16: Kinetics 2 Flashcards
What is reaction rate?
Change in amount of reactants or products per unit time
What does a way to monitor reaction rate have to be?
Pick something that changes as the reaction goes on
Must be continuous monitoring of the reaction over the whole reaction
What are some methods of following the rate of reaction?
Gas volume
Loss of mass
Colour change
Change in pH
Titration
Electrical conductivity
How is gas volume monitored to measure rate of reaction?
Gas collected in the syringe to record how much given off at regular time intervals
To find conc of a reactant, use pV = nRT
How is loss of mass monitored to measure rate of reaction?
System will lose mass if gas is given off
Measure using a balance then use mole calculations to work out moles
How is colour change monitored to measure rate of reaction?
Colorimeter measures absorbance of a particular light wavelength
More conc a solution, the higher the absorbance
What can absorbance from a colorimeter be used to do?
Plot a calibration curve - graph of conc of a known substance against absorbance
Calibration curve used to convert absorbance into concentration
Take small samples at regular intervals and test absorbance
How is pH monitored to measure rate of reaction?
If reaction produces/uses H+ the pH will change
Measure pH at regular intervals and calculate conc of H+
How is titration used to measure rate of reaction?
Take small samples of a reaction at regular time intervals and titrate them using a standard solution
Rate can be measured by change in conc of products or reactants over time
How is electrical conductivity monitored to measure rate of reaction?
If the number of ions changes so will the electrical conductivity
How can a conc-time graph be done?
Measure repeatedly at set intervals during a reaction
Rate at any point and reaction given by gradient at that point of the graph
If curve, draw a tangent to find gradient at that point
Does the sign of the tangent matter?
Negative - measuring reactant
Positive - product concentration
What is the order of reaction with respect to a reactant?
A value which tells you how the reactant’s concentrations affects the rate
What does the order of reaction 0 with respect to X mean?
When [X] changes, the rate stays the same
If [X] doubles the rate of reaction stays the same
What does the order of reaction 1 with respect to X mean?
Rate is proportional to [X]
If [X] doubles the rate will double
What does the order of reaction 2 with respect to X mean?
Rate is proportional to [X]^2
If [X] doubles, the rate will be 2^2 = 4 times faster
How are orders of reactions calculated?
Only from experiments NOT from chemical equations
What is the overall order of reaction?
Sum of the orders of all the reactants
What does wrt mean?
With respect to
What is the graph of rate against [X] when order is 0 wrt X?
Paralell line to the x-axis
What is the graph of rate against [X] when order is 1 wrt X?
Straight line through the origin
What is the graph of rate against [X] when order is 2 wrt X?
Curve through the origin
What is the graph of [X] against time when order is 0 wrt X?
Straight line with negative gradient, starting at (0,k) where k is a positive value
What is the graph of [X] against time when order is 1 wrt X?
Curve from steep to flat, half-life is constant
What is the graph of [X] against time when order is 2 wrt X?
Curve from steep to flat, half life is not constant
How can you tell the difference between a 1 or 2 order graph of conc-time?
Plot a graph of rate-conc for both
1st order gives straight line, 2nd order gives curve
How is half-life calculated from a conc-time graph?
Draw lines across y-axis at points where conc has halved
Useful for saying whether 1st or 2nd order
What is the inital rate of reaction?
Rate at the start of the reaction
What is the inital rates method?
Using the inital rate of an experiment to work out the orders of reaction
What are the steps in the inital rate method?
Carry out separate experiments using diferent inital concs of one reactant
Only change one conc at a time, keep the rest constant
Observe how change in inital concs affects inital rates and work out order of each reactant
What are the pratical techniques for the inital rate methods?
Coninuous monitoring techniques
Clock reaction
How is the coninuous monitoring technique for the inital rate method?
Monitor concentrations or volume continuously over a reaction
Draw conc-time or volume-time graphs
Calculate gradient at time = 0
How is a clock reaction done?
