12.6 Reaction Mechanisms
3 min read•june 25, 2024
Chemical reactions often involve complex mechanisms with multiple steps. Understanding these mechanisms is crucial for predicting reaction rates and outcomes. Let's break down the key components of reaction mechanisms and how they relate to overall chemical processes.
Reaction mechanisms reveal the molecular-level events that occur during chemical changes. By examining , , and rate-determining factors, we can gain insights into reaction kinetics and develop accurate rate laws. This knowledge is essential for controlling and optimizing chemical reactions in various applications.
Reaction Mechanisms
Net reactions vs elementary steps
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- represent overall balanced chemical equation for a reaction may consist of multiple steps shows the overall change in reactants and products (synthesis of ammonia from nitrogen and hydrogen)
- Elementary steps are individual steps in a reaction mechanism represent the actual molecular events that occur during a reaction each step involves a single molecular event or collision (formation of an , breaking of a bond) the sum of all elementary steps equals the net reaction
Molecularity of elementary reactions
- is the number of reactant species involved in an elementary step
- involve a single reactant molecule (decomposition of hydrogen peroxide into water and oxygen, isomerization of cyclopropane to propene)
- involve the collision of two reactant molecules or species (formation of hydrogen iodide from hydrogen and iodine, displacement of copper by zinc in a solution of copper sulfate)
- involve the simultaneous collision of three reactant molecules or species are rare due to the low probability of three-molecule collisions (formation of nitric oxide from nitrogen dioxide and oxygen)
Balanced equations from mechanisms
- Each elementary step in a mechanism must be balanced
- The sum of all elementary steps must equal the balanced net reaction
- Steps for constructing a balanced chemical equation from a mechanism:
- Write each elementary step as a balanced chemical equation
- Cancel out any intermediate species that appear on both sides of the arrows (hydroxide ion in the base-catalyzed hydrolysis of an ester)
- Add the remaining species on each side of the arrows to obtain the balanced net reaction
Rate laws from reaction mechanisms
- The for a reaction can be determined from the slowest step () in the mechanism
- The molecularity of the rate-determining step determines the order of the reaction with respect to each reactant
- Steps for developing a rate law from a mechanism:
- Identify the slowest step (rate-determining step) in the mechanism (dissociation of iodine in the iodination of acetone)
- Write the rate law expression based on the molecularity of the rate-determining step
- For a unimolecular step: rate = (decomposition of N2O5 into NO2 and O2)
- For a bimolecular step: rate = or rate = (if the step involves two molecules of the same species) (formation of HBr from H2 and Br2)
- The is the sum of the exponents in the rate law expression (second-order for the formation of NO2 from NO and O2)
Reaction Energy Profile
- A is a graphical representation of the energy changes during a chemical reaction along the
- The reaction coordinate represents the progress of a reaction from reactants to products
- The highest point on the reaction profile represents the , which is the highest-energy arrangement of atoms during the reaction
- The is the minimum energy required for a reaction to occur, represented by the energy difference between reactants and the
- A lowers the activation energy of a reaction by providing an alternative reaction pathway, without being consumed in the process
Key Terms to Review (27)
Activation Energy: Activation energy is the minimum amount of energy required to initiate a chemical reaction. It represents the energy barrier that must be overcome for the reaction to occur, acting as a catalyst for the transformation of reactants into products.
Activation energy (Ea): Activation energy (Ea) is the minimum amount of energy required for a chemical reaction to occur. It determines the rate at which reactants transform into products.
Bimolecular reaction: A bimolecular reaction is a chemical reaction that involves two reactant molecules colliding and reacting to form products. The rate of such reactions depends on the concentration of both reactants.
Bimolecular Reactions: Bimolecular reactions, also known as second-order reactions, are chemical reactions in which two reactant molecules collide and interact to form the products. These types of reactions are an important part of understanding reaction mechanisms, as they provide insights into the kinetics and dynamics of chemical processes.
Catalyst: A catalyst is a substance that increases the rate of a chemical reaction without being consumed or altered itself. Catalysts play a crucial role in various chemical processes, including reaction mechanisms, catalysis, chemical equilibria, and shifting equilibria.
Catalysts: Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by lowering the activation energy needed for the reaction to proceed.
Elementary reaction: An elementary reaction is a single-step process in which reactants directly form products without any intermediates. Each elementary reaction has its own specific rate law that depends only on the concentration of the reactants involved.
