Kinetics Flashcards
Kinetics
Study of rate and mechanisms of reactions
Kinetic Molecular Theory
Particles move and collide; when effective collisions happen, when enough energy then a reaction occurs
Activation energy(Ea) and what it means
Minimum amount of energy needed for a reaction to occur. It determines the rate and how many collisions should happen for a reaction to occur
What can make a reaction faster?
If the number of effective collisions, temperature, molarity, or pressure increases.
The state of matter can also change how fast the reaction occurs (s<l«g)
Rate of reaction…
how fast the reaction is proceeding
Rate increases when…
number of effective collisions increase
What does an “effective” collision mean?
The particles hit each other, and one sticks to the other
–> A BB <—-
AB B —–>
Catalyst
Speeds up the reaction but is not consumed during it
How do catalysts work?
They lower the activation energy by taking a different path that requires lower energy collisions.
Rate law definition
Specific to a reaction, describes how the concentration of reactants affect the rate of a reaction. Modifies the RDS
What is the unit for k for a zeroth, first, and second order reaction?
zeroth: M · s ⁻¹
first: s ⁻¹
second M ⁻¹ · s ⁻¹
Terms for orders 1,2,3?
unimolecular
bimolecular
termolecular
Mechanisms
Theoretical statement of how a reaction occurs, supported by experimentation
Intermediate
Something that appears in the middle of a reaction and is used up before the end (would cancel out in Hess’s law)
Elementary steps
Collisions that happen to give the overall reaction
What do you call the “slow” step?
The rate determining step (RDS)
Rate of consumption/production formula
rate rxn = (-1/a) · (Δ[A]/Δt) = (-1/b) · (Δ[B]/Δt) = …
Rate of rxn formula
rate rxn = (-1/a) · (Δ[A]/Δt)
How do you determine orders and a rate law
rate rxn = k[A] to the m [B] to the n
(rate #/rate #) = (k[A]# to the x [B]# to the y) / (k[A]# to the x [B]# to the y)
the larger number goes on top
1. Hold one of the values constant so it can cancel out and you can solve for the order
2. Do the same for the other ion
3. To solve for k plug in values along with order
Overall order: add expotents
How to write laws for slow/fast mechanisms?
Rate law for RDS should equal rate law of overall
How to write laws for fast/slow mechanisms?
- Write overall rxn
- Write rate laws for slow and fast steps
- Set rate law of forward and reverse = to each other for fast
- Solve for intermediates
- Substitute into slow step rate law
- The slow step rate should be without intermediates = overall rate law
Graphical rules
- Endo if reactants are higher than products
- Exo if reactants are lower than products
- If one hump then there is one elementary step, 2 humps is two elementary steps, and so on
Where is the transition state on a graph?
The peak of the humps
Where is the intermediate on a graph?
The dip between two humps (transition states)
How does a catalyst affect a reaction? (4 effects)
- Changes reaction mechanism
- Does not change enthlapy
- Increases forward rate
- Increases reverse rate
How is a rate law determined given a mechanism
It must be the same as the rate law of the slow step, and not include intermediates
Where do you mark activation energy and ΔH on a graph?
Ea: From R line to the tallest peak
ΔH: space vertically between R and P lines
What are the axis labels for an energy diagram?
E (energy) on y axis and “reaction coordinate” on x axis
What are the three things you should check if a mechanism is valid?
- Check if the overall reaction is the same as the one you are given
- See if the RDS(slow) law = the experimentally determined law
- If finding the law without any intermediates takes more than two steps, then it is likely not valid
