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Catalysis

Pearson
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If you've ever had a cut on your skin, sometimes you're told to pour hydrogen peroxide on it, and when that happens, you see bubbling. If you use the same hydrogen peroxide bottle roughly year later, and it was stored in your medicine cabinet, and you go and pour it on your skin again, there's not as much bubbling, and it's not nearly as effective. That's because hydrogen peroxide or H2O2 decomposes into water and oxygen. This reaction right here is a very, very slow reaction that takes a long time to form the products. We can use something called a catalyst to try to speed up this reaction. So I have in front of me here is hydrogen peroxide in an acidic solution, and we're going to use sodium bromide, which will produce bromide ions in solution that can act as a catalyst. So I'm going to add some solid sodium bromide to this hydrogen peroxide solution. When I added the sodium bromide, the solution turned brown. This indicates that Br2 is formed. At this point in our reaction, with our brown solution, with Br2 present, can we confirm that the sodium bromide acted as a catalyst? Now let's investigate the chemistry of this particular reaction. We already indicated this hydrogen peroxide reaction that decomposes into water and oxygen. When we have hydrogen peroxide in the presence of an acid and we add Br- in solution, the Br- is going to be colorless. We don't have a colorless solution. Our solution turned brown, indicating that Br2 formed in our solution. If we let this reaction go for a much longer period of time, we're going to see that the Br2 is going to react again with the hydrogen peroxide to give Br-, H+, and O2 in solution. After we let this solution go for about 20 minutes, we can come back and look and see if this solution is colorless. So as we can see, our solution now turned clear, and another observation we can look at is that there's oxygen bubbles forming in this test tube. This is showing that our NaBr acted as a catalyst, and how we can explain that is by looking at a reaction mechanism. In the first step of this mechanism, we added Br- in solution, which is colorless. Okay, that was a quick step going from a Br-, which is colorless, to Br2, which is brown in solution, and we saw that. At this point in the reaction, we cannot say that this is catalyzed, because our catalyst needs to be regenerated in the last step of the reaction mechanism, but as this reaction proceeds for about 15 minutes, we see that the hydrogen peroxide is going to interact with the Br2 to form Br- plus 2H+ plus O2. In this case, our Br- is regenerated in the last step, indicating that this is an effective reagent to add as a catalyst. We can further explain this by looking at a reaction progress diagram plotted versus energy. Here we have our H2O2, becoming water and oxygen, okay? There's going to be an energy of activation barrier that needs to be overcome for this reaction to happen. What a catalyst does is it lowers the energy of activation barrier. So when we add our acid and our Br-, that energy of activation barrier is lowered, and therefore, the reaction will proceed at a much faster rate. In this balloon in front of me, I have hydrogen and oxygen. Even though water is thermodynamically stable than hydrogen and oxygen reacting together, no reaction forms because there's an energy of activation barrier that must be overcome to facilitate this reaction. We are going to provide a spark to this reaction to enable us to get over the energy of activation barrier. [ Silence ] [ Explosion ] Wow, I hope everyone at home was covering their ears because that was a pretty loud explosion. Now let's see what really happened by looking at a reaction progress diagram again. We have hydrogen and oxygen gas. They need some type of energy to overcome the energy of activation barrier to form water. We provided that energy with the flame, but this reaction right here is not catalyzed. All we did was provided enough energy to overcome the energy of activation barrier. Now let's use this same reaction with hydrogen and oxygen, which I have here in this container, and lets catalyze this reaction and see what will happen. [ Silence ] What would you predict would happen when I take a palladium catalyst and dump it into the mixture containing hydrogen and oxygen? [ Silence ] [ Explosion ] So again, we saw a very large flame and a big explosion happen in this particular reaction, but the big difference, we didn't have to use a flame to start this reaction. We were able to use a catalyst in order to facilitate this reaction. So if we go back to our reaction progress diagram, we need the hydrogen and oxygen to go over this energy of activation barrier to form water. In the first reaction, we did that with a flame. In the second reaction, we took this energy of activation barrier, and lowered it with a catalyst. So we didn't need to provide energy into the reaction. We just provided another pathway for the hydrogen and oxygen to form water, indicating that this reaction was catalyzed.
