Section E: kinetics Flashcards

1
Q

what is the rate of reaction for a reaction aA —> bB ?

A

rate = - 1/a dA/dt = 1/b dB/dt

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2
Q

what is the relationship between the stoichiometry of the reaction and the order?

A

there is no connection between the two

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3
Q

what is k and what does is depend on?

A

k is the rate constant and it depends on temperature

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4
Q

zero order reaction

A

rate = - d[A]/dt = k
integrating, ∫d[A] = - k ∫dt
[A] - [A]0 = - kt
therefore [A] = [A]0 - kt

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5
Q

first order reaction

A
rate = - d[A]/dt = k[A]
integrating, ∫1/[A] d[A] = - k ∫dt
ln[A] - ln[A]0 = - kt
therefore ln[A] = ln[A]0 - kt
and [A] = [A]0 e^-kt
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6
Q

second order reaction

A

rate = - 1/2 d[A]/dt = k[A]^2
integrating, ∫1/[A]^2 d[A] = - 2k ∫dt
- 1/[A] + 1/[A]0 = -2kt
therefore 1/[A] = 1/[A]0 + 2kt

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7
Q

how would you use graphs to determine the order of reaction?

A
[A] versus t
linear? zero order
ln[A] versus t
linear? first order
1/[A] versus t
linear? second order
if not, more complex system
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8
Q

what is t1/2?

A

t1/2 is the half life = the time required for the concentration to drop to half its original value

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9
Q

how can t1/2 be calculated?

A

t1/2 = ln2 / k

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10
Q

what is the equation of radioactive decay?

A

N = N0 e^-kt

therefore N = N0 e^-tln2/t1/2

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11
Q

how can you measure the rate of reaction?

A
  • start the reaction t=0
  • measure a property as a function of time
  • convert the measurement into concentration
  • analyse the data
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12
Q

which methods can be used to measure the rate of reaction?

A
  • spectrophotometric methods
  • NMR
  • polarimetry
  • conductivity
  • electrochemical/pH detection
  • changes in pressure
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13
Q

how can spectrophotometric methods be used to measure the rate of reaction?

A
  • UV/vis or IR absorbance ∝ [concentration]
  • fluorescence
  • stopped flow method (fast reaction)
  • flash photolysis (very fast reaction)
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14
Q

how can NMR be used to measure the rate of reaction?

A

NMR integration ∝ [concentration]

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15
Q

how can polarimetry be used to measure the rate of reaction?

A
  • measure changes in optical rotation for chiral molecules

- circular dichroism

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16
Q

how can conductivity be used to measure the rate of reaction?

A

changes in the number of ions (concentration) result in changes in conductivity

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17
Q

explain the initial rates/differential method

A

A + B —> products
rate = [A]^a [B]^b
- isolate variation in [A]
- let [B]&raquo_space; [A] so that [B] is effectively constant (pseudo-order conditions)
- run the reaction at several different [A]0
- determine the initial rate for each curve by drawing tangents
- the ratio of any two rates is equal to the ratio of initial concentrations to the power a:
rate1/rate2 = ( [A]01/[A]02 )^a
- in general, rate = k(obs) [A]^a
- taking logs, log(rate) = log(k(obs)) + a log( [A] )
- this will produce a straight line graph with a slope equal to the order of the reaction

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18
Q

what is k(obs)?

A
  • the observed rate constant

- the real rate constant must be determined properly once the order is known

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19
Q

what is an elementary reaction?

A

a reaction at the molecular level eg. a collision between two molecules or bond breaking within a molecule

20
Q

what types of elementary reactions are there?

A
unimolecular = a single species undergoes a change
bimolecular = two species come together (homo/heteronuclear)
21
Q

what is the rate-determining step?

A

if a reaction proceeds by a number of steps, the rate constant of the slowest step governs the rate of reaction

22
Q

why must we consider elementary steps?

A

most reactions require several steps or events for the reaction to go to completion, progressing via intermediates

23
Q

explain the steady state approximation

A
  • if A —> B —> C , we have rates r1 and r2
  • if r1 B is rate determining
  • [B] remains constant and low
  • we can approximate d[B]/dt = 0
24
Q

what is the Arrhenius equation?

A

k = Ae^-Ea/RT

25
what does the Arrhenius equation demonstrate?
that the rate constant is dependent on temperature
26
what is the Boltzmann distribution?
kinetic energy is evenly distributed among molecules of a sample
27
what is represented by e^-Ea/RT in the Arrhenius equation?
the fraction of molecules at a temperature T with energy greater than Ea
28
what is represented by A in the Arrhenius equation?
the total number of collision per second
29
describe some properties of proteases
- group specificity - will hydrolyse only peptide bonds in a protein - stereochemical specificity - will only hydrolyse peptides made from L-amino acids - often associated with additional molecules eg. coenzymes, prosthetic groups, etc. - often require metal ions
30
who proposed the lock and key model of enzymes?
Daniel Koschland
31
what is the Michaelis-Menten equation?
V = Vmax [S] / (Km + [S] )
32
define 'Km'
- the substrate concentration required to achieve half-maximum velocity (Vmax) - the enzyme is half-saturated - a measure of the enzyme's substrate affinity
33
what is demonstrated by a small Km?
the enzyme has a high affinity for the substrate
34
what is Vmax?
- the maximal velocity | - enzyme is fully saturated
35
what is the effect on the M-M equation when [S] is very large?
it cancels to V = Vmax
36
what is the effect on the M-M equation when [S] is very small?
it cancels to V = Vmax [S] / Km
37
how is Kcat calculated?
Kcat = Vmax / [E]0
38
what is the Kcat?
the turnover number ie. the number of substrate molecules processed per second by one mole of enzyme
39
how is the catalytic efficiency calculated?
catalytic efficiency = Kcat / Km
40
what is the equation of a Lineweaver-Burk plot and how is it plotted?
1 / [V] = Km/Vmax (1 / [S] ) | plot 1/[V] against 1/[S]
41
what is indicated by the y-intercept of a L-B plot?
y-intercept = 1 / Vmax
42
what is indicated by the x-intercept of a L-B plot?
x-intercept = - 1 / Km
43
what is indicated by the gradient of a L-B plot?
gradient = Km / Vmax
44
describe competitive inhibition and its effects on Km and Vmax
- binds to the active site of the enzyme - Vmax is unchanged - Km is decreased since the enzyme cannot bind S if bound to an inhibitor - Km is decreased because an increased concentration of substrate is required to reach Vmax - gradient increases
45
describe non-competitive inhibition and its effects on Km and Vmax
- CHEM1603: binds to a remote site on the enzyme, affecting the activity of the enzyme but not its ability to bind substrate - BIOC1009: binds to both the free enzyme and to the enzyme-substrate complex - Vmax is decreased - Km is unchanged - gradient increases
46
describe uncompetitive inhibition and its effects on Km and Vmax
- binds to the enzyme-substrate complex - Vmax is decreased - Km is decreased - gradient is unchanged