kinetics: rate at which an amount of reactants is converted to products per unit of time
rates of change of concentrations are determined by the stoichiometry in the balanced chemical equation
influenced by:
reactant concentrations
temperature
surface area
catalysts
other environmental factors
rate law: rate of reaction is proportional to the concentration of each reactant raised to a power
power of each reactant: order of the reaction with respect to that reactant
sum of the powers of the reactant concentrations: overall order of the reaction
rate constant: constant of proportionality
temperature-dependent
units reflect the overall reaction order
experimental methods can be used to:
monitor the amounts of reactants and/or products of a reaction over time
determine the rate of a reaction
comparing the initial rates of a reaction is a method to determine the order with respect to each reactant
concentration changes over time
the order of a reaction can be inferred from a graph of concentration of reactant versus time, based on which graph is linear
zeroth order: concentration
first order: natural log (ln) of concentration
second order: reciprocal of concentration
integrated rate law: slopes of the concentration versus time data can be used to determine the rate constant for the reaction
half-life of first-order reaction is constant
radioactive decay processes provide an important illustration of first order kinetics
elementary reaction: single-step reaction with no intermediates
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
collision model
for an elementary reaction to successfully produce products, reactants must successfully collide to initiate bond-breaking and bond-making events
in most reactions, only a small fraction of the collisions leads to a reaction
successful collisions have:
sufficient energy to overcome the activation energy requirements
orientations that allow the bonds to rearrange in the required manner
Maxwell-Boltzmann distribution curve (distribution of particle energies) can be used to gain a qualitative estimate of:
the fraction of collisions with sufficient energy to lead to a reaction
how that fraction depends on temperature
reaction energy profile
elementary reactions typically involve the breaking of some bonds and the forming of new ones
reaction coordinate: axis along which the complex set of motions involved in rearranging reactants to form products can be plotted
energy profile: energy along the reaction coordinate
reactants → transition state → products
activation energy for the forward reaction: difference between the reactants and the transition state
rate of an elementary reaction is temperature dependent
the proportion of particle collisions that are energetic enough to reach the transition state varies with temperature
reaction mechanism: consists of a series of steps (elementary reactions) that occur in sequence
components
reactants
intermediates
products
catalysts
elementary steps when combined should align with the overall balanced equation of a chemical reaction
reaction intermediate: produced by some elementary steps and consumed by others
only present 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
when each elementary step is irreversible or the first step is rate limiting: rate law is set by the molecularity of the rate-limiting step(slowest elementary step)
if the first elementary reaction is not rat limiting, approximations (such as pre-equilibrium) must be made to determine a rate law expression
pre-equilibrium approximation: assuming the reactants and intermediates are in equilibrium, one can solve for the rate of product formation
energy profile: representation of a chemical reaction or process as a single energy pathway as the reactants are transformed into products
knowledge of the energetics of each elementary reaction in a mechanism allows for the construction of an energy profile for a multistep reaction
catalysis
in order for a catalyst to increase the rate of a reaction, the addition of the catalyst must do at least one of:
increasing the number of effective collisions
providing a reaction path with a lower activation energy relative to the original reaction coordinate
the net concentration of the catalyst is constant in a reaction mechanism
the catalyst will frequently be consumed in the rate-determining step and later regenerated in a subsequent step
some catalysts accelerate a reaction by binding to the reactants
orient more favorably
react with lower activation energy
often a new reaction intermediate in which the catalyst is bound to the reactants
some catalysts involve covalent bonding between the catalyst and the reactants
e.g. acid-base catalysis: reactant or intermediate gains or loses a proton
introduces a new reaction intermediate and new elementary reactions involving that intermediate
surface catalysis: reaction or intermediate binds to/forms a covalent bond with the surface
introduces elementary reactions involving the new bound reaction intermediates