If you've ever had a cut on your skin, sometimes you're told to pour hydrogen peroxide on it, and when that happens, you see bubbling. If you use the same hydrogen peroxide bottle roughly year later, and it was stored in your medicine cabinet, and you go and pour it on your skin again, there's not as much bubbling, and it's not nearly as effective. That's because hydrogen peroxide or H2O2 decomposes into water and oxygen. This reaction right here is a very, very slow reaction that takes a long time to form the products. We can use something called a catalyst to try to speed up this reaction. So I have in front of me here is hydrogen peroxide in an acidic solution, and we're going to use sodium bromide, which will produce bromide ions in solution that can act as a catalyst. So I'm going to add some solid sodium bromide to this hydrogen peroxide solution. When I added the sodium bromide, the solution turned brown. This indicates that Br2 is formed. At this point in our reaction, with our brown solution, with Br2 present, can we confirm that the sodium bromide acted as a catalyst? Now let's investigate the chemistry of this particular reaction. We already indicated this hydrogen peroxide reaction that decomposes into water and oxygen. When we have hydrogen peroxide in the presence of an acid and we add Br- in solution, the Br- is going to be colorless. We don't have a colorless solution. Our solution turned brown, indicating that Br2 formed in our solution. If we let this reaction go for a much longer period of time, we're going to see that the Br2 is going to react again with the hydrogen peroxide to give Br-, H+, and O2 in solution. After we let this solution go for about 20 minutes, we can come back and look and see if this solution is colorless. So as we can see, our solution now turned clear, and another observation we can look at is that there's oxygen bubbles forming in this test tube. This is showing that our NaBr acted as a catalyst, and how we can explain that is by looking at a reaction mechanism. In the first step of this mechanism, we added Br- in solution, which is colorless. Okay, that was a quick step going from a Br-, which is colorless, to Br2, which is brown in solution, and we saw that. At this point in the reaction, we cannot say that this is catalyzed, because our catalyst needs to be regenerated in the last step of the reaction mechanism, but as this reaction proceeds for about 15 minutes, we see that the hydrogen peroxide is going to interact with the Br2 to form Br- plus 2H+ plus O2. In this case, our Br- is regenerated in the last step, indicating that this is an effective reagent to add as a catalyst. We can further explain this by looking at a reaction progress diagram plotted versus energy. Here we have our H2O2, becoming water and oxygen, okay? There's going to be an energy of activation barrier that needs to be overcome for this reaction to happen. What a catalyst does is it lowers the energy of activation barrier. So when we add our acid and our Br-, that energy of activation barrier is lowered, and therefore, the reaction will proceed at a much faster rate. In this balloon in front of me, I have hydrogen and oxygen. Even though water is thermodynamically stable than hydrogen and oxygen reacting together, no reaction forms because there's an energy of activation barrier that must be overcome to facilitate this reaction. We are going to provide a spark to this reaction to enable us to get over the energy of activation barrier. [ Silence ] [ Explosion ] Wow, I hope everyone at home was covering their ears because that was a pretty loud explosion. Now let's see what really happened by looking at a reaction progress diagram again. We have hydrogen and oxygen gas. They need some type of energy to overcome the energy of activation barrier to form water. We provided that energy with the flame, but this reaction right here is not catalyzed. All we did was provided enough energy to overcome the energy of activation barrier. Now let's use this same reaction with hydrogen and oxygen, which I have here in this container, and lets catalyze this reaction and see what will happen. [ Silence ] What would you predict would happen when I take a palladium catalyst and dump it into the mixture containing hydrogen and oxygen? [ Silence ] [ Explosion ] So again, we saw a very large flame and a big explosion happen in this particular reaction, but the big difference, we didn't have to use a flame to start this reaction. We were able to use a catalyst in order to facilitate this reaction. So if we go back to our reaction progress diagram, we need the hydrogen and oxygen to go over this energy of activation barrier to form water. In the first reaction, we did that with a flame. In the second reaction, we took this energy of activation barrier, and lowered it with a catalyst. So we didn't need to provide energy into the reaction. We just provided another pathway for the hydrogen and oxygen to form water, indicating that this reaction was catalyzed.