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Reaction mechanisms

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Intros
Lessons
  1. How do reactants become products?
  2. Steps in chemical reactions.
  3. Why one reaction step matters most.
  4. Drawing activated complexes and transition states.
  5. Writing elementary steps and overall reactions.
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Examples
Lessons
  1. Identify products and intermediates in a multi-step chemical reaction.
    The reactions below are some elementary steps in the free-radical substitution reaction between methane and chlorine.

    Cl2  \, →  \, 2Cl·
    CH4 + Cl·  \, →  \, CH3· + HCl
    CH3· + Cl2  \, →  \, CH3Cl + Cl·
    2Cl·  \, →  \, Cl2
    1. Identify the reactants, products and intermediates in this series of reactions.
  2. Write the rate law for a balanced chemical equation.
    The reaction below is the rate determining step of a chemical process.

    2A + 3B  \, →  \, C + 2D

    1. Write the rate law expression for this reaction. What is the order with respect to B?
    2. In an experiment to measure the rate of reaction, by how much would you expect the rate increase if the concentration of B was doubled?
    Topic Notes
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    Introduction to Reaction Mechanisms

    Reaction mechanisms are fundamental to understanding chemical processes at the molecular level. They provide a step-by-step explanation of how reactants transform into products, revealing the intricate dance of electrons and bonds. Our introduction video serves as a crucial starting point, offering a visual and conceptual framework for grasping these complex ideas. By watching this video, students can gain a solid foundation in reaction mechanisms, setting the stage for deeper exploration. Understanding reaction mechanisms is essential in chemistry as it allows scientists to predict outcomes, design more efficient processes, and develop new materials. These mechanisms shed light on the 'how' and 'why' behind chemical reactions, enabling chemists to manipulate and control reactions with precision. Whether you're studying organic synthesis, biochemistry, or materials science, a strong grasp of reaction mechanisms is indispensable for success in the field of chemistry.

    Understanding Reaction Mechanisms

    Reaction mechanisms are fundamental to understanding how chemical processes unfold at the molecular level. Much like organizing a party, complex chemical reactions don't happen all at once but rather through a series of carefully orchestrated steps. This concept is crucial in chemistry and helps explain how substances transform during chemical reactions.

    At its core, a reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. It's akin to a detailed recipe that outlines not just the ingredients (reactants) and final dish (products) but every mixing, stirring, and baking step in between. This stepwise approach is essential because most chemical reactions, especially complex ones, don't occur in a single step as written in a balanced equation.

    To better grasp this concept, let's consider the analogy of organizing a party. Just as a successful party doesn't magically come together in an instant, neither do complex chemical reactions. When planning a party, you might start by sending invitations, then prepare food, decorate the venue, and finally welcome guests. Each step is crucial and occurs in a specific order. Similarly, in a chemical reaction, molecules must first collide (as explained by collision theory), then bonds may break, new bonds form, and intermediates may be created before the final products emerge.

    Simple reaction mechanisms might involve just one or two steps, much like organizing a small gathering with close friends. For instance, the decomposition of hydrogen peroxide into water and oxygen is a relatively straightforward process. However, more complex reactions, like those in organic synthesis or biochemical processes, can involve numerous steps, intermediates, and even catalysts similar to planning an elaborate wedding or a large corporate event.

    An example of a complex reaction mechanism is the ozone depletion process in the Earth's stratosphere. This multi-step reaction involves chlorofluorocarbons (CFCs) breaking down to release chlorine atoms, which then catalyze the destruction of ozone molecules. Each step in this mechanism is crucial to understanding how human-made chemicals can impact the environment on a global scale.

    Understanding reaction mechanisms is vital for several reasons. Firstly, it allows chemists to predict the outcome of reactions and design more efficient synthetic routes for producing desired compounds. Secondly, it helps in identifying potential side reactions or unwanted byproducts. Lastly, comprehending reaction mechanisms is crucial in developing new catalysts and improving existing chemical processes in industries ranging from pharmaceuticals to materials science.

    The study of reaction mechanisms often involves sophisticated experimental techniques and theoretical calculations. Scientists use methods like spectroscopy, kinetic studies, and computational modeling to elucidate the steps involved in chemical processes. By breaking down complex reactions into their constituent steps, researchers can identify rate-determining steps, understand the role of catalysts, and even manipulate reactions to yield desired products more efficiently.

