Chemical Kinetics

  • Nobel prize winners for catalyst and their use in reaction

Introduction to Chemical Kinetics

  • Definition: Study of rates of chemical reactions
  • Importance: Understanding reaction rates, mechanisms, and factors affecting reaction rates

Rate of Reaction

  • Definition: Change in concentration of reactants or formation of products per unit time
  • Unit: Moles per liter per second (mol/L·s)
  • Formula: Δ[C]/Δt = -Δ[R]/Δt = Δ[P]/Δt

Factors Affecting Reaction Rates

  1. Nature of reactants
  1. Concentration of reactants
  1. Temperature
  1. Pressure (for gaseous reactions)
  1. Surface area (for heterogeneous reactions)

Order of a Reaction

  • Definition: Sum of the powers to which the concentrations of the reactants are raised in the rate law equation
  • Examples:
    • Rate = k[A][B]^2
      • Order = 1 + 2 = 3
    • Rate = k[C]^2[D]
      • Order = 2 + 1 = 3

Rate Law

  • Definition: Mathematical representation of the relationship between the rate of a reaction and the concentrations of reactants
  • Example: Rate = k[A]^2[B]

Integrated Rate Law - Zero Order

  • Definition: Relationship between the concentration of reactant and time for a zero-order reaction
  • Equation: [A] = -kt + [A]₀

Integrated Rate Law - First Order

  • Definition: Relationship between the concentration of reactant and time for a first-order reaction
  • Equation: ln[A] = -kt + ln[A]₀

Integrated Rate Law - Second Order

  • Definition: Relationship between the concentration of reactant and time for a second-order reaction
  • Equation: 1/[A] = kt + 1/[A]₀

