ú l o h a
/43/
7.a-c
1. Chemical kinetics
1.a. Photometric study of reaction kinetics
Crystal Violet (Gentian Violet) is an acid-base indicator (transition pH of 0.0 - 2.0),
its blue-colored form is converted by hydroxyl anions in a colorless neutral form
with a half-life of a few minutes.
The rate equation is:
[ ] [ ] [ ]pr
VIOLOHk
dt
VIOLd
v ⋅=−= −
(1.1.)
where 𝑘𝑘 is the rate constant, [ ]VIOL is the concentration of crystal violet in cationic form,
and [ ]−
OH is the concentration of the hydroxide anoints. p and r are the orders of the
reaction with respect to the corresponding reacting ions. In an alkaline environment
where [ ] [ ]00 VIOLOH >>−
(index 0 indicates the initial concentration), the concentration
[ ]−
OH is nearly unchanged during the reaction and it can be included in the rate
constant k′. The rate equation has then the form:
[ ] [ ]p
VIOLk
dt
VIOLd
′=− (1.2.)
where
[ ]k k OH
r'
= −
(1.3.)
thus the constant k'
depends on the concentration [ ]−
OH . Assuming that the order of
reaction is resp. , we obtain the integral forms of equation (1.2.):
[ ] [ ] ( )tkVIOLVIOL t ′−⋅= exp0 resp.: [ ] [ ]
[ ]0
0
1 VIOLtk
VIOL
VIOL t
⋅⋅′+
= (1.4.)
These equations can be rearranged in linear formulas:
tkt
′−= 0[VIOL]ln[VIOL]ln resp.: [ ] [ ] tkVIOLVIOL t ⋅′+= 011 (1.5.)
1=p 2=p

ú l o h a
/44/
7.a-c
The decision whether the partial order 𝑝𝑝 is equal to 1 or 2 can be made by comparing
the values of the correlation coefficients of the lines fitted on experimental
dependencies 𝑙𝑙𝑙𝑙[𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉]𝑡𝑡 resp. 1 [𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉]𝑡𝑡⁄ vs. time 𝑡𝑡 according eqns (1.5.).
The dependence of the velocity constant k on the ionic strenght I is given by the
relationship:
log 𝑘𝑘 = 𝑙𝑙𝑙𝑙𝑙𝑙 𝑘𝑘0 + 1.02⋅𝑧𝑧𝑎𝑎⋅𝑧𝑧𝑏𝑏⋅√𝐼𝐼 (1.6.)
where za and zb are the charges of the reacting particles (including their sign) and k0 is
the rate constant of the reaction at ionic strength equal to zero.
TASK: Determine the orders of reaction with respect to crystal violet cation and
hydroxide anion. Evaluate the rate constants ( k′ and k). Verify the charges of the
reacting ions and determine the rate constant k0 from the dependence of the rate
constant k on the ionic strength I.
LABORATORY AIDS AND CHEMICALS: 0,5·10-4
M crystalline violet solution (CAS No
548-62-9), 0.1 M NaOH, 1 M KNO3, 6 volumetric flasks (25 cm3
), 3 beakers (50
cm3
) 2 scale pipettes (5 cm3
), volume pipette (10 cm3
), spectrophotometer with
accessories, stopwatch.
INSTRUCTIONS: Work at laboratory temperature, which should not be noticeable
changed during the experiment.
Calibration curve of spectrophotometer. Pipette successively 1, 2; 3, 4 and 5 cm3
of
the stock solution of the dye into 6 volumetric flasks (each of 25 cm3
volume) and fill
them with distilled water up to marking. Measure the absorbance of the dye solutions at
the absorption maximum wavelength (574 nm). Note the pipette volume y cm3
of the
dye stock solution, which was applies to prepare a solution exhibiting an absorbance in
the range of 0.7-0.9.
1. Monitoring of reaction for different concentrations of hydroxide.
a) Pipette the volume 2y cm3
of the stock solution of the dye into 25 cm3
volumetric
flask and fill with distilled water up to marking. Prepare volume 25 cm3
of 8·10-3
M
NaOH from the stock solution of the NaOH in the second volumetric flask. Mix the
contents of the volumetric flasks together in beaker and switch the stopwatches in
this moment on.
b) Start absorbance measurement (i.e. decolourisation) of the reaction mixture at 574
nm in 3 minute intervals for 30 minutes.
c) Repeat the measuring by the same way using 25 cm3
of 1.6·10-2
M NaOH.
