DATE :
OBJECTIVE
:
1.
To determine the surface area of carbon powder by studying the adsorption of the soluble dye methylene blue on the powder.
To determine the surface area of carbon powder by studying the adsorption of the soluble dye methylene blue on the powder.
INTRODUCTION
:
The term “adsorption”is used to indicate that the adsorbate
(gas or liquid) is bound onto the adsorbent (solid surface). This is different
from absorbtion, in which the molecule goes into the interior of the solid. In
the case of adsorption, the process is defined as physisorption if the
adsorbate is held by weak, van der Waals type of forces on the surface. If bond
formation between the adsorbate and the adsorbent is observed, the process is
commonly known as chemisorptions. The term adsorption is used to describe the
fact that there is a greater concentration of the adsorbed molecules at the
surface of the solid than in the bulk solution. In general, one uses solid
adsorbents of small size and often with surface imperfections such as cracks
and holes which serve to increase the surface area per unit mass greatly over
the apparent geometrical area. Some examples of adsorbents commonly used in
experiments of this kind are charcoal, silica gel, alumina, zeolites, and
molecular sieves. The adsorption from aqueous solutions of acetic cid on
charcoal will be investigated in the present experiment. The type of
interaction between the adsorbed molecule and the solid surface varies over a
wide range from weak nonpolar van der Waals’ forces to strong chemical bonding.
Chemisorption is highly specific in nature and depends on the chemical
properties of both the surface molecules and the adsorbed molecules. Adsorption
arising from the weaker van der Waals’ and dipole forces is not so specific in
character and can take place in any system at low or moderate temperatures.
This type of adsorption is called physical adsorption and is usually associated
with low heats of adsorption (less than about 10 kcal mol-1). Physical
adsorption forces are similar to those which cause condensation of gases into
liquid or solids. When an adsorbing molecule approaches the surface of the
solid, there is an interaction between that molecule and the molecule in the
surface which tends to concentrate the adsorbing molecules on the surface in
much the same way that a gas molecule is condensed onto the surface of bulk
liquid.
Another respect in which physical adsorption is similar to liquid
condensation is the fact that molar heats are of adsorption are of the same
order of magnitude as molar heats of vaporization. The amount adsorbed per gram
of solid depends on the specific area of the solid, the equilibrium solute
concentration in the solution (or pressure in the case of adsorption from the
gas phase), the temperature, and the nature of the molecules involved. From
measurements at constant temperature, one can obtain a plot of N, the number of
moles adsorbed per gram of solid, versus C, the equilibrium solute
concentration. This is called an adsorption isotherm. Much effort has been
devoted to developing a theory of adsorption which would explain the observed
experimental facts. In some simple systems, a theory derived by Langmuir can be
applied. This theory is restricted to cases where only one layer of molecules
can be adsorbed at the surface. Monolayer adsorption is usually observed in the
case of chemisorption from the gas phase or adsorption from solution. Monolayer
adsorption is distinguished by the fact that the amount adsorbed reaches a
maximum value at moderate concentrations (corresponding to complete coverage of
the surface of the adsorbent by a layer one molecule thick) and remains
constant with further increase in concentration. The Langmuir isotherm can be
derived from either kinetic or equilibrium arguments and is most commonly
applied to the chemisorption of gases.
Chemical adsorption involves the formation of
chemical bonds such as in the surface oxidation of a metal. A new penny
gradually turns to a darker brown color and changes from being shiny to being a
dull color because of the formation of a coating of copper oxide on the surface
of the penny. Physical adsorption involves nonspecific attraction due to weaker
van der Waals or dipole forces and is similar to the condensation of a gas to a
liquid. Chemical adsorption involves higher heats of adsorption (80 to 400
kJ/mol of molecules) and is enhanced by raising the temperature. There is an
energy barrier for chemisorption based on the fact that a chemical bond in the
gas-phase or liquid-phase molecule must be broken first before the molecule or
its fragments can bond to the surface of the solid. Physical adsorption
involves low heats of adsorption (less than 40 kJ/mol) and is enhanced by
lowering the temperature.
MATERIALS
:
12 conical flasks, 6 centrifuge tubes, measuring cylinders, analytical balance, Beckman J6M/E centrifuge, burettes, retort stand and clamps, Pasteur pipettes, iodine solutions, 1% w/v starch solution, 0.1 M sodium thiosulphate solution, distilled water and activated charcoal
12 conical flasks, 6 centrifuge tubes, measuring cylinders, analytical balance, Beckman J6M/E centrifuge, burettes, retort stand and clamps, Pasteur pipettes, iodine solutions, 1% w/v starch solution, 0.1 M sodium thiosulphate solution, distilled water and activated charcoal
PROCEDURE :
Measuring cylinder was used to fill 12 conical flasks (labelled 1-12) with 50ml mixtures of iodine solutions (A and B) as stated in the Table.
