Question 51 in the homework [ENDORSED]

[104422816]

15.51 The following mechanism has been proposed for the reaction between nitric oxide and bromine:
Step 1: NO -> Br2 + NOBr2 (slow)
Step 2: NOBr2 + NO + NOBr -> NOBr (fast)
Write the rate law for the formation of NOBr implied by this mechanism.

The explanation for the answer: k(NO)(Br2) says that this is a second order reaction but i thought second order equation is (a)^2

[Reine Nakamura 1C]

It's a second order reaction because the overall order of the reaction is the sum of the orders for [NO] and [Br2]. In this case both are first order, making the overall order 2.

[Jade Fosburgh Discussion 2C]

Why is the speed of the rxn relevant in this problem? (slow vs fast)

[Chem_Mod]

This is to show that the first step is the most important overall for determining reaction rate, as it is slower than the second, making it rate-limiting.

[Dang Lam]

This is to show that the first step is the most important overall for determining reaction rate, as it is slower than the second, making it rate-limiting.

so which reaction (slow or fast) should we use to write our rate law?