## Catalyst's effect on overall reaction

Arrhenius Equation: $\ln k = - \frac{E_{a}}{RT} + \ln A$

Rebecca Remple 1C
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### Catalyst's effect on overall reaction

Hi all,

In book problem 7.17, the textbook gives us the illustration of a reaction (A->D) and asks whether using a catalyst to accelerate the third step only would change the overall reaction rate. According to the solutions manual, it will not. I understand that the first step is the rate-determining step, but I'm having a hard time visualizing this reaction. Why would shortening the third step, even if it is already fast, not decrease the time it takes for this reaction to occur? If someone could explain this I would really appreciate it. Thank you! :)

-Rebecca

jisulee1C
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Joined: Thu Jul 25, 2019 12:17 am

### Re: Catalyst's effect on overall reaction

since the slow step is the rate determining step, changing the activation energy of an already fast step will not change the rate because it is not changing the rate of the slow step

Nick Fiorentino 1E
Posts: 102
Joined: Wed Sep 18, 2019 12:16 am

### Re: Catalyst's effect on overall reaction

One way to think of it is remembering the saying "a chain is only as strong as its weakest link". So, if you make the fast step faster, it won't change the fact that the slow step is still slow, which makes the reaction slow, or the "chain weak". In order to make the reaction faster, you must increase the speed of the slow step.

Rory Simpson 2F
Posts: 106
Joined: Fri Aug 09, 2019 12:17 am

### Re: Catalyst's effect on overall reaction

If the slow step is the rate determining step, then in order for a catalyst to speed up the reaction it would need to lower the activation energy of that slowest step. Changing the activation energy of the faster steps wouldn't do anything for the overall reaction because the rate is still determined by the slowest step.

Lauren Stack 1C
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Joined: Sat Aug 17, 2019 12:18 am

### Re: Catalyst's effect on overall reaction

In order to consider this reaction, you have to know that the slow step is the rate determining step. If you make the other steps faster, it still does not change the fact that the slow step is still at the same (slow) rate. As the slow step is the sole factor you consider when determining overall rate, the catalyst does not impact the overall rate as it does not impact the slow step rate.

805097738
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### Re: Catalyst's effect on overall reaction

jisulee1C wrote:since the slow step is the rate determining step, changing the activation energy of an already fast step will not change the rate because it is not changing the rate of the slow step

how do you determine the fast and slow step?

jisulee1C
Posts: 149
Joined: Thu Jul 25, 2019 12:17 am

### Re: Catalyst's effect on overall reaction

Usually it is written if it is a slow step but if not assuming it's this problem we are looking at the activation energies. The step that takes the most activation energy will be the slowest step because it requires a lot of energy to overcome the barrier.

ramiro_romero
Posts: 90
Joined: Sat Sep 07, 2019 12:16 am

### Re: Catalyst's effect on overall reaction

Rebecca Remple 1C wrote:Hi all,

In book problem 7.17, the textbook gives us the illustration of a reaction (A->D) and asks whether using a catalyst to accelerate the third step only would change the overall reaction rate. According to the solutions manual, it will not. I understand that the first step is the rate-determining step, but I'm having a hard time visualizing this reaction. Why would shortening the third step, even if it is already fast, not decrease the time it takes for this reaction to occur? If someone could explain this I would really appreciate it. Thank you! :)

-Rebecca

The third step does not determine the rate of the rxn. Therefore, shortening its activation energy has no effect on the rate (determined by the slowest step with the highest activation energy).

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