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A topic from the subject of Chemical Kinetics in Chemistry.

  • 1 year ago
  • 6 min read
Elementary Reactions in Chemistry
Introduction

Elementary reactions are the simplest chemical reactions that can occur. They involve the interaction of two or more atoms, molecules, or ions to form new products. These reactions proceed in a single step and have a single transition state. Elementary reactions are the building blocks of more complex chemical reactions, and they play a fundamental role in understanding the behavior of chemical systems.

Basic Concepts
  • Reactants: The atoms, molecules, or ions that participate in an elementary reaction.
  • Products: The atoms, molecules, or ions that are formed in an elementary reaction.
  • Reaction rate: The rate at which an elementary reaction occurs. This is often expressed as a change in concentration per unit time.
  • Activation energy: The minimum amount of energy that reactants must have in order to undergo an elementary reaction. This energy barrier determines the reaction rate.
  • Molecularity: The number of reactant molecules (or ions or atoms) involved in an elementary reaction. Common molecularities include unimolecular (one reactant molecule), bimolecular (two reactant molecules), and termolecular (three reactant molecules). Termolecular reactions are rare.
Equipment and Techniques

A variety of equipment and techniques can be used to study elementary reactions. These include:

  • Spectroscopy: A technique that uses the absorption or emission of electromagnetic radiation to identify and characterize atoms, molecules, and ions. Different types of spectroscopy (e.g., UV-Vis, IR, NMR) provide information about the structure and concentration of reactants and products.
  • Mass spectrometry: A technique that measures the mass-to-charge ratio of ions to identify and characterize atoms, molecules, and ions. This is useful for determining the composition and structure of reaction products.
  • Chemical kinetics: A technique that measures the rate of chemical reactions to determine the activation energy, rate constant, and other kinetic parameters. Techniques like stopped-flow and flash photolysis are used to study fast reactions.
Types of Experiments

There are many different types of experiments that can be used to study elementary reactions. These include:

  • Single-collision experiments: Experiments in which a single reactant molecule is collided with a target molecule to study the reaction products. These are often conducted in molecular beams.
  • Bulk experiments: Experiments in which a large number of reactant molecules are reacted to study the overall reaction rate. These experiments provide macroscopic rate data.
  • Time-resolved experiments: Experiments in which the reaction products are measured as a function of time to study the reaction mechanism. This allows for the observation of reaction intermediates.
Data Analysis

The data from elementary reaction experiments can be analyzed to determine the reaction rate, activation energy, rate constant, and other kinetic parameters. Arrhenius plots are commonly used to determine activation energy. This information can be used to understand the mechanism of the reaction and to predict its behavior under different conditions.

Applications

Elementary reactions have a wide range of applications in chemistry, including:

  • Combustion: The burning of fuels is a complex process that involves a series of elementary reactions.
  • Atmospheric chemistry: The reactions of pollutants in the atmosphere are a major source of air pollution. Understanding elementary reactions is crucial for developing strategies to mitigate pollution.
  • Pharmaceutical chemistry: The development of new drugs often involves the study of elementary reactions to understand the mechanisms of drug action and metabolism.
  • Catalysis: Many catalytic processes rely on understanding elementary steps at the catalyst surface.
Conclusion

Elementary reactions are the fundamental building blocks of chemical reactions. They play a critical role in understanding the behavior of chemical systems and have a wide range of applications in chemistry and other fields.

Elementary Reactions

Elementary reactions are the simplest chemical reactions that can occur. They involve a single step with only a few reactant molecules directly interacting to form products. The rate of an elementary reaction is directly proportional to the concentration of the reactants raised to their stoichiometric coefficients (the numbers before each reactant in the balanced chemical equation).

Types of Elementary Reactions:

Elementary reactions are classified based on the number of reactant molecules involved:

  1. Unimolecular Reactions: These involve the decomposition or rearrangement of a single molecule. The rate law is first-order. 
    A → products  (Rate = k[A])

    Example: The isomerization of cyclopropane to propene.

  2. Bimolecular Reactions: These involve the collision and interaction of two molecules. The rate law is second-order. 
    A + B → products  (Rate = k[A][B])

    Example: The reaction between hydrogen and iodine: H₂ + I₂ → 2HI

    A + A → products  (Rate = k[A]²)

    Example: The dimerization of nitrogen dioxide: 2NO₂ → N₂O₄

  3. Termolecular Reactions: These involve the simultaneous collision of three molecules. These are rare due to the low probability of three molecules colliding simultaneously. The rate law is third-order. 
    A + B + C → products  (Rate = k[A][B][C])

    Example: The reaction of nitric oxide with oxygen and chlorine: 2NO + Cl₂ → 2NOCl

Rate Laws and Elementary Reactions:

A crucial distinction is that for elementary reactions, the rate law is directly derived from the stoichiometry of the reaction. The exponents in the rate law are equal to the stoichiometric coefficients of the reactants in the balanced equation. This is not true for complex reactions, which involve multiple steps.

Elementary Reactions and Complex Reactions:

Most chemical reactions are not elementary. They are complex reactions composed of a sequence of elementary steps. The overall rate law for a complex reaction cannot be predicted directly from the stoichiometry of the overall balanced reaction. Instead, detailed reaction mechanisms are necessary to understand the rates of complex reactions.

Experiment: Hydrogenation of Ethene
Objective

To demonstrate the elementary reaction of hydrogenation of ethene to ethane.

Materials
  • Ethene gas
  • Hydrogen gas
  • Platinum catalyst (finely divided)
  • Glass tube
  • Bunsen burner
  • Glass wool
  • Thermometer
  • Rubber tubing and stoppers to connect gas sources to the glass tube.
Procedure
  1. Prepare the glass tube by loosely packing a small amount of finely divided platinum catalyst onto a piece of glass wool. Insert this into the center of the glass tube, ensuring gas can flow freely around the catalyst.
  2. Securely attach rubber tubing to both ends of the glass tube, connecting one end to a source of ethene gas and the other to a source of hydrogen gas. Ensure all connections are gas-tight.
  3. Carefully begin a slow and steady flow of ethene gas through the tube.
  4. Position the Bunsen burner under the section of the tube containing the catalyst. Gently heat the tube, monitoring the temperature with a thermometer to avoid overheating.
  5. Simultaneously begin a slow and steady flow of hydrogen gas through the tube.
  6. Observe the reaction. Note any changes in temperature, the appearance of the catalyst, and the potential formation of a new gas (ethane).
  7. After a sufficient reaction time, carefully stop the flow of both gases and allow the apparatus to cool.
Observations

The ethene gas will react with the hydrogen gas in the presence of the platinum catalyst to form ethane gas (C2H6). The balanced chemical equation is: C2H4 + H2 → C2H6. The reaction is exothermic; therefore, a temperature increase should be observed. The platinum catalyst facilitates the reaction without being consumed.

Key Procedures & Safety Precautions
  • The use of a finely divided platinum catalyst is crucial for the reaction to occur at a reasonable rate. The larger the surface area of the catalyst, the faster the reaction.
  • The reaction is exothermic. Carefully monitor the temperature to prevent overheating, which could damage the apparatus or deactivate the catalyst.
  • Safety: Ethene and hydrogen are flammable gases. Ensure adequate ventilation and avoid open flames near the gas sources until the gases are fully connected to the reaction apparatus. Wear appropriate safety goggles and gloves during the experiment.
Significance

The hydrogenation of ethene is an important industrial process used to produce ethane, a valuable feedstock for the production of polyethylene plastics and other chemicals. This experiment demonstrates a fundamental catalytic reaction with significant industrial applications.

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