kinetics ii Flashcards
what is reaction rate ?
.measure of how fast a reaction takes place
.defined as the change in concentration of a reactant or product per unit time
what is collision theory
.for particles to react they must collide with sufficient energy to react and at the correct orrientation
.the minimum energy particles need to react is called the activation energy
describe Maxwell Boltzmann distribution
.particles in substance do not all have the same amount of energy
.energy of particles of gas is shown by a Maxwell Boltzmann distribution
.there are no particles with no energy
.area under curve represents total number of particles
.the peak of the curve gives the the energy that more particles have than any other energy
.number of particles on y axis energy on y axis
what are the factors affecting rate
concentration of a solution, surface area of a solid, pressure of gases ,temperature, catalyst
what affects does concentration have on the rate
.the higher the concentration the faster the reaction (unless that reagent is zero order). because particles are closer together and so there are more frequent and successful collisions
what affects does surface area have on the rate
.the more pieces a solid is broken up to the bigger the surface area the greater the surface area the greater the rate of reaction, because there are more particles exposed at the surface that can be collided with and so there’s more frequent successful collisions
what affect does pressure have on the rate
the higher the pressure the higher the rate, particles have more energy and so greater proportion of the collisions are successful the particles also move faster and so collisions are more frequent therefore there are more frequent successful collisions
what affect does a catalyst have on the rate
a catalyst is a substance that increases the rate of a reaction but is not used up it provides alternative route /mechanism with a lower activation energy and therefore a greater proportion of the collisions are successful
what is order of reaction
power to which the concentration of the reactant is raised in the rate equation.
what is overall order of reaction
sum of the powers of the concentration terms in the rate equation.
The only thing that affects the value of k (the rate constant) is ….
temperature
what are the different ways to measure reaction rate
measuring gas volume
loss of mass
colour change/colorimetry
change in ph
titration
electrical conductivity
how to measure reaction rate by gas volume
collect the gas given off in a gas syringe and record how how much there is at regular time interviews
you can then find out the concentration at each time point by using ideal gas equation
how to measure reaction rate using loss of mass method
if gas is given off then mass is lost you can measure the mass remaining at regular time intervals with a balance
you can use moles equation to work out how much gas you have lost and therefore how many reactants is left
how to measure rate of reaction using colour change method
you can track the colour change using a colorimeter which measure the absorbance, the more concentrated the colour the higher the absorbance
you can measure the change in absorbance by plotting a calibration curve a graph of known concentrations against absorbance
during the experiment take small sample from reaction solution at regular intervals and read the absorbance
use calibration curve to convert the absorbance at each time point into a concentration
how to measure rate of reaction using ph method
if reaction produces or uses up H ions the pH of the solution will change so you could measure the pH at regualr time intervals and calculate the concentration of H
how to measure rate of reaction using titration method
*take small samples of a reaction mixture at regular time intervals
*quench them
*titrate them using standard soloution
*the rate can be found from measuring the change in concentration of the products or reactant over time
how to work out reaction rate from concentration time graph
the rate at any point in the reaction is given by the gradient at that point on the graph
if the graph is a curve you have to draw a tangent to the curve and find the gradient of that
what does the order of reaction tell you
tells you how with respect to a certain reactant how the reactants concentration affects the rate
you can only find the orders of reaction from
experiments
for a concentration time graph what order is a straight line
zero order
for concentration time graph what order is a curve and an even steeper curve
normal curve first order even steeper curve is second order
for rate concentration graph what order is a straight horizontal line
zero order
for rate concentration graph what order is a straight line through origin
first order
for rate concentration graph what order is a curve through origin
second order
what is the half life
half life is the time it takes for half or the reactants to be used up
how do you work out half life from concentration time graph
plot concentration time graph
then draw lines across from the y axis at points where the concentration has halved and read off time taken
what are half lives useful for
identifying first order reactions without having to draw a rate concentration graph because half life is always constant for first order reactions
initial rate method is used to work out?
orders of reactions
initial rate of a reaction is the rate at ?
the start of the reaction
time = 0
how to do initial rates method?
*carry out separate experiments using different initial concentrations of one reactant
*change concentration of only one reactant
*then see how the change in in initial concentration affects the initial rates and work out order of each reactant
*you can do this using continuous monitoring techniques and drawing concentration time graphs
*by calculating the gradient at time=0 you can work out the initial rate
a clock reaction is an example of a
initial rates method
What is clock reaction
*clock reactions measure how long the time taken for amount of product to form Changes as you vary the concentration of one of the reactants
assumptions of carrying a clock reaction
*concentration of each reactant doesn’t change significantly/concentration change of each reactant insignificant
*the temperature stays constant
*when the end point is seen the reaction has not gone too far
describe iodine clock reaction
*A small amount of sodium thiosulfate solution and starch are added to an excess of hydrogen peroxide and iodide ions
*sodium thiosulfate reacts instantaneously with iodine
*once sodium thiosulfate is used up iodine remains in solution turning starch blue black colour this is the end of the clock reaction
*varying the iodide or hydrogen peroxide concentration while keeping the others constant will give different reaction times
what method do you use to find initial rate of iodination of propanone
titrating(the rate of reaction for the iodine propane reaction can be followed by a continuous monitoring titrimetric method )
titration
iodination of propanone method
*monitor reaction by taking samples at regular intervals
*quench (stop reaction) by adding sodium hydrogen carbonate to neutralise acid
*titrate samples against sodium thiosulfate and starch to work out conc of iodine
*need to repeat experiment several times and change the concentration of just one reactant
rate=k[A]ᵐ[B]ⁿ
what is k
the rate constant the bigger it is the faster the reaction
rate=k[A]ᵐ[B]ⁿ
what are m and n
they are orders of reaction
rate=k[A]ᵐ[B]ⁿ
what is the overall order
m+n
rate constant is always the same for certain reactions at ?
