5 Kinetics

Explain the relationship between the rate of a chemical reaction and experimental parameters.

• The kinetics of a chemical reaction is defined as the rate at which an amount of reactants is converted to products per unit of time.
• The rates of change of reactant and product concentrations are determined by the stoichiometry in the balanced chemical equation.
• The rate of a reaction is influenced by reactant concentrations, temperature, surface area, catalysts, and other environmental factors.

Represent experimental data with a consistent rate law expression.

• Experimental methods can be used to monitor the amounts of reactants and/or products of a reaction over time and to determine the rate of the reaction.
• The rate law expresses the rate of a reaction as proportional to the concentration of each reactant raised to a power.
• The power of each reactant in the rate law is the order of the reaction with respect to that reactant. The sum of the powers of the reactant concentrations in the rate law is the overall order of the reaction.
• The proportionality constant in the rate law is called the rate constant. The value of this constant is temperature dependent and the units reflect the overall reaction order.
• Comparing initial rates of a reaction is a method to determine the order with respect to each reactant.

Identify the rate law expression of a chemical reaction using data that show how the concentrations of reaction species change over time. Kinetics UNIT 5

• The order of a reaction can be inferred from a graph of concentration of reactant versus time.
• If a reaction is first order with respect to a reactant being monitored, a plot of the natural log (ln) of the reactant concentration as a function of time will be linear.
• If a reaction is second order with respect to a reactant being monitored, a plot of the reciprocal of the concentration of that reactant versus time will be linear.
• The slopes of the concentration versus time data for zeroth, first, and second order reactions can be used to determine the rate constant for the reaction. Zeroth order: EQN: [A]t − [A]0 = −kt First order: EQN: ln[A]t − ln[A]0 = −kt Second order: EQN: 1/[A]t − 1/[A]0 = kt
• Half-life is a critical parameter for first order reactions because the half-life is constant and related to the rate constant for the reaction by the equation: EQN: t1/2 = 0.693/k.
• Radioactive decay processes provide an important illustration of first order kinetics.

Represent an elementary reaction as a rate law expression using stoichiometry.

• The rate law of an elementary reaction can be inferred from the stoichiometry of the particles participating in a collision.
• Elementary reactions involving the simultaneous collision of three or more particles are rare.

Explain the relationship between the rate of an elementary reaction and the frequency, energy, and orientation of particle collisions.

• For an elementary reaction to successfully produce products, reactants must successfully collide to initiate bond-breaking and bondmaking events.
• In most reactions, only a small fraction of the collisions leads to a reaction. Successful collisions have both sufficient energy to overcome the activation energy requirements and orientations that allow the bonds to rearrange in the required manner.
• The Maxwell-Boltzmann distribution curve describes the distribution of particle energies; this distribution can be used to gain a qualitative estimate of the fraction of collisions with sufficient energy to lead to a reaction, and also how that fraction depends on temperature.

Represent the activation energy and overall energy change in an elementary reaction using a reaction energy profile.

• Elementary reactions typically involve the breaking of some bonds and the forming of new ones.
• The reaction coordinate is the axis along which the complex set of motions involved in rearranging reactants to form products can be plotted.
• The energy profile gives the energy along the reaction coordinate, which typically proceeds from reactants, through a transition state, to products. The energy difference between the reactants and the transition state is the activation energy for the forward reaction.
• The rate of an elementary reaction is temperature dependent because the proportion of particle collisions that are energetic enough to reach the transition state varies with temperature. The Arrhenius equation relates the temperature dependence of the rate of an elementary reaction to the activation energy needed by molecular collisions to reach the transition state.
  • Exclusion: Calculations involving the Arrhenius equation will not be assessed on the AP Exam.

Identify the components of a reaction mechanism.

• A reaction mechanism consists of a series of elementary reactions, or steps, that occur in sequence. The components may include reactants, intermediates, products, and catalysts.
• The elementary steps when combined should align with the overall balanced equation of a chemical reaction.
• A reaction intermediate is produced by some elementary steps and consumed by others, such that it is present only while a reaction is occurring.
• Experimental detection of a reaction intermediate is a common way to build evidence in support of one reaction mechanism over an alternative mechanism.
  • Exclusion: Collection of data pertaining to detection of a reaction intermediate will not be assessed on the AP Exam.

Identify the rate law for a reaction from a mechanism in which the first step is rate limiting.

• For reaction mechanisms in which each elementary step is irreversible, or in which the first step is rate limiting, the rate law of the reaction is set by the molecularity of the slowest elementary step (i.e., the rate-limiting step).
  • Exclusion: Collection of data pertaining to detection of a reaction intermediate will not be assessed on the AP Exam.

Identify the rate law for a reaction from a mechanism in which the first step is not rate limiting.

• If the first elementary reaction is not rate limiting, approximations (such as pre-equilibrium) must be made to determine a rate law expression.

Represent the activation energy and overall energy change in a multistep reaction with a reaction energy profile.

• Knowledge of the energetics of each elementary reaction in a mechanism allows for the construction of an energy profile for a multistep reaction. 104

Explain the relationship between the effect of a catalyst on a reaction and changes in the reaction mechanism.

• In order for a catalyst to increase the rate of a reaction, the addition of the catalyst must increase the number of effective collisions and/ or provide a reaction path with a lower activation energy relative to the original reaction coordinate.
• In a reaction mechanism containing a catalyst, the net concentration of the catalyst is constant. However, the catalyst will frequently be consumed in the rate-determining step of the reaction, only to be regenerated in a subsequent step in the mechanism.
• Some catalysts accelerate a reaction by binding to the reactant(s). The reactants are either oriented more favorably or react with lower activation energy. There is often a new reaction intermediate in which the catalyst is bound to the reactant(s). Many enzymes function in this manner.
• Some catalysts involve covalent bonding between the catalyst and the reactant(s). An example is acid-base catalysis, in which a reactant or intermediate either gains or loses a proton. This introduces a new reaction intermediate and new elementary reactions involving that intermediate.
• In surface catalysis, a reactant or intermediate binds to, or forms a covalent bond with, the surface. This introduces elementary reactions involving these new bound reaction intermediate(s).