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CHM135 > Kinetics > Flashcards

Kinetics Flashcards

1
Q

rates of reaction can be determined using the two formulas

A

A->B
rate of production of B = Δ[B]/Δt
rate of consumption of A = -Δ[A]/Δt

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2
Q

rate =

A

change in concentration/change in time

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3
Q

The General Reaction Rate

A

for a balanced reaction:

aA -> bB + cC

rate = -1/a(Δ[A]/Δt) = 1/b(Δ[B]/Δt) = 1/c*(Δ[C]/Δt)

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4
Q

Initial instantaneous rate vs instantaneous rate

A

initial instantaneous rate is at t=0.
instantaneous rate can be at any point on the graph

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5
Q

describe the rate law in words

A

an equation that shows how the reaction rate depends on the concentration of each reactant

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6
Q

Rate Law

A

aA + bB -> cC + dD

rate = k[A]^m[B]^n

[A], [B] - concentration in M
k - rate constant
m - reaction order in A
n - reaction order in B

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7
Q

what si the overall order of a reaction?

A

the sum of the individual orders

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8
Q

units of rate

A

always M/s

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9
Q

units of k

A

depends on the rate law expression

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10
Q

units of k if:
rate = k
rate = k[A]
rate = k[A][B]
rate = k[A][B]^2

A

M/s
1/s
1/(Ms)
1/(M^2s)

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11
Q

how are the order of reactions (m, n) and the rate constant (k) determined?

A

experimentally

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12
Q

give 2 ways of determining order of reaction and rate constant

A
  1. method of initial rates:
    - vary initial concentration of one reactant at a time
    - measure the initial reaction rate for each reaction
    - solve a system of rate law equations to determine the order
  2. graphical method/integrated rate law
    - monitor the course of a reaction over time
    - plot data
    - the shape of the curve reveals reaction order
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13
Q

0th order reaction

A

A –(k)–> product

[A]t = -kt + [A]0
[A]0 is the initial concentration
[A]t is the concentration at time t

  • plot the experimental [A] vs time
  • if the graph is linear, the reaction is zero-order
  • rate constant k = (-slope)
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14
Q

half life

A

the time required for reactant concentration to reach half of its original value

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15
Q

half life for a 0th order reaction

A

t(1/2) = [A]0/2k
- dependent on the (initial) concentration
- gets shorter over the course of the reaction (each successive half-life is half as long)

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16
Q

how are 0th order reactions possible?

A

in reactions whether the kinetics are governed by the availability of a catalyst

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17
Q

1st order reaction

A

A –(k)–> products

[A]t = [A]0e^-kt
ln[A]t - ln[A]0 = -kt

  • plot the experimental ln[A] vs time
  • if the graph is linear, the reaction is first-order
  • the rate constant k = (-slope)
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18
Q

half life of 1st order reaction

A

t(1/2) = ln(2)/k
- t(1/2) is independent of the concentration
- every half-life is an equal period of time

19
Q

2nd order reaction

A

A –(k)–> products

1/[A]t = kt + 1/[A]0

  • plot the experimental 1/[A] vs. time
  • if the graph is linear, the reaction is second-order
  • the rate constant k = slope
20
Q

half life of 2nd order reaction

A

t(1/2) = 1/k[A]0
- t(1/2) is dependent on the (initial) concentration
- t(1/2) gets longer over the course of the reaction, so each successive t(1/2) doubles in length

21
Q

draw graphs for zeroth order, first order, and second order. also draw straight-line plots used to determine rate constant

A
22
Q

how can we find the overall reaction from the elementary steps?

A

the elementary steps in the true reaction mechanism must sum to give the overall (stoichiometric) reaction

23
Q

reaction intermediate

A

a species formed in one step of a reaction mechanism and consumed in a later step

24
Q

molecularity

A

a classification of an elementary reaction based on the number of molecules (or atoms) on the reactant side of the chemical equation

25
Q

unimolecular reaction

A

an elementary reaction that involves a single reactant molecule
eg rate = k[O3]

26
Q

bimolecular reaction

A

an elementary reaction that results from an energetic collision of two reactant molecules
eg rate = k[O3][O]

27
Q

termolecular reaction

A

an elementary reaction that involves three atoms or molecules (rare)
eg rate = k[O]^2[M]

28
Q

how can you find a plausible mechanism for a reaction?

A
  • elementary steps must add up to the overall reaction
  • mechanism must correlate with the experimental rate law
29
Q

the rate law of the overall reaction must be the rate law of

A

the rate limiting step

30
Q

how do you determine the rate law for a reaction where the rate limiting step is the second step?

A
31
Q

factors affecting reaction rates

A
  1. concentration; increased concentration of reactants = increased rate
    - moving molecules are closer together
    - increased likelihood that reactants will collide
  2. temperature: observed increase in rate wen there is an increase in temperature
    - due to collision and transition state theory
    - molecules collide with energy greater than Ea more often
  3. catalyst: a catalyst provides an alternative reaction mechanism that proceeds faster than the original (uncatalysed) mechanism
32
Q

collision theory

A

for a bimolecular reaction to take place, reactants A and B must collide with proper orientation, and an energy greater than the activation energy, Ea

33
Q

k =

A

Zpf
Z = volumetric collision frequency
p = fraction with correct orientation
f = fraction with sufficient energy

34
Q

k depends strongly on temperature - so which of Z, p, and f depend on temperature?

A

collision frequency (Z)
- increase in temperature -> reactants collide more often (but not that more often)
fraction with correct orientation (p)
- increase T -> no change
fraction with sufficient energy (f)
- increase T -> reactants collide with greater kinetic energy

35
Q

transition state

A

the unstable group of atoms that are the highest-energy species along the pathway from reactants to products

36
Q

activation energy

A

the minimum energy required for a successful reaction

37
Q

f =

A

e^(-Ea/RT)

38
Q

as T increases, how does f change?

A

increases exponentially

39
Q

draw a graph of collision energy vs fraction of collisions for two temperatures

A
40
Q

Arrhenius equation

A

k = Ae^(-Ea/RT)
A = zp : the frequency factor (aka pre-exponential factor)

ln(k) = -Ea/R (1/T) + lnA
ln(k) = y
-Ea/R = m
1/T = x
lnA = b

41
Q

Arrhenius plot

A

y intercept = ln(A)
slope = -Ea/R

42
Q

catalysts

A

a substance that provides an alternative reaction mechanism that has a lower activation energy than the uncatalyzed mechanism -> faster reaction
- usually involved in the rate-limiting step of the new pathway
- often appears in the rate law of the catalysed reaction
- react early in a multi-step mechanism; regenerated in a later step, so not consumed
- do not show up in balanced, overall reaction
- catalysed reaction has the same endo/exothermicity as the uncatalysed reaction

43
Q

Michaelis-Menten kinetics

A

enzymes are catalysts of biological organisms, typically protein molecules with large molecular weights

at low substrate concentration
- first order behaviour: rate increases linearly with [S]

at high substrate concentration
- zeroth order behaviour: once all the enzyme is complexed, rate of reaction saturates

CHM135 (23 decks)

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