5.3 Rate-determining steps and their identification
3 min read•july 22, 2024
Chemical reactions often involve multiple steps, and understanding which step controls the overall speed is crucial. The is the slowest part of a reaction, acting like a bottleneck that limits how fast the entire process can go.
Identifying the rate-determining step helps predict how changes in conditions will affect reaction speed. It's like finding the weakest link in a chain – strengthening other parts won't make much difference until you address the slowest step.
Rate-Determining Steps
Rate-determining step definition
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- Slowest step in a multi-step reaction mechanism
- Has the highest barrier among all steps
- Limits the overall rate of the reaction
- Rate of the rate-determining step determines the rate of the entire reaction
- Increasing rate of rate-determining step increases overall reaction rate (adding a catalyst)
- Changing rate of other steps does not affect overall reaction rate
Significance in reaction rates
- Acts as a bottleneck in the reaction mechanism
- Controls flow of reactants through entire reaction pathway (limiting reactant)
- Overall reaction rate cannot proceed faster than rate of rate-determining step
- Reactants accumulate before rate-determining step (traffic jam)
- Products accumulate after rate-determining step
- Rate law for overall reaction determined by rate-determining step
- Order of reaction with respect to each reactant determined by of rate-determining step ( order, order)
Identification in reaction mechanisms
- Compare rates of each step in reaction mechanism
- Step with slowest rate is rate-determining step
- Identify step with highest activation energy barrier
- Requires most energy to overcome, likely rate-determining step (hill on a race course)
- Look for step with lowest rate constant ()
- Rate constant directly related to rate of step
- Consider molecularity of each step
- Unimolecular steps typically faster than bimolecular or termolecular steps
Effects of changing conditions
- Increasing temperature increases overall reaction rate
- Higher temperature provides more energy to overcome activation energy barrier of rate-determining step (heating a pot of water)
- Changing concentration of reactants involved in rate-determining step affects overall reaction rate
- Increasing concentration of reactants in rate-determining step increases overall reaction rate (adding more fuel to a fire)
- Changing concentration of reactants not involved in rate-determining step does not affect overall reaction rate
- Adding a catalyst can change the rate-determining step
- Catalyst lowers activation energy barrier of a specific step (stepping stool)
- If catalyzed step becomes faster than original rate-determining step, rate-determining step may change
Reaction Mechanisms and Kinetics
Understand the relationship between reaction mechanisms and rate laws
- Reaction mechanism is series of describing how reactants convert to products
- Each elementary step has specific rate law based on molecularity
- Overall rate law for reaction determined by rate-determining step
- Slowest step in mechanism controls overall rate (weakest link in a chain)
- Order of reaction with respect to each reactant determined by molecularity of rate-determining step
- Rate law provides insights into reaction mechanism
- Order of reaction with respect to each reactant suggests molecularity of rate-determining step
- Comparing experimental rate laws with proposed mechanisms helps validate or refute proposed mechanism (scientific method)
Key Terms to Review (11)
Activation Energy: Activation energy is the minimum amount of energy required for a chemical reaction to occur. It represents the energy barrier that reactants must overcome to be transformed into products, linking the concepts of kinetics and thermodynamics in the context of chemical reactions.
Catalysis: Catalysis is the process by which the rate of a chemical reaction is increased by the presence of a substance called a catalyst, which is not consumed in the reaction. Catalysts work by providing an alternative pathway for the reaction with a lower activation energy, allowing reactions to proceed more quickly and efficiently. This concept is critical in various fields including pharmaceuticals, environmental science, and mechanistic analysis.
Collision Theory: Collision theory is a fundamental concept in chemical kinetics that explains how reactions occur and why reaction rates vary. It posits that for a reaction to take place, reactant particles must collide with sufficient energy and the correct orientation. This theory connects to various aspects of reaction dynamics, including the interplay between energy, molecular structure, and the speed of reactions.
Elementary Steps: Elementary steps are the individual, simple reactions that make up a complex reaction mechanism. Each elementary step represents a single transition state and describes a fundamental change in reactants, typically involving one or two molecules. Understanding these steps helps in identifying the overall reaction pathway and determining the rate-determining step, which is the slowest step that controls the reaction rate.
Langmuir-Hinshelwood Mechanism: The Langmuir-Hinshelwood mechanism describes a specific type of surface reaction mechanism in heterogeneous catalysis, where both reactants adsorb onto the catalyst surface before reacting. This process involves the formation of intermediate species on the surface, leading to the production of products through a series of surface reactions. Understanding this mechanism is crucial for analyzing surface reactions and determining the kinetics of catalytic processes, as well as identifying rate-determining steps in the overall reaction.
Molecularity: Molecularity is the number of reactant molecules involved in an elementary reaction, defining the simplest form of a reaction mechanism. This concept helps classify reactions as unimolecular, bimolecular, or termolecular, which provides insight into how the reaction occurs at a molecular level. Understanding molecularity is essential for identifying the rate-determining step in a reaction and analyzing how the number of molecules affects the overall reaction rate.
Multi-step mechanism: A multi-step mechanism refers to a series of elementary reactions that collectively describe the overall process of a chemical reaction. Each step in the mechanism involves distinct transitions between reactants and products, highlighting the complexity of reaction pathways. Understanding these mechanisms allows chemists to identify rate-determining steps and better grasp how molecularity affects the overall reaction rate.
Rate-determining step: The rate-determining step is the slowest step in a reaction mechanism that limits the overall rate of the reaction. This concept is crucial because it helps to identify which specific reaction step controls how fast a chemical reaction occurs, influencing the formulation of rate laws and determining the kinetics of reactions.
Reaction intermediate: A reaction intermediate is a transient species formed during the conversion of reactants to products in a chemical reaction. These intermediates are not present in the initial or final states of the reaction but play a crucial role in understanding the stepwise progression of the reaction mechanism, including consecutive reactions, rate-determining steps, and the comparison between different theories of reaction dynamics.
Steady-state approximation: The steady-state approximation is a method used in chemical kinetics to simplify the analysis of complex reaction mechanisms by assuming that the concentration of intermediates remains constant over the course of the reaction. This approach helps in deriving rate equations and understanding how various steps contribute to overall reaction rates without having to solve differential equations for every intermediate.
Transition state: The transition state is a temporary, high-energy arrangement of atoms that occurs during a chemical reaction, representing the point of maximum energy along the reaction pathway. This state is crucial as it determines the activation energy required for the reaction to proceed and connects the reactants and products through an energy barrier.