Kinetics II Flashcards
What does rate equation show?
The effect of changing the concentration of substances (taking part in a reaction) on the rate
What are we looking at when we measure the rate of a reaction?
Looking at how fast the concentration of one of the reactants is falling (at any one time)
What happens to rate as the reactant gets used up?
As the reactant gets used to as the reaction progresses, the rate will decrease.
Rate units
Mol dm^-3 S^-1
What would it mean if the concentration of A was falling at a rate of
0.004 mol dm^-3 S^-1 ?
(Eg. Concentration of A falls by 0.004 mol dm^-3 every second)
How are orders of reaction always found?
DOING EXPERIMENTS
What do [ ] mean?
concentration in mol dm^-3
What is k?
The rate constant
What do orders of reaction show?
Tells us how changing the concentration of a specific substance affects the rate
Zero order means
Change in concentration has no affect on the rate
1st order means
Change in concentration has a proportional affect on the rate
(Eg. If [A] doubles then the rate doubles) This would mean it’s first order with respect to A)
2nd order
Change in concentration has a squad proportional change on the right
(if [A] doubles then rate quadruples)
How to find the overall order of the reaction?
Add up individual orders
State the general rate equation
Rate = k[A]^a [B]^b
Rearrange the rate equation to work out k
k = rate/[A]^a [B]^b
What is not included in the rate equation?
Things with zero order
Tips for working out units of k
X
How to work out the order of x ?
compare experiments where x is CHANGING and the other reactants are constant
(Order of X = look at where X is changing)
Two steps of given a rates table
XXX
1) work out rate equation
2) use rate equation to rearrange and find missing data in table
3) calculate k (pick a complete row and calculate using rate equation)
4) Use k to find rate of a substance in the table
State the Arrhenius equation
k = Ae ^ -Ea/RT
What does T mean in the Arrhenius equation?
Temperature in Kelvin
What does R mean in the Arrhenius equation?
Ideal gas constant
8.31
What does Ea mean in the Arrhenius equation?
Activation energy - minimum energy for reaction to occur
For this it’s measured in J mol^-1
What does e in the Arrhenius equation mean?
like Pi
Use shift Ln button on calculator
What does the expression
e^-Ea/RT
mean in the Arrhenius equation?
Counts the fraction of molecules in a gas with energies equal to or bigger than activation energy at a particular temperature
What does A mean in the Arrhenius equation?
frequency factor/pre-exponential factor
Term that includes factors such as frequency and orientation of collisions. Varies slightly with temperature and is often taken as constant across small temperature ranges.
What’s the other form of the Arrhenius equation?
The one with the lns in it
ln k = ln A - (Ea/RT)
1) What happens to rate if you change temperature by a small amount? Eg. From 20 to 30 degrees (293K to 303K)
1) k = Ae ^ -Ea/RT
- assume A is constant
- insert values given for e^-Ea/RT
(293K) and value is 3
-insert values given for e^-Ea/RT but using new temperature (303K) and value is 6
Conclusion:
Increasing temp by 10 degrees doubles e^-Ea/RT (fraction of molecules able to react bc they have energy equal to or bigger than activation energy) from 3 to 6
so k doubles (because k is set = e^-Ea/RT)
So RATE OF REACTION DOUBLES
How can Arrhenius be used? (Effect of temperature)
Use Arrhenius equation to show the effect of a change in temperature on the rate constant (and thus rate of reaction)
Explain the steps to use Arrhenius to work out effect on rate a small temperature change has. (4)
1) k = Ae ^ -Ea/RT
2) we want to see how changing the value of T affects k. Whatever happens to k is the same as what is happening to the rate.
3) we don’t know the actual value of A (frequency factor) but because we are making a small temperature change we can assume it stays constant and thus can ignore it
4) so we are just looking at e^-Ea/RT which can tell us what’s happening to the fraction of molecules with energy bigger than or equal to Ea …. which tells us what happens to the rate constant (bc k is set = to e^-Ea/RT)… which tells us what happens to rate of reaction …
How can Arrhenius be used? (Effect of a catalyst)
To show the effect of a change in Ea (due to catalyst) on the rate constant - on the e ^ -Ea/RT (fraction of molecules with energy greater than or equal to Ea)
How to use Arrhenius to show a decrease in Ea from 50Kj mol ^ -1 to 25Kj mol^-1 (due to a catalyst) increases rate of reaction. (4)
k = Ae ^ -Ea/RT
Just look at e ^ -Ea/RT part
Insert values given and Ea of 25 000J (remember to convert to J)
You’ll find with a LOWER Ea of 25 000 the e ^ -Ea/RT is a LARGER number because more particles have energy equal to or greater than Ea
Then do again and insert values given but with Ea 50 000J
(remember to convert to J)
You’ll find with a HIGHER Ea of 50 000 the e ^ -Ea/RT is a SMALLER number because less particles have energy equal to or greater than Ea
You can divide the larger number/smaller number to find how many more times molecules there are that can react
What is the rate determining step?
