadruples it, and doubling both multiplies the rate by 8, which matches a third-order dependence.