Writing Rate Laws of Reaction Mechanisms Using The Rate Determining Step - Chemical Kinetics
TLDRThis chemistry tutorial delves into the intricacies of reaction mechanisms, explaining how to write rate laws for individual steps and the overall reaction. It illustrates the concept of elementary reactions and how they combine to form a complete mechanism. The video distinguishes between catalysts and intermediates, highlighting their roles in speeding up reactions and lowering activation energy. It also covers the molecularity of reactions, including unimolecular, bimolecular, and termolecular types, and guides viewers through examples to identify these components and write rate laws accordingly.
Takeaways
- 🔍 A reaction mechanism is a step-by-step pathway detailing how a particular reaction occurs, with each step being an elementary reaction.
- 📝 The rate law expression for an elementary reaction can be written based on the coefficients of the reactants involved in that step.
- 🔄 To identify a catalyst and an intermediate in a reaction, one must understand their roles: a catalyst is consumed and then produced, while an intermediate is produced and then consumed.
- 🚀 Catalysts speed up chemical reactions by providing an alternative pathway and lowering the activation energy.
- ✍️ The overall reaction is derived by canceling out intermediates and catalysts from the series of elementary reactions.
- 🔢 Molecularity refers to the number of molecules involved in an elementary reaction, which can be unimolecular (one molecule), bimolecular (two molecules), or termolecular (three molecules).
- 📉 The rate of the overall reaction is determined by the slowest step, known as the rate-determining step.
- 📚 The rate law expression for the overall reaction is based on the slowest step and does not include intermediates or catalysts.
- 📉 The order of a reaction is the sum of the exponents of the concentration terms in the rate law expression.
- 🧪 In the provided example, iodide acts as a catalyst and io- as an intermediate in the decomposition of hydrogen peroxide into water and oxygen.
- 📚 The rate law for the overall reaction of the example is first order with respect to O3 and NO2, and second order overall.
Q & A
What is a reaction mechanism in chemistry?
-A reaction mechanism is a step-by-step pathway that describes how a particular chemical reaction occurs. Each individual step in the reaction mechanism is known as an elementary reaction.
What is an elementary reaction?
-An elementary reaction is a single step within a reaction mechanism, representing a basic process that contributes to the overall chemical reaction.
How do you write the rate law expression for an elementary reaction?
-The rate law expression for an elementary reaction is written based on the coefficients of the reactants involved in that step, multiplied by the rate constant (k) for that step.
What is a catalyst in a chemical reaction?
-A catalyst is a substance that speeds up a chemical reaction by providing an alternative pathway for the reactants to become products, without being consumed in the overall reaction.
How can you identify a catalyst in a reaction mechanism?
-A catalyst can be identified by its presence at the beginning and the end of the reaction mechanism; it is consumed first and then produced later in the sequence of reactions.
What is an intermediate in a chemical reaction?
-An intermediate is a species that is produced in one step of a reaction mechanism and consumed in a subsequent step, thus only present during the reaction process.
How can you distinguish between a catalyst and an intermediate in a reaction mechanism?
-A catalyst is consumed first and then produced later, while an intermediate is produced first and then consumed later. This distinction helps in identifying their roles within the reaction mechanism.
What is the molecularity of a unimolecular reaction?
-The molecularity of a unimolecular reaction is one, as it involves a single molecule reacting to form products.
How do you determine the overall order of a bimolecular reaction?
-The overall order of a bimolecular reaction is second order, as it involves two reactant molecules reacting with each other.
What is the rate determining step in a reaction mechanism?
-The rate determining step is the slowest step in a reaction mechanism, which governs the rate of the overall reaction.
How is the rate law expression for the overall reaction related to the rate determining step?
-The rate law expression for the overall reaction is dependent on the rate determining step, which is the slow step in the reaction mechanism.
Outlines
🔍 Understanding Reaction Mechanisms and Rate Laws
This paragraph introduces the concept of reaction mechanisms, which are step-by-step pathways for chemical reactions, with each step being an elementary reaction. The speaker explains how to write rate law expressions for these steps based on the coefficients of reactants. The paragraph also discusses how to identify catalysts and intermediates in a reaction, highlighting the difference between them based on their presence in the reaction sequence. Catalysts are consumed and then produced, while intermediates are produced and then consumed. The role of catalysts in lowering activation energy and speeding up reactions is also mentioned.
📚 Writing Overall Reactions and Determining Molecularity
The second paragraph focuses on writing the overall reaction for a given reaction mechanism, explaining how to cancel out intermediates and catalysts to obtain the net reaction. It delves into the molecularity of reactions, defining unimolecular and bimolecular reactions with examples and their corresponding rate law expressions. The paragraph also touches on termolecular reactions, noting their rarity and slowness due to the difficulty of three molecules colliding with the correct energy and orientation. The molecularity and order of reactions are discussed in relation to the rate law expressions for elementary steps.
🔬 Identifying Rate-Determining Steps and Reaction Components
This paragraph presents a scenario where a two-step reaction mechanism is given, with the first step being slow and the second being fast. The speaker guides the audience through the process of writing the overall reaction, identifying the intermediate (which is produced and then consumed), and determining the rate law for the overall reaction, which depends on the slow, rate-determining step. The paragraph emphasizes that the molecularity of the first step is bimolecular, and the overall order of the reaction is second order, with the rate law expression reflecting this.
🌐 Decomposition of Hydrogen Peroxide with Catalysts
The final paragraph discusses the decomposition of hydrogen peroxide into water and oxygen, accelerated by the presence of an iodide catalyst. It explains how to identify the catalyst and intermediate in the reaction and how to write the overall reaction for the mechanism. The molecularity of each elementary step is identified as bimolecular. The paragraph also explains that the rate law expression for the overall reaction should only include the reactant present in the overall reaction, excluding the catalyst. The rate law expression for the slow step and the overall reaction is provided, highlighting the exclusion of the catalyst from the overall rate law.
Mindmap
Keywords
💡Reaction Mechanism
💡Elementary Reaction
💡Rate Law
💡Rate Constant
💡Catalyst
💡Intermediate
💡Activation Energy
💡Molecularity
💡Uni-, Bi-, and Termolecular Reactions
💡Rate-Determining Step
💡Overall Reaction
Highlights
Reaction mechanisms are explained as a step-by-step pathway for a reaction to occur.
Elementary reactions are the individual steps within a reaction mechanism.
Rate law expressions can be written for each elementary reaction based on reactant coefficients.
The rate constant for each step is denoted differently, such as k1 for the first step.
Catalysts and intermediates can be identified by their consumption and production order in the reaction.
A catalyst is consumed first and then produced later, while an intermediate is produced first and consumed later.
Catalysts provide an alternative pathway and lower activation energy to speed up reactions.
The overall reaction is derived by canceling out intermediates and catalysts from the elementary reactions.
Molecularity of reactions is determined by the number of molecules involved in each elementary step.
Uni-, bi-, and termolecular reactions are differentiated based on the number of reactants involved.
The rate law expression for unimolecular reactions is first order, for bimolecular reactions it's second order.
Termolecular reactions are rare and slow due to the difficulty of three molecules colliding with the right energy and orientation.
The rate of the overall reaction is determined by the rate-determining step, which is the slowest step.
An example is provided to illustrate the identification of intermediates and catalysts, and the writing of the overall reaction.
The molecularity and rate law for each elementary reaction are explained using a specific example.
The rate law for the overall reaction is derived from the slowest step, excluding intermediates and catalysts.
An example problem is presented to practice identifying catalysts, intermediates, and writing rate law expressions.
Transcripts
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