kinetics Flashcards
what is rate of reaction
the change of concentration of reactant or product per unit time
what is rate
amount of reactant used/product made
over time
what is collision theory
for a reaction to occur the particles must collide in the right direction and they must also have a minimum amount of kinetic energy (the Ea)
what is activation energy
the minimum amount of energy required for a reaction to occur
what is the peak on a maxwell boltzmann distribution graph
the most likely energy of a particle in a sample
where is the mean energy on maxwell boltzmann distribution graph
right from the most likely energy
factors of maxwell boltzmann distribution graph
starts at 0,0 as no particles have no kinetic energy
area under curve is equal to total number of molecules
y axis = number of molecules
x axis = kinetic energy
how does temperature affect rate of reaction
particles have on average more kinetic energy when heated
larger proportion of particles will have energy greater than the activation energy
temperature on maxwell boltzmann distribution
curve shifts to the right
peak is lower
area under curve is the same
area under curve beyond Ea increases
Ea stays the same
why do we get a faster rate of reaction when temperature is increased
particles move around more at higher temperatures
there are more frequent successful collisions which are more energetic
why do we get a faster rate of reaction when pressure or concentration is increased
particles are closer together so collide more often
there are more frequent successful collisions and so higher chance of a reaction
what is a catalyst
a substance than increases the rate of reaction by providing an alternative pathway that has a lower activation energy, it remains chemically unchanged
how do catalysts affect industry
1) lowers temperature needed = less money spent on energy and less co2 produced
2) speed up reaction so faster production
3) change properties of a product
4) less waste produced as catalysts allow reactions with better atom economies
what is a heterogeneous catalyst
in a different phase (state) from the reactants
what happens when surface area is increased with heterogeneous catalyst
increase rate of reaction as more particles can react with catalyst at the same time
what is a homogeneous catalyst
in the same phase (state) as the reactants
how do homogeneous catalysts work
they form an intermediate by reactants combining with catalyst to form a product. catalyst is reformed again
how do heterogeneous catalysts work
1) substances adsorb to the surface of catalysts
2) bonds in reactants weaken and break to form radicals and radicals react together to form new substances
3) new molecules desorb from surface of catalyst
how do homogenous catalysts affect energy profile diagrams
they have 2 activation energies (double bump, 1st is higher)
1st - forms intermediate
2nd - intermediate breaks
what is the affect of increased surface area on the rate of reaction
more frequent successful collisions as more surface for particles to collide with
catalyst on Maxwell Boltzmann distribution graph
area under curve is the same
peak is same
Ea shifts left
more particles with Ea
what is order of a reaction
the power to which the concentration of that reactant is raised in the rate equation
when is the rate constant constant
when the temperature is constant
what is rate equation
rate = K [A]a [B]b
zero order reaction
no impact on rate
rate-conc graph straight line
rate=K
first order reaction
conc and rate directly proportional
rate=K[A]
second order reaction
rate proportional to conc squared
rate=K[A]^2
conc time graph zero order
linear
conc time graph first and second order
both curved
second order has kink
define half life
time taken for initial conc of the reactants to decrease by half
what is the half life in first order reaction
stays constant at 100 50 and 25%
what are experimental techniques used to obtain rate data
measuring change in reactant/ product mass/conc over time - produces gas
change in electrical conductivity - loss/gain of ions
change in pH
how can you find rate of reaction from mass change
if gas is produced then mass decreases as reaction proceeds
plot mass-time graph
how can you find rate from volume of gas evolved
measure volume of gas produced over time in reaction
plot volume-time graph
how to use titration to work out rate of reaction
small samples of a reaction mixture can be removed at regular intervals throughout a reaction and titrated to find a concentration of a reactant/product
how to use colorimetry to work out rate of reaction
for a reaction that involves the formation of a coloured species
colorimeter used to measure absorbance of light of solutions at known concentrations
then the absorbance of samples from reaction mixture can be measured to convert calibration curve to conc readings
what is the rate determining step
the slowest step in a multi step chemical reaction
what is the Arrhenius equation
k=Ae^ -Ea/RT
OR
lnK = lnA -Ea/RT
describe Sn1 reaction
the leaving group leaves first
forms a carbocation intermediate
1st step is rate determining so rate equation is conc of haloalkane
describe Sn2 reaction
leaving group leaves while the nucleophile attacks the molecule
rate equation is conc of haloalkane and nucleophile
how to slow down titration to get rate values
-cool it down
-add distilled water
-add chemical to stop reaction (quenching)
if we didn’t slow down reaction the conc would proceed to change as you conduct titration
what is a colorimeter
measures absorbance of light in a coloured sample, if it is more concentrated the colour is darker hence more light is absorbed
propanone plus iodine colour change
brown to colourless
in what conditions will k change in the rate equation
if temp increases then k increases
why does temperature increase k in rate equation
more kinetic energy-> more collisions -> higher rate
concs stay constant so k must increase
what assumptions do you make in clock reaction
conc of reactants doesn’t change significantly
temp remains constant
reaction has not proceeded too far when the end point is seen
why does half life not change in zero order
rate is constant as conc doesn’t affect rate so half life will decrease
why is there another substance in rate equation that isn’t a reactant
it is a catalyst
why do tertiary haloalkanes proceed by sn1
less space around carbon being attacked as it is bonded to multiple other carbons so it is more stable
what happens to rate constant when activation energy gets smaller
gets bigger
as the activation energy drops the rate of reaction increases as more particles have enough energy to collide