Half-Life

  • Definition: Time taken for the concentration of a reactant to decrease by half
  • Equation: t½ = 0.693/k
  1. Catalyst: Definition and Role
  • Definition: Substance that increases the rate of a chemical reaction without being consumed in the process
  • Role: Provides an alternative reaction pathway with lower activation energy
  • Example: Platinum used as a catalyst in the oxidation of hydrogen gas
  1. Mechanism of Catalysis
  • Definition: Sequence of elementary steps that describe how a reaction occurs at the molecular level
  • Steps: Adsorption, surface reaction, desorption
  • Catalysts can increase the rate of reaction by providing an alternative reaction pathway with lower activation energy
  1. Enzymes: Biological Catalysts
  • Definition: Proteins that act as catalysts in biochemical reactions
  • Example: Enzyme catalase speeds up the decomposition of hydrogen peroxide into water and oxygen
  • Enzymes are highly specific and can catalyze reactions under mild conditions
  1. Homogeneous vs. Heterogeneous Catalysis
  • Homogeneous catalysis: Catalyst and reactants are present in the same phase (e.g., reaction in a liquid solution)
  • Heterogeneous catalysis: Catalyst is in a different phase than the reactants (e.g., solid catalyst in a gas or liquid reaction)
  • Examples: Homogeneous catalysts - acid-catalysed esterification reaction; Heterogeneous catalysts - platinum in catalytic converters
  1. Factors Affecting Catalysis
  • Temperature: Higher temperature increases the rate of reaction for both catalyzed and uncatalyzed reactions
  • Concentration: Increasing the concentration of reactants increases the rate of reaction, but only up to a certain point for catalyzed reactions
  • Catalyst Poisoning: Some substances can bind to the catalyst, rendering it inactive
  1. Activation Energy and Catalysts
  • Activation Energy: Minimum energy required for a reaction to occur
  • Catalysts lower the activation energy by providing an alternative reaction pathway with lower energy barriers
  • This increases the rate of reaction and allows reactions to occur under milder conditions
  1. Use of Catalysts in Industrial Processes
  • Catalytic cracking: Breaking down larger hydrocarbon molecules into smaller ones for the production of gasoline
  • Haber process: Production of ammonia by the reaction of nitrogen and hydrogen gases
  • Contact process: Production of sulfuric acid by the oxidation of sulfur dioxide
  1. Selectivity and Catalysts
  • Definition: Ability of a catalyst to selectively promote a specific reaction pathway
  • Catalysts can control the selectivity of a reaction, leading to the desired products while minimizing unwanted byproducts
  • Example: Catalysts used in the production of polymers, where chain length and branching can be controlled
  1. Promoters and Inhibitors
  • Promoters: Substances that enhance the activity of a catalyst
  • Inhibitors: Substances that reduce or completely inhibit the activity of a catalyst
  • Example: Promoter - Additives in a catalyst used for automobile exhaust treatment; Inhibitor - Poisoning of a catalyst by impurities
  1. Catalyst Regeneration and Recycling
  • Catalysts can often be regenerated or recycled after being used in a reaction
  • Regeneration methods: Heat treatment, chemical treatment, physical cleaning
  • Recycling reduces costs and environmental impact by minimizing the need for new catalysts
  1. Energy Diagram for a Catalyzed Reaction
  • Energy diagram illustrates the energy changes that occur during a chemical reaction
  • Activation energy (Ea): Energy required to activate the reaction
  • Catalyst provides an alternative, lower energy pathway (dashed line), reducing the activation energy
  • Overall, the reaction proceeds faster in the presence of a catalyst
  1. Nobel Prize Winners for Catalyst and Their Use in Reaction
  • Fritz Haber and Carl Bosch: Nobel Prize in Chemistry (1918) for the development of the Haber-Bosch process, which uses an iron catalyst to produce ammonia from nitrogen and hydrogen gases
  • Paul Sabatier: Nobel Prize in Chemistry (1912) for his method of hydrogenating organic compounds using a catalyst, known as catalytic hydrogenation
  • Gerhard Ertl: Nobel Prize in Chemistry (2007) for his studies of chemical processes on solid surfaces, including the catalytic reactions on metal surfaces
  1. Industrial Applications of Catalysts
  • Catalytic converters in automobiles: Platinum and palladium catalysts convert harmful exhaust gases (such as carbon monoxide, nitrogen oxides, and unburned hydrocarbons) into less toxic substances
  • Petroleum refining: Catalysts (such as zeolites, platinum, and palladium) are used to convert crude oil into useful products, such as gasoline, diesel, and jet fuel
  • Polymer production: Catalysts (such as Ziegler-Natta catalysts) are used to control the polymerization of monomers, resulting in the production of high-quality plastics and elastomers
  1. Enzyme Catalysis and Enzyme Kinetics
  • Enzymes are biological catalysts that speed up biochemical reactions in living organisms
  • Enzyme kinetics is the study of the rates and mechanisms of enzyme-catalyzed reactions
  • Enzymes exhibit specific catalytic activity and selectivity, allowing precise control over metabolic pathways
  1. Enzyme Kinetics - Michaelis-Menten Equation
  • Michaelis-Menten equation describes the relationship between the initial reaction rate (v₀), substrate concentration ([S]), and enzyme parameters (Km and Vmax)
  • Equation: v₀ = (Vmax [S]) / (Km + [S])
  • Km (Michaelis constant): Substrate concentration at which the reaction rate is half of the maximum rate (Vmax)
  1. Enzyme Inhibition
  • Inhibition occurs when a substance binds to the enzyme, reducing or preventing its catalytic activity
  • Competitive inhibition: Inhibitor competes with substrate for the active site of the enzyme
  • Non-competitive inhibition: Inhibitor binds to a different site on the enzyme, altering its conformation and reducing its activity
  1. Enzyme Inhibition - Examples
  • Aspirin: Inhibits the enzyme cyclooxygenase, reducing the production of prostaglandins and thromboxanes, which are involved in inflammation
  • Statins: Inhibit the enzyme HMG-CoA reductase, reducing cholesterol synthesis in the liver
  • Pesticides and herbicides: Inhibit enzymes necessary for the survival of pests or unwanted plants
  1. Enzyme Regulation
  • Cells regulate enzyme activity to maintain optimal metabolic conditions
  • Allosteric regulation: Binding of a molecule at a site other than the active site, altering the enzyme’s activity
  • Feedback inhibition: End product of a metabolic pathway inhibits an enzyme involved in an earlier step of the pathway
  1. Biocatalysis and Industrial Applications
  • Biocatalysis: The use of natural catalysts (enzymes) for chemical transformations
  • Advantages: High selectivity, mild reaction conditions, reduced environmental impact
  • Industrial applications of biocatalysis: Production of pharmaceuticals, synthesis of fine chemicals, biofuel production
  1. Conclusion
  • Chemical kinetics is the study of reaction rates and mechanisms, essential for understanding and controlling chemical processes
  • Catalysts play a crucial role in enhancing reaction rates and selectivity in various industrial and biological systems
  • The development of catalysts and the understanding of their mechanistic principles have led to significant advancements in chemistry and industry