Chemistry Kinetics

• For the reaction aA + Bb → cC + dD the rate law is k[A]^m[B]^n
• If a rate law is second order (reactant), doubling the reactant quadruples the reaction rate.
• Reaction rates are affected by reactant concentrations and temperature. This is accounted for by the collision model.
• The minimum energy to initiate a chemical reaction is the activation energy.
• Reaction rate data obey an equation devised by Arrhenius.
• Concentration


  Molecules must collide to react.
• Physical State


  Molecules must mix to collide.
• Temperature


  Molecules must collide with enough energy to react.
• Reaction Rate


  Changes in the concentrations of reactants or products per unit time.
• Reactant concentrations decreases while product concentration increase.
• Rate Law


  A mathematical expression that relates the concentrations of reactants to the reaction rate.
• R

  8.314 J/mol*K
• The overall rate of a reaction is related to the rate of the slowest, or

  rate-determining step.
• Catalysts


  • Lowers the energy of activation.
  • Increases the rate of the FORWARD and the REVERSE reactions.
  • A catalyzed reaction yields the products more quickly, but does not yield more product than the uncatalyzed reaction.
  • A catalyst lowers Ea by providing a different mechanism, for the reaction through a new, lower energy pathway.
• Under a specific set of conditions, every reaction has its own characteristic rate, which depends upon the chemical nature of the reactants.
• A reaction was found to be second order in carbon monoxide concentration. The rate of the reaction increases by a factor of 4 if the [CO] is doubled, with everything else kept the same.
• The kinetics of the reaction below were studied and it was determined that the reaction rate increased by a factor of 9 when the concentration of B was tripled. The reaction is third order in B.

  A + B → P
• The kinetics of the reaction below were studied and it was determined that the reaction rate did not change when the concentration of B was tripled. The reaction is zero order in B.

  A + B → P
• A reaction was found to be third order in A. Increasing the concentration of A by a factor of 3 will cause the reaction rate to increase by a factor of 27.
• A reaction was found to be zero order in A. Increasing the concentration of A by a factor of 3 will cause the reaction rate to remain constant.
• For a first-order reaction, a plot of ln [A]t, versus t is linear
• A burning splint will burn more vigorously in pure oxygen than in air because oxygen is a reactant in combustion and concentration of oxygen is higher in pure oxygen.
• One difference between first- and second-order reactions is that the half-life of a first-order reaction does not depend on [A]0 ; the half-life of a second-order reaction does depend on [A]0.
• As the temperature of a reaction is increased, the rate of the reaction increases because the reactant molecules collide with greater energy per collision.
• The rate of a reaction depends on

  • Collision Frequency
  • Collision Energy
  • Collision Orientation
• In the energy profile of a reaction, the species that exists at the maximum on the curve is called the activated complex.
• In general, as temperature goes up, reaction rate goes up regardless of whether the reaction is exothermic or endothermic.
• Adding a catalyst for the reaction will lower the activation energy for a reaction.
• A catalyst can increase the rate of a reaction by providing an alternative pathway with a lower activation energy.
• The primary source of the specificity of enzymes is their shape, which relates to the lock-and-key model.
• Heterogeneous catalysts are used in automotive catalytic converters.
• Nitrogen fixation is a difficult process because nitrogen is very unreactive, largely due to its triple bond
• The number of molecules that participate as reactants defines the molecularity of the reaction.
• Effusion


  the escape of gas molecules through a tiny hole.
• Diffusion


  the spread of one substance throughout a space or throughout a second substance.
• Graham's Law (Thomas Graham)


  The effusion and diffusion rates of gas are inversely proportional to the square of its molar mass.