Unit 5: Kinetics

rate formula

change in amount/change in time

kinetics

the rate at which reactants are converted into products

concentration rates of change

determined by stoichiometry in the balanced chemical equation, always is a positive value

average rate

found by taking the difference in molarity over the difference in time

instantaneous reaction rate

the rate of a reaction at a period of time that is so short that the concentrations of reactants and products change by a negligible amount

initial rate

rate for the first interval on a graph

ways to increase rate of reaction

increase surface area, concentration, temperature, increase pressure/decrease volume (only when reactants are gaseous), add a catalyst

reaction requirements

particles must collide, have proper orientation, and have sufficient energy

rate laws

can only be deducted from experiement

doubling concentration of zeroeth order reaction

rate stays the same

doubling concentration of first order reaction

rate doubles

doubling concentration of second order reaction

rate quadruples

doubling concentration of third order reaction

rate octuples

rate law formula

rate=k [reactant 1]ⁿ [reactant 2]^m

overall reaction order

sum of each reactants order

0th order linear y-axis label

[A]

1st order linear y-axis label

ln [A]

2nd order linear y-axis label

1/[A]

half-life

the time required for the concentration of a reactant to decrease to half of its initial concentration

half-life formula

t_1/2=.693/k

faster reaction

signified by shorter half-life and higher rate constant

steps to determine order

1. is it linear? yes, 0th

2. definite half life? yes, 1st

3. neither apply? 2nd

rate unit for all orders

Ms^-1

0th order k label

Ms^-1

1st order k label

s^-1

2nd order k label

M^-1s^-1

0th and 1st order slope

-k

2nd order slope

k

0th order equation

[A]_t - [A]₀ = -kt

1st order equation

ln[A]_t - ln [A]₀ = -kt

2nd order equation

1/[A]_t - 1/[A]₀ = kt

elementary reactions

series of simple reactions that represent the progress of the overall reaction at the molecular level

reaction mechanism

sequence of elementary steps that leads to product formation

intermediates

species that are not reactants or products (they are formed and then consumed), not included in the overall reaction equation

catalyst

substance that increases the rate of a reaction by stabilizing the transition state or forming a new one, not included in the overall reaction equation

rate determining step

the slowest step of a chemical reaction

first step

if this step is slow, the overall rate for the reaction will be based on the stoichiometry of this elementary reaction

subsequent step

if this step is slow, the reactants up to this step, minus the intermediates, must be added to determine the rate of reaction

reaction profiles

a way to graphically represent a reaction, y-axis is potential energy, x-axis is reaction coordinate

collision theory

for a reaction to occur, a collision between molecules must occur with the correct orientation of molecules and with sufficient energy

activation energy

the minimum amount of energy a reactant must have in order to undergo a reaction

transition state

the intermediate point between two stable compounds

activated complex

the structure that results in the maximum energy point along the reaction path

enthalpy

potential energy difference between the products and the reactants

exothermic

negative enthalpy, releases energy

endothermic

positive enthalpy, gain energy

catalysts or intermediates in slow step

if present in the slow step, include them in the rate law

overall balanced equation

rate law expression cannot be determined from this

elementary step

rate law expression can be determined from this

rate (forward) = rate (reverse)

use with a reversible fast step to determine rate law formula

chemical equilibrium

both the reactants and products are present in concentrations which have no further tendency to change with time

steady state approximation

method used to estimate the overall reaction rate of a multi-step reaction.

largest activation energy

represents the slowest step on a graph

valleys

represent intermediates on a graph

ways catalysts work

altering the rate of reaction by changing the number of effective collisions or altering the chemical pathway to one that requires less energy

homogeneous catalysts

catalysts in the same phase as the reactants and can mix with them uniformly

heterogeneous catalysts

catalysts in a different phase from the reacting particles

enzymatic catalysts

speeds up the reaction by binding with one or more of the reactants either by creating a more favorable orientation or lowering the amount of energy required to react