Introduction: The term "dilatometry" refers to methods in which changes in volume are observed and measured (Dilate = to expand).
Any determination of reaction rate constant requires knowledge of the variation of concentration of reactants (or products) with time. The variation of concentration may be measured either directly by chemical analysis or indirectly by measuring certain physical properties such as pressure volume or conductivity, which can be related easily to the concentration. In dilatometry concentration changes are followed by measurements of volume of the reacting system.
In the present experiment, a homogeneous liquid phase catalytic reaction is studied. The kinetics of homogeneous catalytic reactions is analogous to the kinetics of homogeneous non-catalytic reactions except that the overall rate constant varies linearly with the concentration of the catalyst:
Koverall = Knon-cat + Kcat Ccat
In the present case, Knon-cat is nearly zero and hence :
Koverall = Kcat Ccat
Aim of the Experiment: The aim of the experiment is to determine the Koverall and the catalytic rate constant (Kcat) for the decomposition of diacetone alcohol in aqueous solution, catalyzed by sodium hydroxide at ambient temperature.
(CH3)2 C(OH) CH2 CO CH3 ------- 2CH3 CO CH3
Materials Required:
Diacetone alcohol 10ml
NaOH (aqueous 0.1 N) 100ml
NaOH (aqueous 0.05 N) 100ml
Oxalic acid(standard 0.1 M) 100ml
Dilatometer stop clock measuring jar (100ml) pipette (5ml) beaker (250 ml) phenolphthalein indicator.
Procedure:
1. Measure 100ml of 0.1 N alkali into the 250ml beaker. Add 5 ml of diacetone alcohol by means of pipette. Mix the contents well by means of a glass rod for about one minute.
2. Pour the reacting mixture into the cup of the Dilatometer keeping the stopcock closed.
3. Carefully open the stopcock allowing the reaction mixture into bulb of Dilatometer taking care to see that no air bubbles are retained in the connecting U-tube or in the capillary of the Dilatometer.
4. Adjust the liquid level in the capillary to some arbitrary value at the bottom of the scale. Close the stopcock. Some excess liquid must be left over in the cup at this stage.
5. Note the reading of liquid level and simultaneously start the stopwatch.
6. Continue noting the liquid level readings at intervals of half-minute until the liquid level finally crosses the calibrated portion of the capillary.
7.Repeat the experiment using 0.05 N NaOH (instead of 0.1 N NaOH)
and using the time intervals of 1 min.
8.Standardize the alkali solutions using oxalic acid.
9.Note the room temperature.
Note: Suppose you obtained 18 readings of volumes in one of the experiments.
Then enter the first 9 readings as y and the last 9 readings as y'.
NAMR/SN/24984
READINGS AND CALCULATIONS
---------------------------
Standardisation of NaOH:
Volume of Oxalic acid taken =10 ml
Normality of Oxalic acid = --------------(given)
Titre value with 0.1 N NaOH = -----------ml
Normality of 0.1 N NaOH =
Titre value with o.05 N NaOH = -----------ml
Normality of 0.05 N NaOH =
Dilatometric Readings (1):
Conc. of NaOH =
Sl.No. Time (min.) y (Cm) y’ (Cm) ln (y’ - y)
----------------------------------------------------------------
1
2
3
4
.
.
24
25
Draw a plot of ln (y' - y) versus t, and report modulus of the value of slope as Koverall.
Koverall = ……………………………………………………………………………………min-1
kcat = Koverall / Ccat
………………………………………………………litre / (gm – mole) (mole)
Dilatometric Readings (2):
Conc. of NaOH =
Sl.No. Time (min.) y(Cm) y'(Cm) ln( y' - y )
----------------------------------------------------------------
1
2
3
4
.
.
24
25
Koverall = -----------------min-1
Kcat = Koverall / Ccat = ----------litre / (gm-mole)(min)
Questions for discussion:
1. Show that the above method of calculation is valid only if the expansion factor (t) is a constant.
2. Assuming a first order reaction mechanism for the catalytic reaction deduce proof for the method of calculation.
REFERENCES:
(1) Wilson New combe Denaro and Rickett Experiments in Physical
Chemistry Pergamon (1962)
Any determination of reaction rate constant requires knowledge of the variation of concentration of reactants (or products) with time. The variation of concentration may be measured either directly by chemical analysis or indirectly by measuring certain physical properties such as pressure volume or conductivity, which can be related easily to the concentration. In dilatometry concentration changes are followed by measurements of volume of the reacting system.