specific temperatures
if you rase the temperature what happens to the rate constant
it will rise too
in a series of steps which step becomes the rate determining step
the one with the slowest rate
what are the rules for determining if something is in the rate determining step
*If reactant appears in rate equation it must affect the rate and be in rds
*if reactant doesn’t appear in the rate equation it isnt involved in rds
can catalyst appear in rate equations and rate determining steps
yes
does the rate determining step have to be the first step in a mechanism
no
can the reaction mechanism be predicted from just the chemical equation
no
in terms of rds
the order of a reaction with respect to a reactant shows?
the number of molecules of that reactant which are involved in or before the rate determining step
if a reactions second order with respect to x how many molecules of x will be in rate determining step
2
Cl(g) + O3(g) ⟶ ClO(g) + O2(g) –slow (rate-determining step)
ClO(g) + O(g) ⟶Cl(g) + O2(g) — fast
what is the rate equation
rate = k[Cl*][O3].
CH₃COCH₃(aq) + I₂(aq) + H⁺(aq) ⟶ CH₃COCH₂I(aq) + H⁺(aq) + I⁻(aq)
rate = k[CH₃COCH₃][H⁺]
what should you say about the reaction
*propanone and H+ are in the rate equation so they or something derived from them must be in the rate determining step
*iodine is not in the rate equation so it is not involved until after the rate determine step
*the order of reaction for both propane and H+ is 1 so the rate determine step must use 1 molecule of each
*H+ is a catalyst so it must be regenerated in another step
halogenoalkanes undergo ?
nucleophilic substitution
SN1 only involve ?
1 molecule or ion in the rate determining step
SN2 involve ?
2 molecules 1 molecule and 1 ion or 2 ions in the rate determining step
primary halogenoalkanes only react by what mechanism
SN2
secondary halogenoalkanes can react by what mechanisms
SN1 and SN2
tetitary halogenoalkanes can react only by what mechanism
SN1
what is the Arrhenius equation
k=Ae⁻ᴱᵃ/ᴿᵀ
what do catalyst do
increase the rate of reaction by providing alternate reaction pathway with a lower activation energy
what is homogenous catalyst
catalyst in the same state as the reactant
what is heterogeneous catalyst
catalyst in different state to to the reactants
advantages of of heterogeneous catalyst
*solid heterogenous catalyst provide surface for reaction to take place on catalyst may be mesh or fine powder to increase surface area
*can be easily separated from from the products and leftover reactants
disadvantages of heterogenous catalyst
can be poisoned which means substance clings to the surface more strongly than the reactant does preventing catalyst from getting involved in the reaction its meant to be speeding up
*A small fixed amount of phenol is present which reacts immediately with the bromine as it is produced, thus removing it from solution.
*Explain why it is essential for the amount of phenol to be small compared to the amounts of the reactants for this assumption to be valid.
so the phenol is used up
A series of experiments was carried out where only the concentration of bromide ions present was varied and the solution contained a large excess of BrO3− and H+ ions. The total volume of the mixture was kept constant.
Why was it important that the solution contained a large excess of BrO3− and H+ ions?
so that the concentration of bromide ions is the only variable
Another ‘clock reaction’ is the ‘iodine clock’ reaction, where hydrogen peroxide solution is mixed with a solution containing sodium thiosulfate, potassium iodide and starch.
H2O2(aq) + 2I−(aq) + 2H+(aq) l I2(aq) + 2H2O(l)
Why are the potassium ions omitted from the above equation?
*becuase they are spectator ions
what is the colour change in iodine clock reaction
colourless to blue black colour
Outline a titrimetric method that could be used to measure the change in concentration of compound A with time. Compound A is an alkali, whereas compounds B, C and D are neutral.
A + B —-> C + D
*collect samples of reaction mixture at regular time intervals
*quench the mixture
*Titrate with acid of known
concentration/standard solution using indicator methyl orange
Suggest an explanation, other than human error, for the two anomalous results circled on the graph. (point above line of best fit lines are the highest and further right)
*rate was higher than expected
*reaction was exothermic so energy is released
*so higher proportion of higher collisions
how many steps is a sn2 reaction
one step
in sn2 the rate of reaction depends on
the concentration of both reactants
how many steps is a sn1 reaction
two step reaction
in sn1 which step is the rate determining step
the slow step/breaking step
S₂O₈²⁻ + 2I⁻→
2SO₄²⁻ + I₂
what is evidence for a reaction having more than one step
*because number of particles in rate equation does not match number of particles in that equation
*the number of molecules in equation for that reaction colliding simultaneously is unlikely
*chance of number of reactants in the equation for the reaction colliding is unlikely
why would reactants be in large excess in reaction
*so they do not affect the rate
*to ensure they are virtually constant
*to make sure theres only one variable constant
what further experiment could you do to show a order of reaction with respect to a certain reactant is one and state its effect on the graph
*monitor the reactant over time plot concentration time graph and show that the half life is constant
or
*double the reactant and the rate should change by the same factor or the gradient would double too