Elementary Steps: Elementary steps refer to the fundamental, individual steps that make up the overall reaction mechanism of a chemical process. These are the most basic and irreducible steps that occur during a reaction, involving the breaking and forming of chemical bonds.
Integrated rate laws: Integrated rate laws describe the concentration of reactants as a function of time. They are derived from differential rate laws and are used to determine reaction order and rate constants.
Intermediate Species: An intermediate species, in the context of reaction mechanisms, refers to a short-lived, unstable chemical species that forms during the course of a chemical reaction. These species are not the initial reactants or the final products, but rather transient intermediates that play a crucial role in the overall reaction pathway.
Intermediates: Intermediates are species that appear in the steps of a reaction mechanism but not in the overall balanced equation. They are formed in one elementary step and consumed in a subsequent step.
Molecularity: Molecularity describes the number of reactant molecules involved in an elementary reaction. It is crucial for understanding the step-by-step mechanism of a chemical reaction.
Molecularity: Molecularity refers to the number of reactant molecules that come together in a single elementary step of a reaction mechanism. It is a fundamental concept in understanding the kinetics and dynamics of chemical reactions.
Net Reactions: Net reactions refer to the overall chemical change that occurs in a reaction after accounting for all the individual steps or intermediates involved in the reaction mechanism. They represent the difference between the reactants and products, simplifying the complex sequence of events into a single equation that captures the essence of the transformation. This concept helps chemists understand how multiple elementary steps combine to yield a final result, providing insight into reaction kinetics and thermodynamics.
Overall reaction order: Overall reaction order is the sum of the exponents of the concentration terms in a rate law equation. It indicates how the rate of reaction depends on the concentration of reactants.
Rate Law: The rate law is an equation that describes the relationship between the rate of a chemical reaction and the concentrations of the reactants. It is a fundamental concept in chemical kinetics that helps quantify and predict the speed of a reaction under specific conditions.
Rate-Determining Step: The rate-determining step is the slowest elementary step in a reaction mechanism that controls the overall rate of the chemical reaction. It is the step that limits the speed at which the reaction can proceed and is the crucial factor in determining the reaction rate.
Rate-limiting step: The rate-limiting step is the slowest step in a reaction mechanism that determines the overall reaction rate. It acts as a bottleneck, limiting the speed at which products are formed.
Reaction Coordinate: The reaction coordinate is a graphical representation of the progress of a chemical reaction, depicting the changes in energy as the reactants are converted into products. It is a fundamental concept in understanding the factors that influence reaction rates and the mechanisms by which reactions occur.
Reaction Order: Reaction order is a measure of how the rate of a chemical reaction changes with the concentrations of the reactants. It describes the relationship between the rate of a reaction and the concentrations of the reactants involved, providing insights into the mechanism of the reaction.
Reaction Profile: A reaction profile, also known as a reaction coordinate diagram, is a graphical representation that illustrates the energy changes that occur during a chemical reaction. It provides a visual depiction of the progress of a reaction, including the reactants, products, and any intermediate species or transition states involved.
Termolecular reaction: A termolecular reaction is a chemical reaction that involves three reactant molecules colliding simultaneously to form products. These types of reactions are rare due to the low probability of three particles colliding at the same time with the correct orientation and sufficient energy.
Termolecular Reactions: Termolecular reactions are a type of chemical reaction mechanism where three reactant molecules collide and interact simultaneously to form the products. This is in contrast to more common bimolecular reactions, which involve the collision and interaction of two reactant molecules.
Transition state: The transition state is a high-energy, unstable configuration of atoms during a chemical reaction that represents the point of maximum energy. It is the state through which reactants must pass to be converted into products.
Transition State: The transition state is a critical point in the course of a chemical reaction where the reactants have not yet fully transformed into the products, but rather exist in a high-energy, unstable intermediate configuration. This transient state is a key concept in understanding the factors that affect reaction rates and the mechanisms by which reactions occur.
Unimolecular reaction: A unimolecular reaction is a chemical reaction in which a single reactant molecule undergoes a transformation to yield products. These reactions are typically characterized by first-order kinetics.
Unimolecular Reactions: Unimolecular reactions are a type of chemical reaction where a single reactant molecule undergoes a transformation to form one or more product molecules. These reactions are often the first step in more complex reaction mechanisms and play a crucial role in understanding the kinetics and dynamics of chemical processes.