    In conclusion, reaction mechanisms provide a detailed roadmap of how chemical transformations occur. They reveal the intricate dance of molecules as they collide, break apart, and recombine to form new substances. By understanding these mechanisms, chemists can not only explain observed phenomena but also predict and control chemical reactions, paving the way for innovations in fields ranging from drug discovery to environmental protection. Just as a well-planned party unfolds smoothly through a series of coordinated actions, complex chemical reactions proceed through carefully orchestrated steps, each contributing to the final outcome.

    Rate Determining Step

    The rate determining step (RDS), also known as the rate-limiting step, is a crucial concept in chemical kinetics that plays a pivotal role in understanding and controlling reaction rates. In essence, the RDS is the slowest step in a multi-step reaction mechanism, which ultimately determines the overall speed of the entire reaction. This concept is fundamental to comprehending how chemical reactions progress and how their rates can be manipulated.

    To better grasp the idea of the rate determining step, consider the analogy of traffic flow on a highway. Imagine a series of intersections along the route, each representing a step in the reaction mechanism. The intersection with the longest wait time or the most congestion becomes the rate determining step for the entire journey. Just as this bottleneck controls the overall travel time, the RDS controls the overall reaction rate.

    The significance of the rate determining step in reaction mechanisms cannot be overstated. It acts as a bottleneck, effectively setting an upper limit on how fast the overall reaction can proceed. No matter how quickly the other steps in the mechanism occur, the reaction cannot progress faster than the rate of the slowest step. This principle is crucial for chemists and chemical engineers when designing and optimizing reaction processes.

    Identifying the rate determining step is essential for understanding and controlling reaction rates. One method to determine the RDS involves analyzing the concentration of intermediates or products during the reaction. In a series of consecutive steps, the concentration of an intermediate will be low if it is rapidly consumed in a fast subsequent step. Conversely, if an intermediate accumulates, it suggests that the step consuming it is slow and likely to be the RDS.

    Another approach to identifying the RDS is through kinetic studies. By measuring the reaction rate under various conditions and comparing it to the rates of individual steps, researchers can pinpoint which step is rate-limiting. If changing the concentration of a reactant or catalyst affects the overall reaction rate, it often indicates that this change is influencing the RDS.

    Understanding the rate determining step has practical implications in various fields. In industrial processes, identifying and optimizing the RDS can lead to significant improvements in efficiency and yield. For example, in pharmaceutical manufacturing, enhancing the rate of the slowest step can dramatically reduce production times and costs. Similarly, in environmental chemistry, recognizing the RDS in pollutant degradation processes can help develop more effective remediation strategies.

    The concept of the rate determining step also plays a crucial role in enzyme kinetics and biochemical reactions. In these complex biological systems, the RDS often involves the formation or breakdown of the enzyme-substrate complex. By understanding which step limits the overall reaction rate, biochemists can design drugs or modify enzymes to enhance or inhibit specific biochemical pathways.

    It's important to note that the rate determining step can change under different reaction conditions. Factors such as temperature, pressure, and concentration can alter the relative rates of individual steps, potentially shifting which step is rate-limiting. This dynamic nature of the RDS underscores the importance of comprehensive kinetic studies under various conditions to fully understand a reaction mechanism.

    In conclusion, the rate determining step is a fundamental concept in chemical kinetics that provides valuable insights into reaction mechanisms and overall reaction rates. By identifying and understanding the RDS, scientists and engineers can effectively control and optimize chemical processes across a wide range of applications, from industrial manufacturing to environmental remediation and drug development. The ability to manipulate the rate determining step opens up possibilities for enhancing reaction efficiency, improving product yields, and developing more sustainable chemical processes.

    Activated Complexes and Elementary Steps

    In the realm of chemical kinetics and reaction mechanisms, the concept of activated complexes plays a crucial role in understanding how reactions proceed at the molecular level. Activated complexes, also known as transition states, represent the highest energy configuration that reactants must pass through to form products. These fleeting arrangements of atoms are pivotal in determining reaction rates and pathways.

    To comprehend reaction mechanisms fully, it's essential to treat each elementary step as an individual reaction. Elementary steps are the simplest possible chemical events that occur during a reaction, involving the breaking or forming of chemical bonds. By analyzing these steps separately, chemists can gain insights into the overall reaction mechanism and identify rate-determining steps.

