Catalyst is a substance that increases the rate of reaction by lowering the Activation Energy (Ea) of a reaction.
Catalyzed vs Uncatalyzed Reactions
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concept
Catalyst Concept 1
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A catalyst can be seen as any substance that increases the rate of reaction by decreasing your energy of activation, and also not being consumed in the process. Here we have a catalyzed versus an uncatalyzed reaction being displayed in terms of an energy diagram. Remember, in our energy diagram we have our reactants in the beginning, our products at the end, the difference between the two lines represents the overall change in energy of the chemical reaction. And then if we take a look here, we have at the top of this first hill is our transition state for it, and at the top of this hill is our transition state. The difference between the transition states and the reactant line gives us our activation energy. Remember, a catalyst lowers your energy of activation. If we take a look at the first red curve, we can see that its activation energy is pretty high. So it's safe to assume that this is the energy of activation for uncatalyzed reaction. So here we'd say that this is e a uncat. Now when we add a catalyst, the energy of activation is expected to go down. So the line in blue represents the addition of our catalyst. So this would be activation energy capped or catalyzed. So remember, the height of the activation energy has a direct impact on the speed of the reaction. The higher the activation energy, the longer it takes for my reactants to basically climb and scale this hill to slide down and become a product. So the red line here, we'd say since it's higher up and high activation energy, it would take longer for us to make our products. The catalyzed one, since the activation energy is smaller, the activation energy is smaller, we expect the reactants to have an easier time getting up this hill and sliding down to make our products. So keep in mind, this is what a catalyst does. It lowers our energy of activation so that our reactants can more easily become products.
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example
Catalyst Example 1
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Hey everyone. So in this example question it says, a certain reaction has an enthalpy value of negative 20 kilojoules and an activation energy of 40 kilojoules. A catalyst is found that lowers the activation energy of the reaction by 10 kilojoules. What is the total difference in energy between the products and the transition state? Alright. So they're telling us here our enthalpy value, so this is delta H and we can say here, delta E also can fill in and it's negative 20. So let's say that where our reactants start here at 40 kilojoules, And negative 20 means our products are 20 kilojoules lower, so they're here at 20 kilojoules. Activation energy, remember activation energy is the difference in height between my transition state and my reactant line. So that would mean my transition state which is up here would have to be 80. The difference between these two is what my activation energy is. Now remember, delta H or delta E equals products minus reactants. So it'd be 20 minuteus 40 which is how we got that negative 20. Alright. So now they're saying that we used a catalyst which lowers the energy of activation. So, here we were at 80 for the transition state it gets lowered by 10, so now we're at 70. We're still at 40 for the reactants that doesn't change, and my products are still down here at line 20. So now, we have to calculate the difference. This Delta H or Delta E doesn't change. It's still gonna be negative 20. And then here differences in energy between products and the transition state. Here, if we're looking at what's the total difference in energy between products and the transition state, so we're looking at this total distance here. Alright, so here we'd say total difference in energy between products in the transition state. Products are at 20, transition state is 70. So that would be a difference of 50 kilojoules toll. K. So that's the total difference. We start out with a total difference of 60 kilojoules between products and transition state. We add the catalyst, the overall energy is lowered by that much. It would be 50 here. If we wanted to look at the new activation energy, we could also do that. The activation energy here, the new one will be 70 minuteus the 40 so it'd be 30 kilojoules. And again, delta H or delta U would still be negative 20 kilojoules. So this is all the information we could tell for this particular energy diagram once we've introduced the catalyst.
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Problem
Problem
Which of the following statements is true regarding the energy diagram provided?
i. The reaction is endothermic.
ii. Activation energy would be less than + 10 kJ after a catalyst is added.
iii. The reaction absorbs energy.
iv. Activation energy would be greater than + 10 kJ after a catalyst is added.
a) I only b) II only c) I and III d) II, III and IV