2. Monitoring of reaction for different ionic strenght.
Take 25 cm3
of the dye solution having the same concentration as for measurement 1a
and mix it with 25 cm3
solution containing NaOH and KNO3 (to adjust the ionic strength)
with following concentrations:
a) 1.6·10-2
M NaOH and 4·10-2
M KNO3.
b) 1.6·10-2
M NaOH and 1.2·10-1
M KNO3.
c) 1.6·10-2
M NaOH and 2·10-1
M KNO3.
Monitor the decrease in the absorbance of the reaction mixtures by same way as in the
case 1a.
?


ú l o h a
/45/
7.a-c
DATA ANALYSIS: Make linear regression of the calibration curve. Calculate the
experimental sample concentrations using regression parameters.
1. EVALUATION OF REACTION ORDERS
a) Order with respect to dye (p). Use the results of the experiments 1a,b. Create
Common graph 2: i.e. plot 𝑙𝑙𝑙𝑙[𝑉𝑉𝑉𝑉 𝑉𝑉𝑉𝑉]𝑡𝑡 vs. time t for both concentration of [𝑂𝑂𝑂𝑂−].
Create Common graph 3: i.e. plot 1 [𝑉𝑉𝑉𝑉 𝑉𝑉𝑉𝑉]𝑡𝑡⁄ vs. time t for both concentration of
[𝑂𝑂𝑂𝑂−]. Make linear regressions of all plots and calculate the correlation
coefficients. The order 𝑝𝑝 = 1 is valid if correlation coefficients obtained from linear
regression of the plot in Common graph 2 is closer to value 1.000 than the
coefficients obtained from regression in Common graph 3. Otherwise 𝑝𝑝 = 2 can be
more suitable. Calculate the rate constants k′ from slopes of regression lines (see
linear forms (1.5.)) for both concentrations of hydroxide [𝑂𝑂𝑂𝑂−].
b) Order with respect to [𝑂𝑂𝑂𝑂−] (r): Use two calculated rate constants k′ valid for
two experimental hydroxide concentrations [𝑂𝑂𝑂𝑂−]. They give together two
equations of type (1.3.) with two unknowns r and k. Find the unknowns and round
off the value 𝑟𝑟 to the whole number. The rounded order 𝑟𝑟 should be used for
recalculus of rate constant 𝑘𝑘.
2. Evaluation of influence of ionic strenght
a) Use the results of the experiments 1a,b and 2a,b,c. Calculate ionic strength 𝐼𝐼 for
all reaction mixtures (use eqn 6.5). Create Common graph 4: i.e. plot 𝑙𝑙𝑙𝑙[𝑉𝑉𝑉𝑉 𝑉𝑉𝑉𝑉]𝑡𝑡
(or 1 [𝑉𝑉𝑉𝑉 𝑉𝑉𝑉𝑉]𝑡𝑡⁄ if 𝑝𝑝 = 2) vs. time t for all experiments. Make linear regressions of all
experimental dependences and calculate the rate constants k` and 𝑘𝑘.
b) Create Graph 5: i.e. plot 𝑙𝑙𝑙𝑙 𝑙𝑙(𝑘𝑘) vs. √𝐼𝐼 . Make linear regression of data in Graph 5
and find the parameters of the regression line. Apply eqn (1.6.) and calculate
values 𝑘𝑘0 and product 𝑧𝑧𝑎𝑎⋅𝑧𝑧𝑏𝑏 . Compare the product with value 𝑝𝑝⋅𝑟𝑟.
REPORT: Calibration table 1: for all standard solution 𝑠𝑠: dye concentration and
absorbance 𝐴𝐴𝑠𝑠. Calibration graf 1: plot 𝐴𝐴𝑠𝑠 vs. concentration. Table 2: for
experiment 1a,b: time t, values At , [ ]tVIOL , ln[ ]tVIOL , [ ]tVIOL1 . Common graph 2
and 3: see data analysis. Tabulka 3: for experiment 2a,b,c: time t, values At , [ ]tVIOL
and ln[ ]tVIOL (or [ ]tVIOL1 ). Common graph 4: see data analysis. Table 5: for all
experiment: concentrations [OH]
and [NO3
],
I, I , k'
, k, log(k). Graph 5: dependence
klog on I . Next: orders of reaction p, r. Calculated value k0 and product zA .zB.