Measure time taken for a set amount of product to form, this changes as you vary the conc of one of the reactants
Sudden increase in conc of a certain product as limiting reactant is used up
Usually easily observable end point (colour change) when desired amount of product forms
Quicker the clock reaction finishes the faster the inital rate
What are the assumptions of a clock reaction?
Conc of each reactant doesn’t change significantly over the time period of the clock reaction
Temperature stays constant
When end point is seen, reaction has not proceeded too far
What are the reactions being monitored in an iodine clock reaction?
H2O2 + 2I- + 2H+ → 2H2O + I2
2S2O32- + I2 → 2I- + S4O62-
What is the thiosulphate ion?
S2O32-
What is starch used as for iodine clock reactions?
Indicator - turns blue/black in the presence of iodine
How are the two reactions linked in the iodine clock reaction?
H2O2 + 2I- + 2H+ → 2H2O + I2
2S2O32- + I2 → 2I- + S4O62-
Iodine made in first is used in second reaction until thiosulphate is used up
Then starch will turn blue-black as iodine is in the solution
What is changed in the iodine clock reaction?
Hydrogen peroxide or iodide concentrations gives different times for the colour change
Time it takes for reaction to occur with conc of reactants allows calculation of inital rate with respect to iodine or hydrogen peroxide
How do you do the iodine clock reaction in the lab to calculate order with respect to potassium iodide (KI)?
Rinse a pipette with sulphuric acid then use to transfer small amount of known conc to a clean beaker
Add distilled water to beaker and a few drops of starch
Measure a set amount of KI and sodium thiosulphate of known conc using different burettes and add to beaker
Measure H2O2 using burette and start stopwatch when added to beaker
Stop stopwatch when colour change (colourless -> blue/black) which is the end point
Record time with quantities of sulphuric acid, water, KI, sodium thiosulphate
Repeat varying KI volume, keep sulphuric acid and thiosulphate and H2O2 and vary water to keep overall volume constant
How is the inital rate found from the iodine clock reaction?
Time took to reach the end point used to find inital rate
Comparing inital rates means you can find reaction order with respect to KI
What is an example of the conitnuous monitoring titrimetric method?
The iodine-propanone reaction, uses a H+ acid catalyst
CH3COCH + I2 → CH3COCH2I + H+ + I-
What is a titrimetric method?
Uses titrations throughout the experimnets at regular intervals to find info about the reaction
What is the procedure of the titrimetric method for the iodine-propanone reaction?
Taking samples at regular intervals
Quench/stop reaction by adding sodium hydrogencarbonate to neutralsie the acid
Titrate each sample against sodium thiosulphate and starch to calcualte conc of the iodine
Repeat the experiment several times, changing the conc of one reatant
What is the rate equation for the reaction: A + B → C + D ?
Rate = k[A]m[B]n
k = rate constant
m and n - orders of reaction
When looking at the rate equation, what is the formula for the overall order?
m + n
How does the rate of reaction change as k increases?
Rate of reaction increases as k increases
What are not usually included in rate equations?
Spectator ions
e.g. elements with 0 order are left out of the equation
How is the data from inital rates method used to calculate orders?
Look at change to rate of reaction when one reactant doubles and others remain constatn
If rate doubles, reaction is 1st order wrt that reactant
If rate increases by 4 times, reaction is 2st order wrt that reactant
If rate doesn’t change, reaction is 0 order wrt that reactant
What is the rate-determining step?
The slowest step in a multi-step reaction
How many steps are there in reaction mechanisms?
Can be one or many steps
Each step has a different rate
What is the rate-determining step sometimes called?
Rate-limiting step
What can the rate equation be used to work out?
The mechanism of the chemical reaction
How can the rate-determining step be worked out from the rate equation?
If a reactant appears in the rate equation it must be in the rate-determining step as it affects the rate
If it isn’t in the equation, then its not in the rate-determining step
What is important to remember about rate-determining steps and mechanisms?
