Ch.14 - Chemical Kinetics

Problem 81b

Chapter 14, Problem 81b

Many metallic catalysts, particularly the precious-metal ones, are often deposited as very thin films on a substance of high surface area per unit mass, such as alumina 1Al2O32 or silica 1SiO22. (b) How does the surface area affect the rate of reaction?

Verified step by step guidance

Step 1: Understand the role of catalysts. Catalysts are substances that increase the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy. They do this by offering a surface where the reactants can come together to react.

Step 2: Recognize the importance of surface area. The greater the surface area of the catalyst, the more space there is for reactants to come into contact with the catalyst. This increases the number of successful collisions between reactant particles and the catalyst, which in turn increases the rate of reaction.

Step 3: Apply this understanding to the given problem. In the case of metallic catalysts deposited as thin films on substances like alumina or silica, the high surface area of these substances allows for more catalyst to be exposed to the reactants. This increases the number of successful collisions and thus the rate of reaction.

Step 4: Understand the practical implications. This is why catalysts are often deposited as thin films on high surface area substances - it allows for the most efficient use of often expensive catalyst materials, while maximizing the rate of reaction.

Step 5: Remember that while catalysts increase the rate of reaction, they do not affect the position of the equilibrium or the enthalpy change of the reaction. They simply allow the reaction to reach equilibrium more quickly.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Surface Area to Volume Ratio

The surface area to volume ratio is a critical factor in chemical reactions, particularly for catalysts. A higher surface area allows more reactant molecules to interact with the catalyst at any given time, increasing the likelihood of collisions and, consequently, the rate of reaction. This principle is especially important in heterogeneous catalysis, where the catalyst is in a different phase than the reactants.

Catalysis

Catalysis refers to the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst. Catalysts work by providing an alternative reaction pathway with a lower activation energy, allowing more reactant molecules to convert into products more quickly. In the context of metallic catalysts, their effectiveness is often enhanced by their high surface area, which facilitates more active sites for reactions.

Reaction Rate

The reaction rate is a measure of how quickly reactants are converted into products in a chemical reaction. It can be influenced by several factors, including concentration, temperature, and the presence of a catalyst. In the case of catalysts with high surface areas, the increased availability of active sites leads to a higher frequency of effective collisions, thereby accelerating the overall reaction rate.

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Open Question

In solution, chemical species as simple as H+ and OH- can serve as catalysts for reactions. Imagine you could measure the [H+] of a solution containing an acid-catalyzed reaction as it occurs. Assume the reactants and products themselves are neither acids nor bases. Sketch the [H+] concentration profile you would measure as a function of time for the reaction, assuming t = 0 is when you add a drop of acid to the reaction.

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The oxidation of SO2 to SO3 is accelerated by NO2. The reaction proceeds according to: NO2(g) + SO2(g) → NO(g) + SO3(g) 2 NO(g) + O2(g) → 2 NO2(g) (a) Show that, with appropriate coefficients, the two reactions can be summed to give the overall oxidation of SO2 by O2 to give SO3. (d) Is this an example of homogeneous catalysis or heterogeneous catalysis?

Textbook Question

The addition of NO accelerates the decomposition of N2O, possibly by the following mechanism: NO1g2 + N2O1g2¡N21g2 + NO21g2 2 NO21g2¡2 NO1g2 + O21g2 (b) Is NO serving as a catalyst or an intermediate in this reaction?

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The enzyme carbonic anhydrase catalyzes the reaction CO2(g) + H2O(l) ↔ HCO3⁻(aq) + H⁺(aq). In water, without the enzyme, the reaction proceeds with a rate constant of 0.039 s⁻¹ at 25 _x001E_C. In the presence of the enzyme in water, the reaction proceeds with a rate constant of 1.0 * 10⁶ s⁻¹ at 25 _x001E_C. Assuming the collision factor is the same for both situations, calculate the difference in activation energies for the uncatalyzed versus enzyme-catalyzed reaction.

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The enzyme urease catalyzes the reaction of urea, 1NH2CONH22, with water to produce carbon dioxide and ammonia. In water, without the enzyme, the reaction proceeds with a first-order rate constant of 4.15 * 10-5 s-1 at 100 C. In the presence of the enzyme in water, the reaction proceeds with a rate constant of 3.4 * 104 s-1 at 21 C. (c) In actuality, what would you expect for the rate of the catalyzed reaction at 100 C as compared to that at 21 C?

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