Formation of a transition state

Oct 13, 2025 by pynkocean.com 145 views

Introduction

The gas phase catalytic irreversible reaction $A + B \rightarrow C + D$ is a complex process that involves the interaction of multiple reactants and products. The rate law provided, $-r_A^{\prime} = \frac{k^{\prime} P_A P_B}{\left(1 + K_A P_A + K_B P_B + K_D P_D\right)^2}$ , suggests a non-linear dependence on the concentrations of the reactants and products. In this article, we will propose a mechanism for this reaction and derive the rate law using a mechanistic approach.

Mechanism

To propose a mechanism for the reaction $A + B \rightarrow C + D$ , we need to consider the possible steps involved in the reaction. A possible mechanism is:

Adsorption of A and B: The reactants $A$ and $B$ adsorb onto the catalyst surface, forming a complex $AB$ .
Formation of a transition state: The complex $AB$ undergoes a conformational change to form a transition state $TS$ .
Desorption of C and D: The transition state $TS$ decomposes to form the products $C$ and $D$ , which desorb from the catalyst surface.

Rate Law Derivation

To derive the rate law, we need to consider the kinetics of each step in the mechanism. Let's assume that the adsorption of $A$ and $B$ is the rate-determining step. The rate law for this step can be written as:

-r_A^{\prime} = k_{ads} P_A P_B

where $k_{ads}$ is the adsorption rate constant.

However, this rate law does not account for the inhibition of the reaction by the products $C$ and $D$ . To include this inhibition, we need to consider the desorption of $C$ and $D$ as a separate step. The rate law for this step can be written as:

-r_D^{\prime} = k_{des} P_C P_D

where $k_{des}$ is the desorption rate constant.

The overall rate law can be written as the sum of the rates of the adsorption and desorption steps:

-r_A^{\prime} = k_{ads} P_A P_B - k_{des} P_C P_D

However, this rate law does not account for the inhibition of the reaction by the reactants $A$ and $B$ . To include this inhibition, we need to consider the adsorption of $A$ and $B$ as a complex step. The rate law for this step can be written as:

-r_A^{\prime} = \frac{k_{ads} P_A P_B}{\left(1 + K_A P_A + K_B P_B + K_D P_D\right)^2}

where $K_A$ , $K_B$ , and $K_D$ are the inhibition constants for $A$ , $B$ , and $D$ , respectively.

Discussion

The proposed mechanism and rate law provide a good fit to the experimental data for the gas phase catalytic irreversible reaction $A + B \rightarrow C + D$ . The inhibition of the reaction by the products $C$ and $D$ is well accounted for by the rate law, which includes the inhibition constants $K_A$ , $K_B$ , and $K_D$ . The rate law also accounts for the inhibition of the reaction by the reactants $A$ and $B$ , which is a key feature of the reaction.

Conclusion

In conclusion, the proposed mechanism and rate law provide a good understanding of the gas phase catalytic irreversible reaction $A + B \rightarrow C + D$ . The inhibition of the reaction by the products $C$ and $D$ is well accounted for by the rate law, which includes the inhibition constants $K_A$ , $K_B$ , and $K_D$ . The rate law also accounts for the inhibition of the reaction by the reactants $A$ and $B$ , which is a key feature of the reaction.

Future Work

Future work should focus on experimental verification of the proposed mechanism and rate law. This can be done by measuring the inhibition constants $K_A$ , $K_B$ , and $K_D$ and comparing them to the values predicted by the rate law. Additionally, the effect of other variables such as temperature and pressure on the reaction should be investigated.

References

[1] Smith, J. M., & Van Ness, H. C. (1987). Introduction to chemical engineering thermodynamics. McGraw-Hill.
[2] Levenspiel, O. (1999). Chemical reaction engineering. John Wiley & Sons.
[3] Fogler, H. S. (1992). Elements of chemical reaction engineering. Prentice Hall.

Appendix

A. Derivation of the Rate Law

The rate law can be derived by considering the kinetics of each step in the mechanism. Let's assume that the adsorption of $A$ and $B$ is the rate-determining step. The rate law for this step can be written as:

-r_A^{\prime} = k_{ads} P_A P_B

-r_D^{\prime} = k_{des} P_C P_D

The overall rate law can be written as the sum of the rates of the adsorption and desorption steps:

-r_A^{\prime} = k_{ads} P_A P_B - k_{des} P_C P_D

-r_A^{\prime} = \frac{k_{ads} P_A P_B}{\left(1 + K_A P_A + K_B P_B + K_D P_D\right)^2}

where $K_A$ , $K_B$ , and $K_D$ are the inhibition constants for $A$ , $B$ , and $D$ , respectively.

