Chemistry Outcomes Review: Chapter 11

Reaction Pathways

In this final chapter, we tried to bring together much of what you already had learned in order to propose chemically reasonable pathways for chemical reactions. In addition, we introduced new concepts, based on measurements of reaction rates, to help in developing and understanding reaction pathways. We found that rates of reaction can be described in terms of rate laws that involve the concentrations of species in the reaction system and a proportionality constant, the rate constant. The species in the rate law are those that enter the reaction pathway before or during the rate-limiting step for the reaction. The rate constants for reactions are temperature dependent and their increase with increasing temperature is described by the Arrhenius equation.

The form of the Arrhenius equation is a consequence of the necessity for reactants to encounter one another in the right orientation and with enough energy to overcome an activation energy barrier and of the distribution of energy among encounters in the reacting system. Absorption of light by a reactant molecule is a way to initiate reactions that require more energy than is available thermally at room temperature.

We have to keep in mind that only reactions that are thermodynamically favorable, under the conditions specified, can occur spontaneously. Whether they will actually be observed to occur depends upon the kinetics of the reaction pathway(s) available.

Your understanding of the ideas in this chapter by reviewing these expected outcomes of your study.

You should be able to:

  • Determine the initial rate of a reaction in units of Ms-1 from data for the concentration (or a property that is directly proportional to concentration, such as gas pressure or absorbance) of a reactant or product as a function of time [Section 11.2].
  • Use initial rate and concentration data to determine the order of a reaction with respect to the concentrations of species in the solution and write the rate law for the reaction [Section 11.3].
  • Explain in words and/or with drawings how the rate of a reaction depends only on the rate of the rate-limiting step in the reaction pathway [Section 11.3].
  • Derive the rate law for a reaction whose pathway (mechanism), including knowledge of the rate-limiting step, you are given [Sections 11.4 and 11.5].
  • Propose a pathway (mechanism) for a reaction that is similar or analogous to one whose pathway you know [Sections 11.4 and 11.5].
  • Use data for concentration (or a property that is directly proportional to concentration) of a reactant or product as a function of time for a first order reaction to determine the rate constant and half life for the reaction [Section 11.5].
  • Use rate constants and/or half lives for radioactive decay to determine how long a sample of a radioisotope has been decaying from some known initial radioactivity [Section 11.5].
  • Determine whether a reaction is zeroth or first order in the concentration of some species by plotting appropriate functions of its concentration vs. time [Section 11.5].
  • Explain what is meant by "flooding" a reaction and use rate and concentration data for a reaction that is flooded with respect to a species to find the order of reaction with respect to the concentration of that species [Section 11.5].
  • Use the temperature variation of the rate constant (or variables directly proportional to the rate constant, such as rates with the same concentrations of all species) to determine the activation energy for a reaction [Section 11.6].
  • Use rate constant and activation energy values to determine the Arrhenius frequency factor for a reaction [Section 11.6].
  • Construct and interpret an activation energy diagram, given information about the activation energy and enthalpy change for the reaction [Section 11.6].
  • Describe how encounters between reactants in solution differ from collisions between reactants in the gas phase [Section 11.6].
  • Describe the origin of the temperature dependence of the rate constant and why the dependence is so strong when the average energy of molecular encounters does not increase so rapidly [Section 11.6].
  • Explain how light can initiate reactions that would not otherwise occur at low (room) temperature [Section 11.7].
  • Describe how competing photochemical and thermal reactions can lead to a steady state concentration of a reactive species in a system [Section 11.7].
  • Explain why some reactions that are highly favored thermodynamically are not observed to occur [Section 11.8].
  • Describe factors that can be changed to provide favorable kinetics for spontaneous reactions that are not otherwise observable [Section 11.8].
  • Relate the temperature dependence of an equilibrium constant to the temperature dependences of the forward and reverse rate constants for the reaction [Section 11.8].