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The rate of a reaction between A + B was followed spectroscopically at three different wavelengths, and the percent absorption at each wavelength was plotted as a function of time. Which of the following mechanisms is consistent with the spectral data?

  • (a) $\ce{A + B -> C}$
  • (b) $\ce{A + B -> D, D -> E}$
  • (c) $\ce{A + B -> F, A + B -> G}$
  • (d) $\ce{A + B -> H, H + A -> I}$
  • (e) none of the above

The correct answer is (b), which I can believe, but not sure why.

So, it can't be (a), because in $\lambda_2$, the curve went up then down, indicating the presence of an intermediate. However, I'm not sure why this can't be (d) or (c) as well. As long as F, G, and H are active at $\lambda_2$, then wouldn't all of the reactions show a similar curve?

I'm not sure what to do with $\lambda_1$ and $\lambda_3$, as I think they just tell me that A + B is consumed, and that the product and/or intermediates aren't active at either $\lambda_1$ or $\lambda_3$.

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Let's see. You are right in that it can't be (a). Now consider (c): A and B are consumed, while F and G accumulate. Where is that intermediate that goes up, then down?

As for (d), I believe there is no way we can tell that from (b) based on the picture alone, without looking at the numbers. See, suppose $\lambda_1$ actually measures B, and A is in huge excess; what would be the qualitative difference between the two? I can't see any.

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