ShmoopTube

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And here’s your Shmoop du jour, brought to you by humps, also known as “lovely science [Camel appears in a desert]

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lumps.”

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Okay, here’s our question:

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Which of the following is true according to the following reaction profile?

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And here are your potential answers: Okay, so though it may just look like a random [Boy drawing]

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squiggle a kindergartener drew, what you’re looking at is actually our reaction profile

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curve, the focus of today’s problem.

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Figure out what the deal with this curve is and we figure out the problem.

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If only that was the way all problems in life worked… [Woman discussing problems with a reaction curve]

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Believe it or not, a reaction profile isn’t the side view of a reaction.

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It’s a graphical representation of the energy pathway through a chemical reaction. [Example of a reaction profile graph]

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The horizontal axis shows the reaction progress, which means how close the reaction is to completion.

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Just like in a chemical reaction equation, the left side of this graph represents the

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reactants, and the right side represents the products. [Graph showing reactants and products of a reaction]

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So as long as you’re used to reading left-to-right, it should be pretty easy.

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Sorry, manga enthusiasts and Hebrew-speakers.

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Anyway, the vertical axis is the energy of the reactant and product species.

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You can think of it as the species in this reaction riding that curve like a roller coaster [People riding a roller-coaster]

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as they’re transformed from the reactants into the products.

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Woo!

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Hope you didn’t eat a big lunch.

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Since there are two humps here, we know that the reaction occurs in two steps. [Two humps of reaction curve highlighted]

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And that maybe the kindergartner who drew this doodle is a camel-lover.

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Anyway, the three local minima represent the reactants, intermediates, and products.

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Each local maximum represents a transition state.

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So let’s dive back into those answers. [People dive into a river]

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Is this an endothermic reaction?

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An endothermic reaction is a reaction that absorbs energy as heat from the surroundings.

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From the reaction profile curve, we can see that the energy of the products is lower than

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the energy of the reactants.

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That means that the reaction releases energy into the surroundings in the form of heat. [Exothermic reaction definition]

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Which means two things: It’s great to take on ski trips, AND it’s

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exothermic, not endothermic.

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So we can give option B the cold shoulder. [Letter B zapped and frozen]

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Is the first step of the reaction the rate-determining step?

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Well, out of this reaction’s two steps, the rate-determining step will be the one

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that has the highest activation energy barrier to overcome.

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The barrier to the first step is the difference in energy between the reactants and the first

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transition state.

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The barrier to the second step is the difference in energy between the intermediates and the

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second transition state.

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In this case, the second barrier is larger, so the second step is rate-determining. [Finger points to second transition state]

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Looks like we’ve just determined that option C is not our guy.

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How about D?

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Was a catalyst used to decrease the activation energy of the reaction? [Catalyst in court with a judge]

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Well, we have no way to tell whether a catalyst was already involved in this reaction or not.

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Not enough information, not our answer.

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So that means that A - “There is an intermediate formed in the reaction,” is the correct

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answer.

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We figured that out like a minute ago, but it’s good to consider the other options

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just to be sure.

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Well…that and we didn't have any other cool plans for today, and we figured we'd force [Two guys sitting on a sofa]

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you to spend more time with us.

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That's what friends do, right?

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…Right?

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