Lecture 5 1st p

• Reversible Reaction


  A chemical reaction in which products formed react to the original reactants
• Equilibrium


  The concentration of the products and the reactants are not changing
• Le Chatelier's Principle


  States that a system at equilibrium will shift to relieve stress and restore equilibrium under a new set of conditions
• Chemical Kinetics


  The study of reaction rates and reaction mechanisms
• Reaction Rate


  The study of change during a chemical reaction
• Factors affecting Reaction Rate

  • Concentration of Reacting Species
  • Reaction Temperature
  • Presence or Absence of Catalysts
  • Nature of the Reactants
• Rate of Reaction


  The change in concentration over the change in time
• Rate Constant


  A specific value for each reaction, determined experimentally, that changes with temperature
• For gas-phase reactions, use partial pressure (PA) instead of concentration [A]
• As concentration of reactants increases


  The likelihood that reactant molecules collide increases
• As temperature increases


  Reactant molecules have more kinetic energy and move faster, colliding more often and with greater energy
• Catalysts


  Speed up reactions by changing the reaction mechanism
• The Ideal Gas Law is PV = nRT
• Elementary Reaction


  Assumed to occur in a single step and to pass through a single transition. No intermediates are formed.
• Composite Reaction


  A chemical reaction which involved rate constant of more than single elementary reaction. (Rate of reaction must determined experincably)
• Unimolecular reaction


  • A→P
  • Rate = k [A]
  • First order reaction
• Bimolecular reaction


  • A+B →P
  • Rate = k [A] [B]
  • Second order reaction
• First-order Processes


  1. In [A] = -kt + In [A]o

  2. When In [A] is plotted as a function of time, a straight line results
• Second-order Processes


  1. 1/[A] = kt + 1/[A]o

  2. The plot of 1/[A] vs time yields a straight line
• Rate Equation: a+bB→cC +dD

  If n=1 and m=0, the reaction has first order dependence on [A]

  If n≠1 and m≠0, then the reaction is nth order in [A] and mth order in [B]

  Overall order = n + m
• Order of reaction


  The experimentally determined dependence of the rate of reaction on the concentration of the reactants
• Molecularity


  Determined by the number of species included in the collision to form the activated complex in the elementary reaction
• Elementary Reactions


  • Unimolecular
  • Bimolecular
  • Termolecular (extremely rare)
• Zero order reaction


  • V = k
  • [A] = [A]o - kt
  • Units: mol.L-1.s-1
• First order reaction


  • V = k[A]
  • [A] = [A]o e^(-kt)
• Second order reaction


  • V = k[A][B]

  [A] = 1/(1/[A]o + kt)
• Temperature increase


  Rate constant increases

  Reaction rate increases
• Arrhenius Equation


  k = A exp(-Ea/RT)

  Ea = Activation Energy

  A = Pre-exponential factor
• Catalysts increase the rate of reaction by decreasing the activation energy of the reaction and changing the mechanism by which the process occurs
• Enzyme catalysis


  The substrate fits into the active site of the enzyme much like a key fits into a lock