Solution A : Iodine (0.05M), Solution B : Potassium Iodide (0.1M)
FLASK
|
VOLUME OF SOLUTION A (ML)
|
VOLUME OF SOLUTION B (ML)
|
1 and 7
|
10
|
40
|
2 and 8
|
15
|
35
|
3 and 9
|
20
|
30
|
4 and 10
|
25
|
25
|
5 and 11
|
30
|
20
|
6 and 12
|
50
|
0
|
For flasks 1-6
One
to two drops of starch solution was added as an indicator. 0.1 M sodium
thiosulfate solution was used to titrate until the colour of the solution
changes from dark blue to colourless. The volume of the sodium thiosulphate use
was recorded.
Set
2 : concentration of iodine in solution A at equilibrium (C)
For flasks 7-12
0.1
g of activated charcoal was added. The flasks was caped tightly. The flask was
swirled every 10 minutes for one hour and half. After one hour and half, the
solution was transferred into centrifuge tubes and accordingly was labelled.
The solution was centrifuged at 3000 rpm for 5 minutes and the resulting
supernatant was transferred into new conical flask. Each conical flask was
labelled. Steps 1, 2, and 3 was repeated as carried out for flasks 1-6 in Set 1
Results :
Results :
For Flask 7 to 12
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| Click to enlarge |
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| Click to enlarge |
DISCUSSION:
From the graph of N against C, we got a negative slope graph. This indicates that the less the amount of iodine is adsorbed onto the charcoal (less N), then the balance concentration of solution ,C will be increased.
From the graph C/N against C, we got a straight line graph that passes through the y-intercept. The 1/Nm is the gradient has the value of 22250 g mol-1 and the Nm value is 4.49438 x 10-5 mol-1 g. The 1/KNm is -76 g L-1 and K is -292.76 L g-1.
The number of iodine molecule adsorbed on the 0.1g of charcoal :
4.49438 x10-5 X 0.1 X 6.023x1023 = 2.707x1018 molecules
The surface area of the charcoal:
2.707x1018 X 3.2x10-19 ÷ 0.1 = 8.66 m2/g
How we determine experimentally that equilibrium has been reached after shaking for 2 hours?
We can see the colour changes in the flask. The initial colour of iodine is dark brown colour. When the reaction proceed, the colour become light (light brown) and remain the unchanged until a long period of time because some of the iodine is being adsorbed onto the charcoal. This is when the reaction is equilibrium.
For flasks 1-6, we did the titration of iodine with the sodium thiosulfate to standardized the concentration of iodine in solution A. The average actual concentration is 0.0445 M. For flasks 7-12, we put 0.1g charcoal into solution each to adsorbed the iodine and this has reduced the iodine left in the balance solution when doing the titration. This can be shown by the calculated concentration of the iodine in the table. There are much lesser than the actual concentration.
The results and readings obtained from the result are not as accurate as the theoretical one as we studied. This could be due to several reasons. Firstly, the time for swirling the flask with charcoal is reduced from 2 hours to 1.5 hours. This may cause some of the iodine did not completely adsorbed onto the charcoal. Thus, balance concentration C were affected.
Next, the different person shaking the flask might affected the results too as different people uses different strength and ways of shaking the flask. This had cause the reaction did not achieved equilibrium.
Moreover, parallax error which the eye of the observer is not perpendicular to the meniscus of the solution when measuring and taking reading might affect the results obtained too.
Furthermore, when we titrating the centrifuged solution of iodine left, we did not have controlled sample to compared the complete end point of the reaction. This might cause we added extra or less sodium thiosulfate into to analyte and affected the concentration calculated.
Besides, some of the charcoal might split when it was added into the small mouth of the conical flask. This will greatly reduced the amount of charcoal to adsorb the iodine. More iodine will be presented in the analyte during titration.
PRECAUTION:
1. We must let the reaction to continue for 2 hours instead of 1.5 hours to get more accurate results.
2. We must ask the same person to shake the flasks all the time to obtain a more consistent and accurate results.
3. We must avoid parallax error when measuring and taking reading for solution by placing our eye perpendicular to the meniscus of the solution.
4. We must have a controlled sample to compare the end point of titration so that we would know when the reaction achieved equilibrium.
5. We must be very careful when we add the charcoal into the flask by using filter funnel to avoid spillage of charcoal and hence obtained accurate results.
CONCLUSION:
The actual concentration of iodine in solution A is 0.0445M. The Nm value is 4.49438x10-5 g mol-1. The number of iodine adsorbed onto the monomolecular layer is2.707x1018 molecules. The surface area of charcoal is 8.66 m2/g.
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