The SLOW step
X
The slow step is the rate determining step
Do rate experiments
Get rate equation
Things NOT in the rate equation (because they are zero order) are not affecting the overall rate. Things which are in the slow step affect overall rate so this means these must be in the fast step.
X
Speeding up the fast step is possible but it won’t have a noticeable affect on the overall rate of reaction,
which is governed by the speed of the slow step.
What does there have to be if you wanna measure rate of a reaction using the rate determine step?
There has to be a large difference between the rates of the various steps
(So there has to be a slow step)
What does the rate equation tell you?
What is taking part in the slow step
How to work out a mechanisms from rates?
XX
Do rat experiment
Get rate equation
Determine what is involved in slow step from rate equation
If only one organic compound then likely to be Sn1
Find carbocation in first step to confirm
Example
What kind of mechanism is the reaction if the rate equation is
k[CH3CH2-Br][OH-] ?
Both the organic compound and OH- must be taking part in the slow step because they are in the rate equation
so Sn2
What molecularity is Sn2?
Bimolecular
What molecularity is Sn1?
Unimolecular
What if the reaction is a
Fast step with an equilibrium arrow
Slow step
A + B X
A + X —> C
Flawed explanation
Rate = k[A][X]
This should mean A and X are in the slow step at the start but it starts with a fast step…
X is formed from A + B
So rate is k[A][A][B]
Flawed
Simplistic view
Doesn’t work on all cases
5 factors that can affect rate of reaction
SCCTL
Surface area of reactants
Concentration of reactants
Temperature
Catalyst
Lights important for photo chemical reactions
Five experimental methods for following reactions
MVPTC
Mass changes
Volume changes
Pressure changes
Titration – to find concentration of reaction mixture at a particular time
Colormetric measurements
How can you follow mass changes as a way of determining the rate of reaction?
Mass changes that take place during a reaction
Can be followed by placing the reaction vessel on a balance and reading the mass at specific intervals
How can you follow volume changes as a way of determining the rate of reaction?
Follow the reaction by measuring the volume of gas collected at specific times
Work in concentration of reactants; so use calculations to work out concentration of reactant remaining for the volume of gas produced u recorded (limit to this method bc calc is not straightforward)
Plot graph of concentration against time
How can you follow pressure changes as a way of determining the rate of reaction?
Used for following the reactions of gases
Connect reaction flask to a pressure meter and measure the pressure and given intervals
How can you follow titrations as a way of determining the rate of reaction?
Used to follow reaction of an acid or alkali
Pipette samples of reaction mixture removed from reaction vessel
Place in flask
quench
titrate to find concentration of the reaction mixture at that particular time
How can you use a colorimetric measurements as a way of determining a rate of reaction?
Copper (II) sulphate example
Reaction can be followed if reactant or product is coloured
Colorimeter measures light intensity
Pass light through filter which goes through the sample which then goes to the meter.
Colour of light sense ged by selecting particular coloured filter.
Eg. Copper(II)sulphate solution use red filter because it absorbs red light.
The more concentrated the solution is at a given time, the more of the red light it will absorb.
Commonly quoted example of using colorimetry to follow the rate of a reaction
Iodine + propanone
Solutions starts brown, fades orange to yellow to colourless as iodine is used up.
Colorimeter curve
Colorimeter larger absorbance = more concentrated
Plot calibration curve (results from absorbance of solutions with known concentrations)
To find an order of a reaction you must be working in…
Concentration of the reactants
Limit to following the course of a reaction by looking at the volume of gas given off
To find an order of a reaction you must work in the concentrations of the reactants
So you would have to work out the concentration of the reactants remaining in the solution for each volume of the gas you record - difficult calculation
X
Follow a particular reaction from start to finish
Either take samples of mixture at different intervals and so titrations to see how concentration of reagent is changing
OR
measure physical property (eg. Volume of gas produced) as reaction continues
Examples of initial rate reactions
Iodine Clock reactions
The thiosulphate - acid reaction
Catalytic decomposition of hydrogen peroxide
What is continuous monitoring
Determining the rate by following the reaction over a period of time
Ways of determining rate of a reaction?
1) Follow course of a single reaction (by looking at a physical property changing or doing titrations to work out how concentration changes)
2) initial rate method
Iodine Clock reactions
The thiosulphate - acid reaction
Catalytic decomposition of hydrogen peroxide