In the present experiment, a homogeneous liquid phase catalytic reaction is studied. The kinetics of homogeneous catalytic reactions is analogous to the kinetics of homogeneous non-catalytic reactions except that the overall rate constant varies linearly with the concentration of the catalyst:
Koverall = Knon-cat + Kcat Ccat
In the present case, Knon-cat is nearly zero and hence :
Koverall = Kcat Ccat
Aim of the Experiment: The aim of the experiment is to determine the Koverall and the catalytic rate constant (Kcat) for the decomposition of diacetone alcohol in aqueous solution, catalyzed by sodium hydroxide at ambient temperature.
(CH3)2 C(OH) CH2 CO CH3 ------- 2CH3 CO CH3
Materials Required:
Diacetone alcohol 10ml
NaOH (aqueous 0.1 N) 100ml
NaOH (aqueous 0.05 N) 100ml
Oxalic acid(standard 0.1 M) 100ml
Dilatometer stop clock measuring jar (100ml) pipette (5ml) beaker (250 ml) phenolphthalein indicator.
Procedure:
1. Measure 100ml of 0.1 N alkali into the 250ml beaker. Add 5 ml of diacetone alcohol by means of pipette. Mix the contents well by means of a glass rod for about one minute.
2. Pour the reacting mixture into the cup of the Dilatometer keeping the stopcock closed.
3. Carefully open the stopcock allowing the reaction mixture into bulb of Dilatometer taking care to see that no air bubbles are retained in the connecting U-tube or in the capillary of the Dilatometer.
4. Adjust the liquid level in the capillary to some arbitrary value at the bottom of the scale. Close the stopcock. Some excess liquid must be left over in the cup at this stage.
5. Note the reading of liquid level and simultaneously start the stopwatch.
6. Continue noting the liquid level readings at intervals of half-minute until the liquid level finally crosses the calibrated portion of the capillary.
7.Repeat the experiment using 0.05 N NaOH (instead of 0.1 N NaOH)
and using the time intervals of 1 min.
8.Standardize the alkali solutions using oxalic acid.
9.Note the room temperature.
Note: Suppose you obtained 18 readings of volumes in one of the experiments.
Then enter the first 9 readings as y and the last 9 readings as y'.
NAMR/SN/24984
READINGS AND CALCULATIONS
---------------------------
Standardisation of NaOH:
Volume of Oxalic acid taken =10 ml
Normality of Oxalic acid = --------------(given)
Titre value with 0.1 N NaOH = -----------ml
Normality of 0.1 N NaOH =
Titre value with o.05 N NaOH = -----------ml
Normality of 0.05 N NaOH =
Dilatometric Readings (1):
Conc. of NaOH =
Sl.No. Time (min.) y (Cm) y’ (Cm) ln (y’ - y)
----------------------------------------------------------------
1
2
3
4
.
.
24
25
Draw a plot of ln (y' - y) versus t, and report modulus of the value of slope as Koverall.
Koverall = ……………………………………………………………………………………min-1
kcat = Koverall / Ccat
………………………………………………………litre / (gm – mole) (mole)
Dilatometric Readings (2):
Conc. of NaOH =
Sl.No. Time (min.) y(Cm) y'(Cm) ln( y' - y )
----------------------------------------------------------------
1
2
3
4
.
.
24
25
Koverall = -----------------min-1
Kcat = Koverall / Ccat = ----------litre / (gm-mole)(min)
Questions for discussion:
1. Show that the above method of calculation is valid only if the expansion factor (t) is a constant.
2. Assuming a first order reaction mechanism for the catalytic reaction deduce proof for the method of calculation.
REFERENCES:
(1) Wilson New combe Denaro and Rickett Experiments in Physical
Chemistry Pergamon (1962)