    When depicting activated complexes, chemists often use dashed lines to represent bonds that are in the process of breaking or forming. This visual representation helps illustrate the transient nature of the transition state. For instance, in a simple SN2 reaction, the activated complex would show the nucleophile approaching the electrophilic center with dashed lines, while the leaving group's bond is also represented by a dashed line, indicating its imminent departure.

    Examples of activated complexes can be found in various types of reactions. In a Diels-Alder cycloaddition, the activated complex shows partial bonds forming between the diene and dienophile. For a simple acid-base proton transfer, the activated complex depicts the proton situated between the acid and base, connected by dashed lines to both species. In enzyme-catalyzed reactions, the activated complex often involves the substrate bound to the enzyme's active site in a high-energy conformation.

    Understanding activated complexes is crucial for predicting reaction outcomes and designing more efficient chemical processes. By studying these high-energy intermediates, researchers can develop catalysts that lower activation energies, thereby increasing reaction rates. Additionally, computational chemists use transition state theory to model and predict reaction kinetics based on the properties of activated complexes.

    When interpreting activated complexes, it's important to consider factors such as bond angles, molecular geometry, and electron distribution. These characteristics can provide valuable information about the reaction mechanism and help explain observed kinetics and product distributions. For example, in an E2 elimination reaction, the activated complex reveals the stereochemistry of the transition state, which determines whether the reaction follows an anti-periplanar or syn-periplanar pathway.

    Activated complexes also play a significant role in understanding solvent effects on reaction rates. The arrangement of solvent molecules around the transition state can stabilize or destabilize the activated complex, thereby influencing the reaction's activation energy. This concept is particularly important in organic synthesis and biochemical reactions, where solvent choice can dramatically affect reaction outcomes.

    In conclusion, the study of activated complexes and elementary steps is fundamental to unraveling the intricacies of chemical reactions. By breaking down complex mechanisms into individual steps and visualizing transition states, chemists can gain deep insights into reaction pathways, kinetics, and thermodynamics. This knowledge not only enhances our understanding of chemical processes but also empowers researchers to design more efficient and selective reactions for various applications in chemistry and related fields.

    Combining Elementary Steps

    Understanding the process of combining elementary steps to construct the overall reaction equation is crucial in chemical kinetics. This approach allows chemists to break down complex reactions into simpler, more manageable parts, providing insights into reaction mechanisms and the role of intermediates. In this section, we'll explore the step-by-step process of combining elementary steps and identify key components such as reactants, products, and intermediates.

    Elementary steps are the simplest molecular events that occur during a chemical reaction. These steps cannot be broken down further and typically involve the collision of two or three molecules. When combined, these steps form the overall reaction equation. The process of combining elementary steps is essential for understanding reaction mechanisms and predicting reaction rates.

    Intermediates play a crucial role in reaction mechanisms. These are species that are formed during the reaction but do not appear in the overall equation. They are created in one elementary step and consumed in a subsequent step. Identifying intermediates is key to understanding the pathway of a reaction and can provide valuable insights into reaction kinetics and mechanisms.

    To combine elementary steps and construct the overall reaction equation, follow these steps:

    1. Identify all elementary steps in the reaction mechanism.
    2. Write out each elementary step, including reactants, products, and any intermediates.
    3. Identify the overall reactants (species present at the beginning) and products (species present at the end).
    4. Cancel out any intermediates that appear on both sides of the equations.
    5. Combine the remaining species to form the overall reaction equation.

    Let's illustrate this process using the reaction between carbon monoxide (CO) and nitrogen dioxide (NO2). This reaction proceeds through the following elementary steps:

    1. NO2 + NO2 NO3 + NO
    2. NO3 + CO NO2 + CO2

    To identify reactants, products, and intermediates:

    • Reactants: CO and NO2 (present at the beginning)
    • Products: CO2 and NO (present at the end)
    • Intermediates: NO3 (formed in step 1, consumed in step 2)

    To construct the overall equation, we combine the elementary steps and cancel out the intermediates:

    NO2 + NO2 NO3 + NO
    NO3 + CO NO2 + CO2

    Canceling out NO3 and one NO2, we get the overall reaction equation:

    NO2 + CO NO + CO2

    This process demonstrates how combining elementary steps and identifying intermediates leads to the overall reaction equation. It's important to note that while intermediates don't appear in the final equation, they are crucial for understanding the reaction mechanism and kinetics.

