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MATE40005 > Kinetics > Flashcards

Kinetics Flashcards

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1
Q

Define Rate of Reaction

A

Change in concentration of some species with time

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2
Q

What can affect the rate of a reaction?

A

Concentration
Surface Area
Temperature
Catalyst

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3
Q

What other systems could measure the change in concentration with time?

A

Titration
Partial Pressure CHange
Spectrophotometry

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4
Q

How can you normalise the rate for this reaction below?
aA + bB —> cC + dD

A

-(1/a) d[A]/dt = (1/d) d[D]/dt

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5
Q

What happens to the probability of two reactants to react if the concentration decreases?

A

With sufficient thermal energy, the activation energy will also decrease

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6
Q

What is a rate law?

A

A rate law is an equation that expresses the rate of a reaction
As a function of concentrations
Of the species present in the overall chemical equation
For the reaction at some time

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7
Q

Give the general term for the rate law

A

Rate = k[A]^x[B]^y
k being the rate constant
x, y are the partial orders of reaction

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8
Q

what is lowercase k?

A

the rate constant, a fundamental kinetic parameter
dependent on temperature
smaller k will be above larger k
exponential slope with a smaller gradient

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9
Q

what does the rate law tell us?

A

reaction mechanism

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10
Q

for an elementary reaction what can you deduce?

A

the chemical equation via inspection
e.g. A + B + C —-> products
Rate = k[A][B][C]

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11
Q

what is a limiting problem with kinetic measurements?

A

they cannot establish the reaction mechanism
more evidence is required on nature of intermediates
rates of intermediate reactions are often required

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12
Q

what is kinetics?

A

study of how fast chemical reactions occur

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13
Q

what is molecularity?

A

the number of species involved in the individual molecular process
in which reactants are transformed into products

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14
Q

what happens in a unimolecular reaction compared to a bimolecular reaction?

A

unimolecular - the molecule shakes itself apart, and arranges into a new shape
bimolecular - two molecules collide and exchange energy, atoms

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15
Q

what do integrated rate laws allow you to do?

A

predict the concentration of a species at any time after the start of the reaction
help find the rate constant and order of the reaction

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16
Q

what is the first order integrated rate law?

A

ln[A]t = -kt + ln[A]0

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17
Q

what is something to note for a first order reaction about their half life?

A

same length of time and is independent of [A]

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18
Q

what can you do using first order kinetics for C14?

A

number of half lives elapsed, you can determine the age of fossils

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19
Q

what happens in second order reactions in terms of their half life?

A

as concentrations decreases, the half life of second order reactions increases

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20
Q

what is a zeroth order reaction?

A

rate is independent of the concentration of reactants
surface area of heterogenous catalyst is the main factor controlling rate

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21
Q

how can you find the rate constant?

A

via finding the rate law, you can plots and it’s usually the gradient

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22
Q

how do you find the order of a reaction using intergrate rate laws?

A

trial and error
not sufficiently accurate to distinguish between first and second order
reaction has to measured over several half lives

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23
Q

what is an opposing reaction?

A

usually considered where equilibrium has been reached
at equilibrium A ↔ B, K = [B]/[A]

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24
Q

what are concurrent reactions?

