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Main content
Current time:0:00Total duration:7:30
AP.Chem:
ENE‑1 (EU)
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ENE‑1.A (LO)
,
ENE‑1.A.1 (EK)
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ENE‑1.A.2 (EK)

Video transcript

a catalyst is a substance that increases the rate of a reaction but it itself is not consumed in the overall reaction so let's look at the decomposition of hydrogen peroxide so h2 o2 is hydrogen peroxide and when it decomposes you get water and also oxygen this reaction occurs at room temperature however it's very slow so this is a slow reaction so to speed it up you need to add a catalyst and if you're doing a demonstration like the famous elephant's toothpaste demonstration in general chemistry you need to add a source of iodide ions so this is one of the catalysts that you could use so you could use potassium iodide or sodium iodide so you add a source of iodide anions and that makes this reaction fast so your iodide anion is your catalyst it increases the rate of a reaction let's take a look at the mechanism for the reaction when we add our iodide anion as our catalyst so in the first step of the mechanism you can see we have h2o 2 and our iodide catalyst and this forms the hypo iodide ion so this is our intermediate so the hypo iodide anion is our intermediate and we also are given the information that this first step of the mechanism is the slow step and the second step of the mechanism right we have another molecule of hydrogen peroxide reacts with our intermediates our hypo iodide ion and we get our oxygen and this step is fast remember for a mechanism a possible mechanism must have elementary steps that add up to the overall reaction so if we add our two steps together we should get our overall reactions we're going to add all of our reactants together so that would be h2o 2 plus I minus plus another h2o 2 plus I Oh minus and that should give us our product so our products circle all of them over here right we have h2o + i o- + h2o + o2 + i - so we have a lot going on there let's see what we can cancel out so what do we have on both sides well we can cancel out the iodide right that's on the left and that's on the right that's our catalyst it increases the rate of the reaction but it's not consumed you can see we're using it in the first step but the iodide anion is regenerated in the second step so overall our catalyst is not consumed and then we have our intermediate right the hypo iodide ion is on the left side and on the right side so we can cancel that out our intermediate is created in the first step but then it's consumed in the second step so what are we left with here we'll be left with 2 h2o - right so we have 2 h2o 2 for our reactants and then on the right we would have 2 h2o s right so two waters and also oxygen so plus o2 so we get back we get back our original reaction or overall reaction also a possible mechanism must be consistent with the experimental rate law for the overall reaction and we've seen how to do that in an earlier video to write your rate law you need to first recognize the rate determining step in your mechanism and the rate determining step is the slow step in a mechanism so step one is our rate determining step and we can write the rate law for the reaction from the rate determining step which we know is an elementary reaction right this is an elementary reaction it's by molecular so let's go ahead and write down our rate law right so the rate of the reaction should be equal to our rate constant K so whatever the rate constant happens to be for this alright times the concentration of H 2 O 2 so we have times the concentration of H 2 O 2 and since we have a coefficient of 1 remember for an elementary reaction we can turn that coefficient in to an exponent so we have to the first power and then we also have the concentration of iodide anions so the concentration of i- and once again our coefficient is a 1 here so since this is an elementary reaction we take our coefficient and turn it into our exponent and this is the rate law that we would predict using the rate determining step and this is in agreement with the experimental rate law for our overall reaction so this is a possible mechanism for our overall reaction the decomposition of hydrogen peroxide so how does our catalyst actually increase the rate of our reaction well let's look at an energy profile for our uncatted eyes 'dry action so the conversion of hydrogen peroxide into water and oxygen and here's the uncatalyzed version we start with a certain energy for our reactants and then we know at the top here that represents the energy of the transition state and the difference between those two would be our activation energy right so that represents our activation energy for our uncatted eyes 'dry action when we add the source of iodide ion right when we add our catalyst right this actually provides a different mechanism a lower energy mechanism and we know that mechanism occurred in two steps so let me go ahead and sketch this the energy of the reactants is the same right it's the same level but we're going to decrease the activation energy so let me just go ahead and draw this in here so it might look something like this I'm sure I'm not drawing it perfectly so let's say that's what our energy profile looks like with the addition of our catalyst right so this this would be the transition stage for the first step of our mechanism and you can see the activation energy right has decreased alright so here we have a lower activation energy so remember from an earlier video if you decrease the activation energy you increase the rate of your reaction which you can see with the Iranians equation so that's what the catalyst does and then we reach this point right here so this this valley if you will would represent the energy of the intermediate the hypo iodide ion and then we have a second activation energy for the second step of our mechanism so this would be the first step of our mechanisms like right EA one and this would be the activation energy for the second step of our mechanism which is EA two so the activation energy for the first step is higher because that's our rate determining step so again the catalyst does not affect the energy of your reactants or of your products right it's still the same energy for both of those what the catalyst does is decrease the necessary activation energy which increases the rate of your reaction
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