    In more complex reactions, you may need to use coefficients to balance the equations properly. Always ensure that the number of atoms of each element is conserved in the overall equation. This approach not only helps in understanding reaction mechanisms but also in predicting reaction rates and equilibrium constants.

    By mastering the process of combining elementary steps, chemists can gain deeper insights into reaction pathways, identify rate-determining steps, and design more efficient chemical processes. This skill is invaluable in fields such as organic synthesis, catalysis, and biochemistry, where understanding reaction mechanisms is crucial for developing new materials, drugs, and industrial processes.

    Rate Laws and Reaction Mechanisms

    Rate laws are fundamental concepts in chemical kinetics that describe the relationship between the rate of a chemical reaction and the concentrations of reactants. These mathematical expressions are crucial for understanding and predicting reaction rates, making them essential tools in the study of reaction mechanisms. Rate laws provide valuable insights into the molecular-level processes occurring during chemical reactions and help scientists elucidate the steps involved in complex reactions.

    The connection between rate laws and reaction mechanisms lies in the rate-determining step, which is the slowest step in a multi-step reaction mechanism. This step controls the overall rate of the reaction and is typically used to derive the rate law. By identifying the rate-determining step, chemists can simplify complex reaction mechanisms and focus on the most critical factors influencing the reaction rate.

    To derive a rate law from the rate-determining step, scientists consider the molecularity of the step and the concentrations of the species involved. For example, in a unimolecular rate-determining step, the rate law will be first-order with respect to the reactant. In a bimolecular rate-determining step, the rate law will be second-order overall, with first-order dependence on each reactant. This approach allows chemists to propose and test reaction mechanisms based on experimental rate data.

    The importance of rate laws in understanding and predicting reaction rates cannot be overstated. These equations enable scientists to:

    • Quantify the effect of concentration changes on reaction rates
    • Determine reaction orders and rate constants
    • Predict how reaction rates will change under different conditions
    • Design and optimize chemical processes in industrial settings
    • Investigate the mechanisms of complex reactions

    Rate laws come in various forms, depending on the type of reaction mechanism. Some common examples include:

    1. First-order rate law: Rate = k[A], where the reaction rate depends on the concentration of a single reactant A.
    2. Second-order rate law: Rate = k[A][B], where the rate depends on the concentrations of two reactants A and B.
    3. Zero-order rate law: Rate = k, where the rate is independent of reactant concentrations.
    4. Mixed-order rate law: Rate = k[A]m[B]n, where m and n are not necessarily integers.

    These rate laws can be derived from various reaction mechanisms. For instance, a first-order rate law might result from a unimolecular decomposition reaction or a reaction with a large excess of one reactant. A second-order rate law could arise from a bimolecular reaction between two different species or the collision of two identical molecules. Zero-order rate laws are often observed in reactions where a reactant is in such excess that its concentration remains effectively constant throughout the reaction.

    In more complex reaction mechanisms, such as those involving catalysts or multiple steps, the rate laws may take on more intricate forms. For example, in enzyme-catalyzed reactions, the rate law often follows the Michaelis-Menten equation, which describes how the reaction rate varies with substrate concentration. In chain reactions, such as free radical polymerization, the rate law may involve the square root of initiator concentration due to the termination step.

    Understanding rate laws and their relationship to reaction mechanisms is crucial for chemists, chemical engineers, and researchers in various fields. By analyzing rate laws, scientists can gain insights into the molecular-level processes occurring during reactions, design more efficient chemical processes, and develop new catalysts and materials. Furthermore, rate laws play a vital role in pharmaceutical research, environmental science, and materials development, where controlling reaction rates and understanding mechanisms are essential for creating innovative solutions to complex problems.

    Conclusion

    In this article, we've explored the fascinating world of reaction mechanisms in chemistry. We've learned how these step-by-step processes explain chemical reactions at the molecular level, highlighting the importance of understanding intermediates and transition states. The concept of the rate determining step was introduced, emphasizing its crucial role in controlling overall reaction speed. Grasping these fundamental principles is essential for any aspiring chemist or science enthusiast. We encourage you to rewatch the introductory video for a comprehensive overview of these concepts. To deepen your understanding, practice solving mechanism problems and explore more complex reactions. Remember, mastering reaction mechanisms is key to unlocking the secrets of chemical processes. Whether you're a student or a professional, further study in this area will enhance your ability to predict and control chemical reactions. Don't hesitate to engage with additional resources and join discussions to broaden your knowledge in this exciting field of chemistry.

