CHEMICAL KINETICS

• Chemical Kinetics


  Deals with the study of the rates of chemical reactions and factors which affect the reaction rates
• Kinetics


  Studies the rate at which chemical process occurs
• Besides information about the speed at which reaction occur, kinetics also sheds light on reaction mechanism (exactly how the reaction occurs)
• Reaction Rates


  The speed at which the reactants disappear or the speed at which the products appear
• In reversible reactions, as products accumulate, they can begin to turn back into reactants
• Early on the rate will depend on only the amount of reactants present. We want to measure the reactants as soon as they are mixed.
• Factors Affecting Reaction Rates

  • Concentration of reactant
  • Temperature
  • Catalyst
  • Surface area of a solid reactant
  • Pressure of gaseous reactants or products
• Concentration of reactant


  Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time t
• Temperature


  At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy
• Generally, as temperature increases, so does the reaction rate because the rate constant k is temperature dependent
• Collision Theory


  When two chemicals react, their molecules have to collide with each other (in a particular orientation) with sufficient energy for the reaction to take place
• Kinetic Theory


  Increasing temperature means the molecules move faster
• Catalyst

  A substance that increases the rate of a chemical reaction without itself being consumed
• Catalysts increase the rate of a reaction by decreasing the activation energy of the reaction
• Catalysts change the mechanism by which the process occurs
• Heterogeneous catalyst


  One that is present in a different phase as the reacting molecules
• Homogeneous catalyst


  One that is present in the same phase as the reacting molecules
• Catalysts operate by lowering the overall activation energy, Ea, for a reaction
• Catalysts can operate by increasing the number of effective collisions
• A catalyst usually provides a completely different mechanism for the reaction
• Reaction Rate Law


  The equation that expresses the rate of a reaction as a function of the concentration of the involved species (e.g. reactant, products, catalysts)
• Rates of reactions increase as concentration increase since there are more collisions occurring between reactants
• Rate Law


  v = k [A]^m [B]^n..., where [A], [B] are reactant concentrations, m and n are reaction orders, and k is the rate constant
• Rate laws, rate constants and orders are determined experimentally
• The order of a reactant is NOT generally related to its stoichiometric coefficient in a balanced chemical equation
• Elementary Reactions


  Reactant order reflects molecularity (# of molecules involved in reaction)
• Order of Reactions


  Zero order: change in concentration has no effect on rate

  First order: doubling concentration doubles rate

  Second order: doubling concentration quadruples rate
• Half-life


  The time taken for the concentration of a reactant to drop to half its original value
• For a first-order process, the half-life does not depend on the initial concentration [A]0