Questions for kinetics module
Level
Knowledge
Comprehension
Question
What units are commonly used for a
second order rate constant?
What units are commonly used for a
first-order rate constant?
When crystal violet (CV) reacts with
hydroxide ion, it forms a transparent
carbinol.
CV (purple) + NaOH → triphenylmethyl
carbinol (clear)
The rate expression for the reaction is
Answer
Molar per second (but any concentration
per unit time combination is OK)
Seconds-1 (or any reciprocal time unit)
When [OH-]>>[CV]
d [carbinol]
 k2 [CV][OH  ] . Under
dt
Application
Analysis
what circumstances is the reaction
pseudo-first order in CV?
For a pseudo-first order reaction
monitored by optical absorption, the
absorbance change 10 seconds into the
reaction is -0.005 absorbance units per
second. At 20 seconds, the absorbance
change is -0.004 absorbance units per
second. What was the initial rate?
For an enzyme that catalyzes reaction
according to the Michaelis-Menten
mechanism, is it possible to measure
both KM and Vmax in the same
experiment?
A = A0e-kt.
dA
  kA0 e  kt
dt
ln (-rate) = ln kA0 – kt. (note handling of
sign so we don’t try to take the log of a
negative number) We do not know A0 or
k. Insert known values:
0.005 = ln kA0 – k*10
0.004 = ln kA0 – k*20
0.001 = 10 k. k = 0.001/10 s-1 = 1×10-4 s-1.
-0.005 s-1 = -1×10-4 s-1A0exp(-1×10-4 s1
*10).
-0.005 = -1×10-4A0*0.999. A0 = 50
At t=0, dA/dt = -kA0 = -50*1×10-4 s-1 = 0.05 s-1
The rate law is
d [product] Vmax [S]

.
dt
K M  [S]
If [S] changes significantly during the
reaction, the rate will change, and we
can get 2 (or more) rates at 2 (or more)
values of [S], at which point it’s just
algebra to find both. While the module
does not discuss this, a common way to
evaluate the parameters is called a
Lineweaver-Burke plot, where one takes
the reciprocal of both sides of the rate
equation and obtains
KM
1
1


 d [product]  Vmax Vmax [S]


dt


Synthesis
Evaluation
Both PO43- and SiO44- can react with
acidified molybdate to make yellow
heteropolymolybdate complexes. If one
starts with excess molybdate, what
would you expect the rate of absorbance
change to be, starting with a mixture of
silicate and phosphate? How would you
determine if your rate law corresponded
to experimental reality?
For a particular pH, it takes 15 minutes
for phosphate reacting with molybdate
to reach stable absorbance (where dA/dt
due to noise exceed dA/dt due to
reaction). What questions would you ask
to decide whether to use reaction rate or
equilibrium absorbance to measure
[PO43-]?
One would expect the reaction to be
pseudo-first order in each reactant. For
absorbance at some appropriate
wavelength,
dA
 k P [PO34 ]  k S [SiO 44 ]
dt
One might make a set of phosphate
solutions, a set of silicate solutions, and a
set of solutions with both, measure
dA/dt, and determine the two rate
constants and if those constants held in
the mixture of the two ions.
a) Is there a background absorbance?
If so, prefer kinetics.
b) Is detection limit for kinetics
measurement adequate?
c) Is selectivity of equilibrium
measurement adequate?
d) Does either method have adequate
precision?
e) Is the faster experiment time for
kinetics (< 15 minutes) useful?