Chemical Kinetics

• A reaction may be thermodynamically favored but not kinetically favored
• Kinetics


  Studies the rates of a chemical process
• Reaction mechanism


  The study of kinetics gives insights into the reaction mechanism
• Rate of a reaction


  A measure of how fast a reaction takes place
• Rate of a reaction

  Often expressed as a change in amount or concentration of a substance (reactant or product) per unit time
• Chemical kinetics


  The study of rates of reactions and factors that affect them
• Rates of reactions


  Can be determined by monitoring the change in concentration of either reactants or products as a function of time
• The (- ve) sign is used because the concentration is decreasing
• Instantaneous Rate


  Change in the concentration of reactants or products at a given instant of time
• Factors that affect rates of reactions

  • Nature of reactants
  • Surface area of Reactants
  • Concentration of Reactants
  • Temperature
  • Catalyst
• Catalyst


  A substance that increases the reaction rate without undergoing chemical change
• Catalyst


  • Provides an alternative reaction mechanism faster than mechanism in absence of catalyst
  • Lowers the activation energy for a chemical reaction
• Homogenous Catalyst


  Reaction mixture and catalyst both are present in the same phase
• Heterogeneous catalyst


  Catalysts exist in a different phase than the reaction mixture
• Processes using heterogeneous catalysts

  • Haber-Bosch process for the synthesis of ammonia
  • Fischer–Tropsch process to produce a variety of hydrocarbons
• Heterogeneous catalysts


  • May be used as fine particles, powders, granules
  • May be deposited on the solid support (supported catalysts)
  • May be used in bulk form (unsupported catalysts)
• Collision Model Theory


  • Contact is necessary between reactants
  • Contact must lead to breaking of bonds (need sufficient energy)
  • The reactants must have proper orientation
• If a reaction occurs, during the collision, the particles that separate are different from those that collide
• Particles collide, molecules slow down, kinetic energy decreases. Energy can't disappear that means potential energy of the particles must increase
• Activation Energy (Ea)


  Minimum amount of energy required for reaction
• Catalyst


  • Provides a path for the reaction with a lower activation energy
  • When a catalyst is present, more molecules possess the minimum amount of energy needed for effective collisions
• Initial Rates


  Rates measured at the beginning of the reaction, which is dependent on the initial concentrations of reactants
• Instantaneous Rates


  Rates measured at any point during the reaction
• Average Rates


  An overall rate measured over a period or time interval
• Average rate decreases as the reaction proceeds
• The slope of a line tangent to the curve at any point is the instantaneous rate at that time