Rate-determining step (RDS) doesn’t have to be the first step in the mechanism
Reaction mechanism can’t usually be predicted from just the chemical reaction
What does RDS stand for?
Rate-determining step
What does the order of a reaction with respect to a reactant show?
Shows the number fo molecules of that reactant which are involved in or before the rate-determining step
What is an SN1 reaction with halogenoalkanes?
Reaction involves 1 molecule or ion in the RDS
What is an SN2 reaction with halogenoalkanes?
Reaction involves 2 molecules, 1 molecule and 1 ion, or 2 ions in the RDS
What type of nucleophilic substitution do primary halogenoalkanes undergo?
SN2
What type of nucleophilic substitution do secondary halogenoalkanes undergo?
Both SN1 and SN2 mechanisms
What type of nucleophilic substitution do tertiary halogenoalkanes undergo?
SN1
How does SN2 occur?
Single, rate determining step
Nucleophile back attacks and forms a negatively charged transition state
Then it forms the products
What is the rate equation for SN2 reaction with OH- as a nucleophile?
Rate = k[R-X][OH-]
As there is only one step, both are in the rate equation
What are the orders in SN2 mean?
Rate = k[R-X][OH-]
Rate is dependent on conc of both reactamnts
One OH- and R-X must be involved in RDS which fits the SN2 mechanism
Why do primary halogenalkanes undergo SN2?
A lot of space around the carbon so more space for OH- to attack
What is the rate equation for a tertiary halogenoalkane?
Rate = k[R-X]
Where R-X is a tertiary halogenalkane
What is the SN1 mechanism?
Step 1 is RDS (slow) = R-X bond breaks heterolytically, forms X- and carbocation
Step 2 is fast = OH- attacks postive charge of carbocation
Why can the OH- not attack tertiary halogenoalkanes?
Reaction happens as very little space
Steric repulsion due to the number of electrons
What is the Arrhenius equation?
k = A e-Ea/RT
k = rate constant
A = arhennius constant
Ea = activation energy
R = gas constant (8.31)
T = temperature in kelvin
Do you have to learn the arrhenius equation?
No it is given in the exam
What occurs to k as Ea increases in the Arhennius equation?
As Ea increases, k decreases
Large Ea means a slow rate of reaction
What occurs to k as temperature increases in the Arhennius equation?
As T increases the value for k increases
Hence rate of reaction increases
What is the Arhennius equation in log form?
ln k = ln A - Ea/RT
What is an Arhennius plot?
Graph of ln k against 1/T
Produces a graph with a gradient of -Ea/R
Gradient used to find activation energy
How is the Arhennius plot related to y = mx + c?
ln K = -Ea/R * 1/T + ln A
y = m*x + c
How is rate proportional to time?
Rate is proportional to 1/time
Rate = k * 1/time
How can time from a clock reaction be used in the Arhennius equation?
Rate is proportional to 1/ time
1/time can be used instead of k
Describe the line produced by a Arrhenius plot?
Straight line with negative gradient
Gradient = -Ea/R
When finding Ea from a Arrhenius plot, what steps should be taken?
gradient = -Ea/R
Ea = -(gradient*R)/1000
/1000 is done to convert from J/mol to kJ/mol
How does a catalyst affect the rate of reaction?
Increases rate of reaction by providing an alternative reaction pathway with a lower activation energy
Catalyst is chemically unchanged
What are the two types of catalyst?
Heterogeneous
Homogeneous
What is a homogeneous catalyst?
Catalyst in the same state as the reactant
What is a heterogeneous catalyst?
A catalyst in a different physical state from the reactants
What is a poison with reference to heterogeneous catalysts?
Poison is a substance that clings to a catalysts substance stronger than a reactant does
This prevents the catalyst binding to it and speeding up the reaction
Give an example of a poison on a catalyst?
Sulphur can poison the iron catalyst in the Haber process
When should you stop the stopwatch when quenching a reaction?
When all of the solution has been added to the beaker with the quenching mixture present
