chemical kinetics
speed or rate of a reaction and its mechanism (do not confuse with the equilibria or extent)
rate
the change in concentration of reactants or products overtime (mol/L/s)
rate equation
rate = - 1/a (delta [A]/delta t) = -1/b (delta [B]/delta t) = 1/c (delta [C]/delta t) = 1/d delta [D]/delta t)
concentration and rate
- substances must come into contact to react
- more particles collisions, more reactions
- rate usually increases with concentration
physical state and rate
- for heterogeneous reactions (two different phases) collisions between reactants only occur at interfaces between phases
- number of collision between the reactants per unit time depends on surface area of more condensed phase
- increases surface area!!
temperature and rate
- increase temperature, increases average kinetic energy of particles
- as KE increases, particles move faster and collide more frequently with greater energy
- increases rate
catalysts and rate
catalysts participate in a chemical reaction and increase the rate of the reaction withoutt undergoing a net chemical change itself
- highly selective
- often determine product by only speeding up one aspect of a reaction
average rate
the reaction rate between two points
instantaneous rate
using a tangent you can find the rate or gradient at any point
rate law
rate = - 1/a (delta [A]/delta t) = k [A]^n
rate: mol/L/sec
k is the rate constant and is characteristic of conditions
n is the rate order and is determined experimentally
first order
rate directly proportional to concentration of a reactant
k[A]^1
- if k is doubled, [A] is doubled
- if [A] decreases, rate decreases
- plotted against time, it is non-linear
- rate is fastest at beginning
- k is s-1
second order
rate is proportional to the square of [A]
k[A]^2
if [A] doubles, rate quadruples
initially faster than first and then slows
k is L/mol/s
zero order
rate is independent of concentration
k[A]^0 = k
- [reactant] graph is linear of -k
- [product] is linear of +k
molL/s
overall reaction order with more than one reactant
k[A]^n[B]^m
overall = n+m
determining rate order: method of initial rates
- preform the reaction a number of times and vary the conditions (iodine clock)
- look at if the rate doubles, quadruples etc. when concentration is changed
- can determine k with algebra
determining rate order: integrated rate law
use if:
- initial concentration of a reactant
- concentration of a reactant overtime
- several measures of [] between times
integrated rate law: zero order
[A]t = [A]0 -kt
linear [A] vs time
integrated rate law: first order
ln[A]t = ln[A]0 -kt
area under graph decreases linearly
linear if you plot ln[A] against t
integrated rate law: second order
1/[A]t = 1/[A]0 + kt
linear if 1/[A] vs t
half life for first order
half life: time it takes to halve a concentration to halve
t1/2 = ln2/k
- independent of [A]
- successive half lives do not change as [A] changes
radioactive decay
the emission of a particle or photon that results from the spontaneous decomposition of the unstable nucleus of an atom
- loss of a particle
- loss of beta particle
- emission of y radiation
- rate is independent of chemical and physical form of the isotopes or temperatures
- first order process
- isotopes with shot half lives decay faster
carbon dating
- living things have constant C14:C12 ratio
- C14 becomes N14 + beta- when they die
- the half life is 5700 +/- 30 years
- comparing ratio with that of living organisms helps determine age
half life for second order
t1/2 = 1/k[A]0
half life for zero order
t1/2 = [A]0/2k