    Understanding the rate determining step is crucial for controlling chemical reactions. Additionally, practicing solving mechanism problems will help you gain a deeper insight into the subject. By mastering these concepts, you will be better equipped to predict and control chemical reactions effectively.

    Reaction Mechanisms: How Do Reactants Become Products?

    How do reactants become products? Steps in chemical reactions.

    Step 1: Understanding Reaction Mechanisms

    To begin with, it's essential to understand what a reaction mechanism is. A reaction mechanism is a detailed step-by-step description of how a chemical reaction occurs. It outlines the sequence of elementary steps that lead from reactants to products. Each step involves the breaking and forming of bonds, and the entire mechanism provides a comprehensive picture of the reaction process.

    Step 2: The Rate Determining Step (RDS)

    One of the critical concepts in reaction mechanisms is the Rate Determining Step (RDS). The RDS is the slowest step in the reaction mechanism and thus determines the overall rate of the reaction. Identifying the RDS is crucial because it helps in understanding which step controls the speed of the entire process. Chemists often abbreviate the Rate Determining Step as RDS.

    Step 3: Collision Theory Recap

    Before diving deeper into reaction mechanisms, it's helpful to revisit the collision theory. According to collision theory, for a reaction to occur, reactant particles must collide with sufficient energy and the correct orientation. This means that not all collisions result in a reaction; only those with the right conditions do. For example, if molecule A and molecule B are to form a product AB, they must collide with enough energy and in the correct orientation to form the new bond.

    Step 4: Energy and Orientation in Collisions

    The energy and orientation of colliding particles are crucial factors in determining whether a reaction will occur. Both reactant molecules need to have enough energy to overcome the activation energy barrier. Additionally, they must collide in a specific orientation that allows the formation of the product. This concept is straightforward for simple reactions involving two molecules, but it becomes more complex for reactions involving multiple molecules.

    Step 5: Complexity in Multi-Molecule Reactions

    In many chemical reactions, multiple molecules are involved, sometimes as many as 10 or 15. The probability of all these molecules colliding simultaneously with the right energy and orientation is extremely low. Therefore, such reactions do not occur in a single step. Instead, they proceed through a series of intermediate steps, each involving a smaller number of molecules.

    Step 6: Sequence of Steps in Reaction Mechanisms

    Chemists believe that complex reactions occur through a sequence of elementary steps. Each step involves the formation of intermediates, which are species that appear in the middle of the reaction but are not present in the final products. The entire sequence of steps from reactants to products is known as the reaction mechanism. This stepwise process allows for the gradual transformation of reactants into products.

    Step 7: Real-Life Analogy

    To better understand the concept of reaction mechanisms, consider the analogy of organizing a party. Organizing a party involves several steps: inviting guests, deciding on the venue, arranging food and drinks, and hosting the event. Each step must be completed in a specific order, and some steps may take longer than others. Similarly, in a chemical reaction, each step in the mechanism must occur in a specific sequence, and some steps may be slower and more critical than others.

    Step 8: Importance of Individual Steps

    Just as in organizing a party, some steps in a chemical reaction mechanism are more important than others. The slower steps, in particular, play a crucial role in determining the overall rate of the reaction. These slower steps are often the Rate Determining Steps (RDS) and dictate the speed at which the entire reaction proceeds. Understanding which steps are rate-determining helps chemists control and optimize chemical reactions.

    Step 9: Conclusion

    In summary, reactants become products through a series of elementary steps known as the reaction mechanism. Each step involves collisions between reactant molecules with sufficient energy and correct orientation. Complex reactions proceed through multiple steps, forming intermediates along the way. The Rate Determining Step (RDS) is the slowest step and controls the overall reaction rate. By understanding these steps, chemists can better understand and manipulate chemical reactions.

    FAQs

    1. What is a reaction mechanism?

      A reaction mechanism is a step-by-step description of how a chemical reaction occurs at the molecular level. It explains the sequence of elementary steps that lead from reactants to products, including any intermediates or transition states formed along the way. Understanding reaction mechanisms is crucial for predicting reaction outcomes, designing efficient synthetic routes, and developing new catalysts.

