Kinetics Flashcards
Define thermodynamics
it tells us whether a reaction can proceed spontaneously but doesn’t inform us about the rate at which the reaction proceeds
Whats kinetics?
it looks at how the enrgy is distributed as the reaction proceeds
Information for kinetics can’t be gained theretically, why is this the case? how is information gained instead?
It has to be gained experimentally, as plugging information into equations doesn’t tell you anything about the kinetics of something
whats speed/ velocity?
the distance something has travelled in a given time
Whats the rate of reaction?
the amount of material produced or consumed in a given time
Label the key aspects on this energy profile
Whats the difference between the reactants energy and products energy?
∆G
Why are transition states so unstable?
due to their high energies
Whats one of the main problems that arises when studying reaction rates?
the transition states and intermediates cannot usually be probed directly as they are very short lived
Whats the thing we can only measure to calculate reaction rates?
we can only measure the concentration of the reactants and/or the products with time, and use this information to calculate the reaction rate
What are the 2 key enzymes reaquired in this reaction?
What cofactor is useful for this reaction to occur also?
the cofactor, zinc, is needed for the enzyme to work as it helps to hold the enzyme, alcohol and NAD+ together in the correct position
why must acetaldehye be degraded quickly?
its toxic
whats is rate defined as?
the change of the concentration of a species as a function of time
On a concentration-time graph, what does the gradient tell you?
d[reactant/product]/ dt
the rate at the particular point during the reaction
Many concentration-time relationships are non-linear what does this mean for the rate?
the rate will change as a function of time (the rate is not the same throughout the reaction)
What are the two generalisations of the rate constant?
Write any equations that support these generalisations
- The rate usually decreases if the concentration of the reactants are decreased
d[products] / dt = K[A]X[B]Y
K= the rate constant
X and Y= the orders of the reaction
- The rate always increases with increasing temperature
K= A exp (-Ea / RT)
A and Ea can be treated as constants over small temperature ranges
What are the 2 main reaction rate theories?
- collision theory
- transition state theory
Whats meant by a simple reaction?
A reaction that occurs in a single step with no intermediates
Whats the collision theory?
What it focusses on/ assumptions
- this theory focusses on the molecular nature of the reaction and provides more insight into the behaviour of individual molecules
- assumption of theory; the reactants and products must collide before the reaction occurs
- they must collide with enought energy to overcome the activation energy barrier for the reaction
do spontaneous reactions have activation energy?
yes
What is the rate of collision detemined by and how does this effect the rate of reaction?
- the concentration of products and reactants (more molecules increase the chance of collision)
- molecule size (bigger molecules have more chance of collision)
- speed (molecules moving faster are more likely to collide )
Whats the equation for rate of collision?
rate of collision= πr2VR[A][B]
Whats the formula for rate and for K in the Collision theory law ?
Rate= k [A][B]
K= πr2VR exp (-Ea /RT)
What is rate related to?
The reactants
What are the units of rate?
moldm-3s-1
Is the reaction rate constant K, a constant value? if not then what makes the value change?
It’ s not constant, it can vary depending on temperature, adding catalyst or changing the catalyst for example
What does the order of a reaction tell us?
What happens in the rate determining step of a particular mechainsm
Tell me the rate equation and units of K for the following types of reactions…
- A –> B order :0
- A –> B order: 1
- A –> B order: 2
- A + B –> C order: 2
A –> B order: 0
rate equation: K
units for K: mol dm-3 S-1
A –> B order: 1
rate equation: K[A]
units for K: S-1
A –> B order: 2
rate equation: K[A]2
units for K: mol-1 dm3 S-1
A + B –> C order: 2
rate equation: K [A][B]
units for K: mol-1 dm3 S-1
what two methods can be used to determine the reaction order and rate constant?
How do you know/ decide what method to use?
- differential or direct method
- integration method
To decide what method to use, look at the system with several measurements and look at the concentration as a function of time
What occurs in the direct method?
- A rate equation is produced for the reaction
A –> Products
- logs are taken from each side of the reaction
Log (rate) = log (k [A]a)
Log (rate)= alog[A] + log (k) (this is in form y= mx +c)
- this then puts the rate equation in the form y= mx + C
Y= log(rate)
M= a
X= log[A]
C= log(k)
- this can then be plotted on a graph and then extrapolated
Draw rate-concentration graphs for zero, first and second order reactions
Draw the conc-time graphs for zero, first and second order reactions
Draw the rate of reaction- concentration graph for zero, first and second order reactions
What is a first order reaction?
if doubling the concentration of a reactant has a double effect on the rate
What is a second order reaction?
If doubling the concentration has a quadrupling effect on the rate
Whats a third order reaction?
