Episode 25 starts with the definition of a catalyst and describes how we can identify the catalyst in a set of elementary steps.
Episode 25 starts with the definition of a catalyst and describes how we can identify the catalyst in a set of elementary steps (1:22). Catalysts work by increasing the number of successful collisions, by providing a pathway with a lower activation energy (2:18) or by providing an intermediate with more favorable orientation (2:50). Catalysts can be homogenous, hence in the same phase, or heterogenous in different phases (4:04). Enzymes are bio-catalysts (5:12).
Question: (5:53) Which of the following solid catalysts is most effective?
A. Fine powered nickel
B. Granular nickel
C. inch-long nickel
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Hi and welcome to the APsolute Recap: Chemistry Edition. Before we start this topic, I want to let you know that Episodes 26 and 28 are Listener’s choice episodes! If you have a topic, skill, or question you want a recap of - please let us know. You can contact us through our website, theapsoluterecap.com, or reach out to us on Instagram, Facebook or Twitter! Today’s episode will recap Catalysis.
Lets Zoom Out:
Unit 5 - Kinetics
Topic 5.11 - Catalysis
Big idea - Energy
Let’s start with a recap of episode 23 : What are the factors that can increase the rate of a reaction - aka have more collisions? We can increase the concentration of the reactants, we can increase the temperature - which is proportional to kinetic energy, we can increase the surface area or we can use a catalyst. But what is a catalyst and how does it increase the rate of a reaction?
Let’s zoom in:
Our “question of the day” in episode 24 actually started our discussion about catalysts: We asked you, how you can identify a catalyst in a multistep reaction? The correct answer is that they show up on the reactant and the product side. But let’s take a closer look at that. Catalysts are often consumed in the rate-determining step and regenerated in a subsequent step. Therefore, they show up on both sides of the equation and are usually written on top of the yield arrow. The net concentration of the catalyst is constant.
So how does a catalyst work? A catalyst often increases the number of effective collisions. As we’ve discussed before, for collisions to be effective, the molecules have to have the right orientation and/or a minimum of energy. Catalysts don’t increase the overall number of collisions, but they increase the number of successful collisions. One way to achieve that is by providing a reaction path that has a lower activation energy. The activation energy is, so to speak, the energy needed to get our reaction started. It is the minimum energy with which the particles have to successfully collide. If this energy is lower, because a different pathway is enabled, more collisions are successful! Careful: The catalyst does NOT lower the activation energy. It provides a DIFFERENT pathway with a lower activation energy.
The second approach a catalyst can use to increase the number of successful collisions is by binding with a reactant and providing a reaction intermediate that has a more favorable orientation. This can be in addition to providing a different pathway.
Forming a new intermediate to increase the rate of reaction can happen with homogenous and heterogenous catalysts. When using a homogenous catalyst, the catalyst and the reactant are in the same phase, for example, both dissolved in the same solvent. In chemistry, these are often acid-base catalysts, in which a new intermediate forms when the reactant either gains or loses a proton. This leads to a different elementary step and therefore a different pathway.
During heterogenous catalysis, the catalyst and the reactant are in two different phases: like with gas and solid. Common solid catalysts are Nickel, Platinum or Palladium and the reaction happens on the metal surface. The metal can form covalent bonds with a reactant or an intermediate and provide a new intermediate. An example for a heterogenous catalyst is the catalytic converter in a car! It has a fine layer of platinum, palladium or rhodium that enables the conversion of poisonous gases like carbon monoxide and nitrogen oxides to less environmentally harmful gases.
Another example that might be familiar to you, where we form a new reaction intermediate, are enzymes! In your biology class, you probably have heard about these “bio-catalysts”. Enzymes can be homogenous or heterogenous catalysts. Check out The APsolute RecAP Biology Edition to learn more.
To recap:
Catalysts are often consumed in the rate determining step and regenerated in a subsequent step. Therefore, their net concentration is constant. They increase the rate of a reaction by increasing the number of successful collisions by providing a different reaction path. This is often tied to the formation of a new intermediate. Catalysts can be homogenous or heterogenous.
Today’s Question of the day is about Catalysts.
Question: Which of the following solid catalysts is most effective?
A. Fine powered nickel
B. Granular nickel
C. inch-long nickel