    2. What is the rate determining step in a reaction mechanism?

      The rate determining step (RDS) is the slowest step in a multi-step reaction mechanism. It acts as a bottleneck, controlling the overall speed of the reaction. No matter how fast the other steps are, the reaction cannot proceed faster than the RDS. Identifying the RDS is crucial for understanding and manipulating reaction rates in various chemical processes.

    3. How are rate laws related to reaction mechanisms?

      Rate laws are mathematical expressions that describe how the rate of a reaction depends on the concentrations of reactants. They are closely linked to reaction mechanisms, particularly the rate determining step. The rate law of a reaction often reflects the molecularity and reactant dependencies of the RDS. By analyzing rate laws, chemists can propose and test reaction mechanisms, providing insights into the molecular-level processes occurring during reactions.

    4. What are activated complexes in chemical reactions?

      Activated complexes, also known as transition states, are high-energy, unstable arrangements of atoms that form during a chemical reaction. They represent the highest energy point along the reaction coordinate, where bonds are in the process of breaking and forming. Understanding activated complexes is essential for predicting reaction rates and outcomes, as well as for designing catalysts to lower activation energies.

    5. How do you combine elementary steps to determine the overall reaction?

      To combine elementary steps and determine the overall reaction, follow these steps: 1) Identify all elementary steps in the mechanism. 2) Write out each step, including reactants, products, and intermediates. 3) Identify overall reactants and products. 4) Cancel out intermediates that appear on both sides of the equations. 5) Combine the remaining species to form the overall reaction equation. This process helps in understanding complex reactions and identifying key intermediates in the reaction pathway.

    Prerequisite Topics for Understanding Reaction Mechanisms

    When delving into the fascinating world of reaction mechanisms in chemistry, it's crucial to have a solid foundation in certain prerequisite topics. These fundamental concepts serve as building blocks that enable students to grasp the intricacies of how chemical reactions occur at the molecular level. Two key prerequisite topics that play a significant role in understanding reaction mechanisms are molecular geometry and activation energy.

    Molecular geometry is essential for comprehending reaction mechanisms because it provides insight into the three-dimensional arrangement of atoms within molecules. This spatial understanding is crucial when analyzing how molecules interact and bond during chemical reactions. By mastering molecular geometry, students can better visualize the orientation of reactants and predict how they might come together to form products. Additionally, knowledge of molecular geometry helps in understanding the concept of steric hindrance, which can significantly influence reaction rates and pathways.

    Another vital prerequisite topic is activation energy. This concept is fundamental to reaction mechanisms as it represents the minimum energy required for a chemical reaction to occur. Understanding activation energy allows students to grasp why some reactions happen spontaneously while others require an input of energy. It also explains the role of catalysts in reaction mechanisms, as these substances work by lowering the activation energy barrier, thus facilitating faster reactions.

    The interplay between molecular geometry and activation energy is particularly important in reaction mechanisms. The spatial arrangement of atoms (molecular geometry) can affect the energy required for molecules to collide effectively (activation energy). For instance, molecules with complex geometries might require higher activation energies to overcome steric barriers and react.

    By thoroughly understanding these prerequisite topics, students can more easily interpret reaction coordinate diagrams, predict reaction outcomes, and explain the steps involved in various reaction mechanisms. Molecular geometry provides the spatial context for visualizing how reactants approach each other, while knowledge of activation energy helps in understanding the energetics of bond breaking and formation during reactions.

    In conclusion, mastering these prerequisite topics is not just about memorizing facts; it's about developing a deeper understanding of the fundamental principles that govern chemical reactions. This knowledge forms the foundation upon which more complex concepts in reaction mechanisms can be built. Students who invest time in solidifying their understanding of molecular geometry and activation energy will find themselves better equipped to tackle the challenges of reaction mechanisms and advance their overall comprehension of chemistry.

    In this lesson, we will learn:

    • The definition of reaction mechanism, transition state and intermediate.
    • How to interpret reaction mechanisms in steps and recognize the rate-determining step (RDS) in a chemical process.
    • To understand rate laws as expressions of the rate-determining step.

    Notes:

    In this lesson, we will learn:

    • Collision theory states that the reactant particles must collide with sufficient energy and correct orientation. What about with complicated reactions?
      • If a chemical reaction involves 5, 10, or even more reactant molecules converting to different products, what is the chance that all of them will collide in the same space at the same time? It’s extremely unlikely.
        However, complex reactions do happen at respectable rates. This suggests that reactions do not happen all in one step – reactions often happen in a sequence of steps which, combined, form the overall reaction. The specific steps in how reactants form the products is called the reaction mechanism.

