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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: spacemonkey on December 14, 2018, 10:46:18 AM

Title: Rate Law Final Exam Question
Post by: spacemonkey on December 14, 2018, 10:46:18 AM

Hello forum - I was hoping you could tell me if my professor is correct here. I lost 20 points out of 250 on this question, but from my understanding my answer was correct, as he asked for the rate law of the mechanism rather than the final reaction. Normally I wouldn't care, but this question literally cost me my A in the class. I only needed 10 more points on the final to get an A, so I just wanted to be thorough and make sure he didn't screw me here (I got the impression that he didn't like me very much).

(https://i.imgur.com/R6cY1kQ.jpg)

https://imgur.com/a/2Vp2DPN (https://imgur.com/a/2Vp2DPN)

Title: Re: Rate Law Final Exam Question
Post by: mjc123 on December 14, 2018, 12:53:26 PM

I'm afraid you're wrong. It asks for the rate law [sc. of the reaction, what other kind is there?] consistent with the proposed mechanism, not the "rate law of the mechanism", whatever that may mean. Your equation is right for the slow step taken alone, but it is not legitimate as a rate law because it involves an intermediate, and you need the rate law in terms of the starting materials. As the first step is a fast equilibrium between NO, Br₂ and NOBr₂, you can express [NOBr₂] in terms of [NO] and [Br₂], and this is what you should have done.