Chemical Kinetics

chemical kinetics

the study of the changes in concentrations of reactants or products as a function of time

factors that affect the rate

conentration

physical state

temperature

the use of a catalyst

how concentration can affect the rate

molecules must collide in order to react

how physical state can affect the rate

molecules must mix in order to collide

how temperature can affect the rate

molecules must collide with enough energy to react

reaction rate

the change in concentration of a reactant or a product per unit time

average rate

the change in concentration of reactant (or products) over a finite period of time

instantaneous rate

the reaction rate at a particular time, given by the slop of a tangent to a plot of reactant concentration vs. time

rate law

an expression in which the reaction rate will depend only on the conentration of the reactants

general form for rate law

rate = k[A]^m

aA --> products

reaction order

a postive or negative exponent, for a reactant, for which the concentration is raised to in a rate law

differential rate law

shows how the rate of reaction depends on the concentration

integrated rate law

shows how the concentrations of species in the reaction depend on time

determining a rate law

1) to determine the form of rate law

2) determine experimentally the power to which each reactant concentration must be raise in the rate law

overalll reaction order

the sum of the orders for the various reactants

energy diagram

a method of depicting potential energy for a system during a reaction as a smooth curve

catalyst

a substance that initiates or accelerates a chemical reaction without itself being affected or consumed

endothermic

occurring or formed with absorption of heat

exothermic

occurring or formed with release of heat

reaction intermeadiate

a specie that forms during a reaction, but are consumed along the process

collision theory

For a reaction to occur, the particles must collide, they must collide with the appropriate orientation, and they must collide with sufficient energy.

how to improve the chances of meeting the requirements for collision theory to occur

increased conentration

increased surface area

add catalyst

state of matter

increased temperature

first-order reaction

means as the concentration doubles, the rate also doubles

integrated rate law for a first-order reaction

ln[A] = -kt + ln[A (initial) ]

integrated rate law for a second-order reaction

1/[A]t = kt + 1/[A (initial) ]

integrated rate law for a zero-order reaction

[A]t = -kt + [A (initial) ]

rate law for a zero-order reaction

rate = K

(straight)

rate law for a first-order reaction

pick a point, find the slope of the tangent to said point

the slope will equal k

instantaneous

activated complex

an unstable arrangement of atoms that exists momentarily at the peak of the activation-energy barrier; an intermediate or transitional structure formed during the course of a reaction

natural log

A logarithm with base e (2.71...). More specifically, the exponent of the power to which e must be raised to equal a given number

rate law for second-order reaction

k = the slope of the linear plot

slope for a zero-order reaction

slope for a first-order reaction

m < 0

slope for a second-order reaction

m > 0

plot needed to give a straight line for a zero-order reaction

[A] vs time

plot needed to give a straight line for a first-order reaction

ln[A] vs time

plot needed to give a straight line for a second-order reaction

1/[A] vs. time

relationship of rate constant to the slope of a straight line for a zero-order reaction or a first-order reaction

k = -m

relationship of rate constant to the slope of straight line for a second-order reaction

k = m

half life for a zero-order reaction

t(1/2) = 1/(k[A])

half life for a first-order reaction

t(1/2) = ln2/k

half life for a second-order reaction

t(1/2) = 1/k[A initial]

reaction mechanism

a series of simpler reactions that sum to the overall reaction that are influenced by elementary steps, molecularity, and rate

elementary reactions

simple reactions that describe a single molecular event in a proposed reaction mechanism

molecularity

the number of reactant particles involved in a proposed reaction mechanism

unimolecular reaction

an elementary reaction that involves the decomposition or rearrangement of a single particle

bimolecular reaction

an elementary reaction involving two reactant species

termolecular reaction

an elementary reaction that involves three reactant molecules

rate-limiting or rate-determining step

the slowest step in a reaction mechanism and therefore is the step that limits the overall rate of the reaction

traits of a reaction intermediate

usually unstable, relative to the reactants and products

molecules with normal bonds

sometimes stable enough to be isolated

activation energy

the minimum amount of energy required for a molecule to react

how reactant concentration affects collision theory

the more moles of reactant present, the increased likelihood of a collision

rate depends on the product of the reactant

how temperature affects collision theory

-increasing temperature increases the average speed of the reactants, which increases the frequency of the collisions.

how molecular structure affects collision theory

collisions are effective or noneffective

how an effective collision affects collision theory

it increases the rate as particles line up in such a way that a reaction occurs upon collision

transition state theory

a theory stating that species form that are neither a product nor reactant, but something in between

rate constant, k

an experimentally determined proportionality constant that gives some measure of the intrinsic "reactivity" of the reaction

integration

an operation used in the calculus whereby the integral of a function is determined

the difference between rate constant and rate of reaction?

The rate of a reaction is the change in concentration with respect to time of a product. The rate equals the rate constant times the concentrations of the reactants raised to their orders. A rate constant is a proportionality constant in the rate law that is a measure of the intrinsic reactivity of the reaction. The rate constant does not depend on the concentrations of reactants but the rate does (if it is not zero order).