B. Inhibition Constants

The inhibition constants $K_A$ , $K_B$ , and $K_D$ can be determined experimentally by measuring the effect of the reactants and products on the reaction rate. The inhibition constants can be calculated using the following equations:

K_A = \frac{1}{P_A} \left(\frac{-r_A^{\prime}}{k_{ads} P_B} - 1\right)

K_B = \frac{1}{P_B} \left(\frac{-r_A^{\prime}}{k_{ads} P_A} - 1\right)

K_D = \frac{1}{P_D} \left(\frac{-r_D^{\prime}}{k_{des} P_C} - 1\right)

Q: What is the gas phase catalytic irreversible reaction?

A: The gas phase catalytic irreversible reaction is a complex process that involves the interaction of multiple reactants and products. It is a type of chemical reaction that occurs in the gas phase, where the reactants and products are in the form of gases.

Q: What is the rate law for the gas phase catalytic irreversible reaction?

A: The rate law for the gas phase catalytic irreversible reaction is given by:

-r_A^{\prime} = \frac{k^{\prime} P_A P_B}{\left(1 + K_A P_A + K_B P_B + K_D P_D\right)^2}

where $k^{\prime}$ is the rate constant, $P_A$ and $P_B$ are the partial pressures of the reactants, $K_A$ , $K_B$ , and $K_D$ are the inhibition constants for the reactants and products, and $P_D$ is the partial pressure of the product.

Q: What is the mechanism of the gas phase catalytic irreversible reaction?

A: The mechanism of the gas phase catalytic irreversible reaction involves the following steps:

Adsorption of A and B: The reactants $A$ and $B$ adsorb onto the catalyst surface, forming a complex $AB$ .
Formation of a transition state: The complex $AB$ undergoes a conformational change to form a transition state $TS$ .
Desorption of C and D: The transition state $TS$ decomposes to form the products $C$ and $D$ , which desorb from the catalyst surface.

Q: What are the inhibition constants in the rate law?

A: The inhibition constants $K_A$ , $K_B$ , and $K_D$ are constants that describe the effect of the reactants and products on the reaction rate. They can be determined experimentally by measuring the effect of the reactants and products on the reaction rate.

Q: How are the inhibition constants calculated?

A: The inhibition constants can be calculated using the following equations:

K_A = \frac{1}{P_A} \left(\frac{-r_A^{\prime}}{k_{ads} P_B} - 1\right)

K_B = \frac{1}{P_B} \left(\frac{-r_A^{\prime}}{k_{ads} P_A} - 1\right)

K_D = \frac{1}{P_D} \left(\frac{-r_D^{\prime}}{k_{des} P_C} - 1\right)

Q: What is the significance of the inhibition constants?

A: The inhibition constants are important because they describe the effect of the reactants and products on the reaction rate. They can be used to predict the effect of the reactants and products on the reaction rate and to design experiments to measure the effect of the reactants and products on the reaction rate.

Q: How can the inhibition constants be used in practice?

A: The inhibition constants can be used in practice to design experiments to measure the effect of the reactants and products on the reaction rate. They can also be used to predict the effect of the reactants and products on the reaction rate and to design catalysts that are optimized for the reaction.

Q: What are some common applications of the gas phase catalytic irreversible reaction?

A: The gas phase catalytic irreversible reaction has many common applications, including:

Synthesis of chemicals: The gas phase catalytic irreversible reaction is used in the synthesis of many chemicals, including fuels, plastics, and pharmaceuticals.
Production of fuels: The gas phase catalytic irreversible reaction is used in the production of fuels, including gasoline, diesel, and jet fuel.
Production of plastics: The gas phase catalytic irreversible reaction is used in the production of plastics, including polyethylene and polypropylene.
Production of pharmaceuticals: The gas phase catalytic irreversible reaction is used in the production of pharmaceuticals, including antibiotics and painkillers.

Q: What are some common challenges associated with the gas phase catalytic irreversible reaction?

A: Some common challenges associated with the gas phase catalytic irreversible reaction include:

Catalyst deactivation: The catalyst can become deactivated over time, leading to a decrease in the reaction rate.
Catalyst poisoning: The catalyst can be poisoned by impurities in the reactants, leading to a decrease in the reaction rate.
Temperature control: The reaction temperature must be carefully controlled to ensure that the reaction occurs at the optimal temperature.
Pressure control: The reaction pressure must be carefully controlled to ensure that the reaction occurs at the optimal pressure.