A

they occur in parallel, can be thought as competing reactions

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25
For a reaction where A ---> B ----> C, what happens when k1>>>k2 and vice versa?
If k1>>>k2, then the second step is the RATE DETERMINING STEP there will be a significant buildup of intermediates as it rapidly formed in reaction 1 If k2 >>> k1 then the first step becomes the rate determining step as B is formed, it is rapidly converted to C so there i no buildup of B
26
What is the steady state approximation?
Used to determine many rate laws of complex systems Including enzyme kinetics
27
What are the terms for the steady state approximation to be used?
A reactive intermediate R is present at low and constant concentrations through the course of the reaction Therefore d[R]/dt = 0 in the rate equations
28
What is an intermediate?
Any species that does not appear in the overall reaction but that has been involved in the mechanism
29
what is a good analogy to think of the rate determining step for?
a slow ferry crossing between two fast motorways the rate at which traffic can reach its destination is detmined by the rate at which the ferry crosses
30
what do we assume in collision theory?
there is no definite structure composition
31
what is the activated complex theory?
the formation of an activated species which has a definite composition for which the strcuture is approximately known temporary stage at the top of the hill before it turns into products, from reactants
32
what is the importance of the activated complex theory?
shows that even a complex series of events displays arrhenius like behaviour the concept of activation energy is applicable
33
what do catalysts do?
increases the reaction rate by providing an alternative reaction path with lower activation energy without undergoing any permanent chemical change
34
what is a homogenous and heterogenous catalysts?
homogenous - same phase as reactants heterogenous - different phase as reactants
35
what are the factors to consider when thinking of catalysts?
activity efficiency stabiity selectivity removability price surface area lifetime
36
what is an enzyme?
a complex protein structure that catalyses a chemical reaction by lowering the activation energy
37
what is the michaelis menten equation?
if [S] >> Km, substrate is in large excess, then the rate is zeroth order ans so only depends on [t]0 if [s] << Km then the reation is first order, both [E]0 and [S]
38
what happens in catalytic hydrogenation?
h2 gas is adsorbed onto the catalysts surface H-h bonds are weakend and easier to break therefore activation energy is lowered found to be zero order
39
what is a thermal explosion?
due to rapid increase in reaction rate with temperature
40
what is a chain branching explosion?
chain branching steps in a reaction, number of chain carriers grows exponentially rate of reaction may cascade into an explosion
41
what does the collision theory state?
a reaction only occurs if two molecules collide with a certain minimum kinetic energy along their line of approach
42
what is a reaction energy profile?
a graph showing the variation in potential energy as one reactant molecules approaches another and the products seperate
43
what is an activated complex?
cluster of atoms close to the potential energy maximum that may lead to formation of products
44
what is the transition state?
cluster of atoms at the potential energy maximum that any further distortion will lead to the formation of products
45
what is collsion frequency and what is it proportional to ?
collision frequency is directly proportional to [A][B]
46
what does the collsion theory suggest?
the pre-exponential factor, is the constant proportionality between the concentration of the reactants and the rate at which the reactant molecules collide
47
what is the arrhenius equation?
k = Ae^(-Ea/RT)
48
Radiochemical Activation
X-rays, high energy electrons, and alpha particles are typical high energy radiation types Gives rise to electrons, ions or highly excited molecules
49
Photochemical Activation
One quantum activates one molecule d[A]/dt = constant x I0 the photo sensitise is excited to an upper electronic state and this energy can be transferred on collision
50
what is unusual temperature dependence?
indicated a more complex reaction mechanism a reaction with no temperature dependence has an activation energy of zero
51
52
what is diffusion?
a process in in homogenous multicomponent systems that aims to equilibrate the system by mass transport
53
what is case hardening?
diffuse carbon atoms into the host iron atoms at the surface c atoms lock planes from shearing
54
what is doping?
doping silicon with phosphorus to create an n-type semiconductor
55
what is fick's first law?
the diffusive flux is proportional to existing concentration gradient
56
what is steady state diffusion?
takes place at a constant rate once the process starts the number of atoms crossing a given interface is constant with time
57
what is non steady state diffusion?
time dependent process in which the rate of diffusion is a function of time
58
what is the diffusion flux?
J = m/At mass, over cross sectional area and time
59
What is the difference between interdiffusion and self diffusion?
Interdiffusion - alloys, atoms tend to migrate from regions of large concentrations Self diffusion, - an elemental solid, atoms migrate
60
What are the terms for diffusion to occur?
Empty adjacent site Atom must have sufficient energy to break bonds with its neighbour atoms Accomodate somem lattice distortion during displacement
61
How do you measure D?
PLace materials to be measure in contact (a diffusion couple) Place the couple in a furnace at tempature Measure concentration profiles and plot data to get penetration curve Use fick's laws to obtain D
62
What are the factors influencing diffusion?
SIze of the diffusing species Concentration of the diffusing species Number of vacanies and defects Magnitude of concentration gradient Nature of the diffusion mechanism
63
Diffusion equation
Like arrhenius, D= D0e(-Qd/RT)
64
What are the two condition that must be met for an atom to move?
There must be an empty adjacent site THe atom must have suffieicent energy to break bonds with its neighbouring atoms and accommodate lattice distortion during the displacement
65
The rate of a reaction in solution depends on the solvent in which the reaction is occuring. Discuss why
Solvent changes Kd, K-d so it can change rate if diffusion controlled Or switch from diffusion to activation controlled
66
What is a reason for a difference in diffusivity?
As activated process, only a fraction of atoms at any given time has sufficient energy to move to neighbouring vacancies
67
rank the relative magnitudes bulk, surface, dislocation, and grain boundary rank their speed in diffusion from fastest to slowest surface > grain boundary > dislocation > bulk
bulk < disloaction < grain boundary < surface
68
what relates the activation energy to the distortion
the larger the moleclule, the larger the distortion and therefore the higher the Qd
69

MATE40005 (4 decks)

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