If doubling the concentration results in an eight-fold increase in rate
whats K’ ?
Pseudo which is a first order rate constant
Whats the integrated method?
the rate equation is made form the components of the reaction where they are then integrated and converted to a linear format in order to plot as a straight line of a graph
What time of graph does the integration method lead to being able to produce?
A concentration- time plot
For the following reaction write the rate equation with rate being written as a differential? and then the integrated rate equaiton your would get from this?
A –> products
Write the following integrated rate equation in the form y=mx + c
Write the following secord order reaction in linear form from the integrated rate equation…
A –> product
Rate = -d[A]/dt rate= k[A]2
put these two equations together and solve by integration
-d[A]/dt = k[A]2
1/[A]t – 1/[A]0 = kt (then put in form y=mx +c)
Y= 1/ [A]t
M= K
X= t
C= 1/[A]0
Whats more accurate, the integrative method or the direct method and why?
The integrative method is more accurate than the direct method as the kinetic parameters are determined directly from a relationship between time and a function of the concentration
The characteristic kinetic plot is just what you need to plot in order to get a straight line
Units of rate constant is variable
Rate is always the same
Learn this table off by heart
Whats a chemical half life?
How is it denoted?
The time taken for a chemical species to fall to half its initial concentration
[A] –> [A] / 2
A half life is denoted as: t1/2
What order reaction has a constant half life?
first order reactions have a constant half life
When a reaction is faster, is there a shorter or longer half-life?
shorter
What is the rate of reaction proportional to?
The rate constant, thus the larger the rate constant the shorter the half life
How does the half life change with second order cases?
it gets longer and longer as time goes on as the reaction is slowing down
Does this graph show a zero, first or second order reaction?
You can’t tell which one it is apart from the fact its not first as the half life isn’t constant
Whats the half life eqaution for a second order reaction?
What does the equation suggest?
t1/2 = 1 / k[A]0
This inverse relationship suggests that as the initial concentration of reactant is increased, there is a higher probability of the two reactant molecules interacting to form a product
Also implies that since the half-life is longer when the concentration are low, species decaying according to second order kinetics may exist for longer amount of time if their initial concentration are small
What are the steps in order to work out the kinetics of an unknown mechanism?
- start with the overall reaction
- guess some mechanism(s)
- derive corresponding rate laws (don’t need to derive as they’re given)
- compare with experimental data
- and repeat from step 3 as needed
if you dont get a straight line then something has gone wrong in the experiment or the mechanism is too complex (not zero, first or second)
Whats the RDS?
The rate-determining step (RDS) is a single step in the mechanism which is much slower that there other steps
it can be thought of like a ‘bottle neck’ (nothing can happen until its happened, it influences the overall reaction)
Look at SN1 and SN2 reactions and facts about their RDS
Observations are made from experimental observations of what…
The rate laws
intermediates of the reaction
Whats the collision theory formula and what does it tell us?
Rate= T [concentration]n
- Rate is increasing dramatically
- T: if rate is increasing dramatically, something on the other side of the equation must be increasing too. Must be the temperature
- [Concentration]n: concentration is not changing much
What are the two forms of Arrhenius equation in 1889 and what does each part stant for/ units
Form one: K = Ae(-Ea’/KBT)
Form two: K= Ae(-Ea/RT)
K= rate constant of the reaciton
A= pre-exponential factor (normalising factor)
E’a= the activation energy for the reaction (J)
Ea= the activation energy for the reaction (J mol-1)
T= temperature (K)
KB= Boltzmann’s constant
R= gas constant
In Arrhenius’ equaiton there is an exponential factor, what does this mean for the results/effects?
its not linear and the effects are dramatic
For the Arrhenius equation, complete these relationships…
if K increase T _____
If K decreases T _____
if K decreases the Ea _____
If K increases the Ea _____
if K increase T increases
If K decreases T decreases
if K decreases the Ea increases
If K increases the Ea decreases
convert the following equation into a linear format…
K= Ae-Ea/RT
Write out in form y= mx+c
lnK = lnA - Ea / RT
y= ln K
m= -Ea/R
x= 1/ T
c= ln A (used to work out pre-exponential factor)
How do you work out the Ea from a graph?
Ea= - gradient x R
If T was raised by 10˚c, then the rate would double
What two ways can the arrhenius equation be written in order to form a general temperature between temperature and rate and write out the general relationship formula…
K1= Ae(-Ea/RT1)
K2= Ae(-Ea/RT2)
General relationship:
ln [K2/K1] = Ea/R {1/T2 - 1/T1}
NOTE; units of A are identical to units of K
Temperature is always in kelvin
Take the natural log of the K2/K1 ratio