    • Just as the reaction rate for an overall reaction can be measured, the rate of individual or elementary steps in reactions can be studied and measured.
      • Even though chemical reactions can have many steps, there is normally one elementary step that is orders of magnitude slower (hundreds or thousands of times) than the other steps. This is the rate-determining step that practically sets the overall rate because this step is the rate bottleneck of the rest of the reaction.
      • Studying individual steps of a reaction is very complicated, but the concentration of reactants, intermediates (substances formed and used up entirely during the reaction) and products can be measured. This helps identify the rate determining step (RDS).
      • Because the RDS is so much slower than the other steps, this is the only step that ‘bottlenecks’ reactants, leading to a noticeable concentration of these reactants if the reaction is analyzed while taking place.
      • In the same way as the RDS bottlenecks, the other steps have relatively no influence – increasing concentration of reactants in the non-rate-determining steps doesn’t affect the overall rate. The rate is solely determined by the concentration of reactants in the rate-determining step (RDS).

    • To write an overall reaction equation, all the elementary steps must be combined and the intermediates cancelled out from both sides.
      • EXAMPLE: Consider the reaction below which has two elementary steps:
        Step one: A→B+C\mathrm{A \to B + C}
        Step two: A+B→2C\mathrm{A + B \to 2C}
        In the above reaction, C is called an intermediate because it is formed entirely and used up entirely during the reaction.
        • It is only an intermediate if every single molecule of the substance is produced and then used up in the middle of the reaction. There will be no trace of it in the overall reactants or products.
        Intermediates and elementary steps must still balance when combining to get an overall equation – use this and the fact that intermediates entirely cancel out to balance these equations!
        The overall equation for the example above would be worked out like this:
        Step one: A→B+C\mathrm{A \to B + C}
        Step two: A+B→2C\mathrm{A + B \to 2C}
        Combined: 2A+B→B+3C\mathrm{2A + B \to B + 3C}
        Cancel out: 2A+B→B+3C \mathrm{2A + \cancel{B} \to \cancel{B} + 3C}
        Overall equation: 2A→3C\mathrm{2A \to 3C}

      • The elementary steps of the reaction show the mechanism - practice looking at the difference between overall equations and elementary steps. The overall equation and elementary steps are often contradictory:
        • The two elementary steps of the example show a molecule of reactant A decomposes to form product C and intermediate B, which then reacts with another reactant A molecule to form two molecules of product C.
        • However, the overall equation suggests two molecules of reactant A react together to form three molecules of product C.
        If you need to describe the mechanism, you must discuss the elementary steps!

    • Each elementary step can be treated as an individual process in terms of the activated complex that the molecules make when they collide.
      • If you need to draw an 'activated complex' for an elementary step of a reaction then use dashed lines to represent bonds in the middle of being broken and formed.

    • When drawing energy diagrams of reaction mechanisms, show each step individually – each elementary step will have a flat section indicating the intermediate(s), and an activation energy barrier.
      • Each activation energy 'hump' is the boundary between the chemical substances changing, and one step of the reaction moving into the next. The number of 'humps' should be the same as the number of reaction step.

    • As said above, when finding reaction rate there is usually only one elementary step that is important because one is much slower than all the other steps – the rate-determining step (RDS). The rate determining step is used to derive the rate law.
      • The rate law is a mathematical expression that relates rate of reaction to the concentration of reactants in the rate determining step. Generally:
        For the reaction with rate determining step: xA+yB→productsxA + yB \to products

        Rate=k[A]x[B]yRate = k[A]^x[B]^y

        Where k=rate  constantk = rate\;constant
        x = rate order with respect to A
        y = rate order with respect to B

        As the equation shows, the rate is found by multiplying a rate constant k (a constant with a unique value for every chemical reaction) by the concentration of the reactants ([A] and [B]) in the rate-determining step raised to the power of their molar ratio (x and y respectively).

    • There are some types of reactions where the rate law is easy to find – simple reactions involving only one (unimolecular) or two (bimolecular) reactant molecules.
      • In unimolecular reactions (most decomposition reactions), the reaction is wholly dependent on one molecule and the breaking of the bonds. The rate law for unimolecular reactions (whether an elementary or overall step) is always:
        A→productsA \to products

        Rate=k[A]Rate = k[A]