The Evaluation of the Effect of Molecular Weight on the Rate of Diffusion

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The Evaluation of the Effect of Molecular Weight on the Rate of Diffusion


ABSTRACT

Two tests, glass tube test and agar-water gel test, were performed in order to determine the effect of molecular weight on the rate of diffusion. Simultaneously, two cotton plugs each soaked in HCl and NH4OH were put into the two edges of the glass tube in the glass tube set-up. NH4OH with the lower molecular weight at 35.04 g/mole diffused faster resulting in the creation of a white ring on the circumference of the glass closer to the side of HCl which has the higher molecular weight at 36.46 g/mole. A petri dish containing agar-water gel with three identical wells was prepared for the agar-water gel set-up. At the same time, a drop of potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7) and methylene blue were dropped individually on the wells. Potassium permanganate (KMnO4), having the lowest molecular weight, presented the biggest diameter at 14 mm and diffused at the fastest rate at 0.366 mm/min. Hence, the lower the molecular weight, the faster the rate of diffusion.

INTRODUCTION

Diffusion comes from the Latin word “diffundere” which means ‘to spread out’. An example of diffusion is the dropping of a droplet of ink in a tub of water without disturbing provides a simple manifestation of diffusion. After some time the color will have proliferated a few millimeters and after several days the coalescence will be undeviatingly colored. (Mehrer and Stolwijk, 2009). Diffusion is the disposition for molecules to expand evenly into the feasible space. (Campbell and Reece, 2011).

The common predilection of molecules to carry out from an area of high concentration to one of lower concentration is called diffusion. (Thomas, et. al., 2012) Diffusion is also the voluntary progress of particles from one area of high amount to an area of low amount. It happens via arbitrary kinetic movement.(Michael Muller, 2004). The first standardized contemplation was all because of Thomas Graham, a Scottish Chemist. He was acknowledged as the “leading chemist of his generation” because he also invented dialysis, that he explained as a process of partition by diffusion under the aegis of a membrane (1854). Graham’s research work on diffusion in gases was completed from 1828 to 1833 and he acquainted his results in 1831 to the Royal Society of Edinburgh. In 1833, his work was later published in the Philosophical Magazine. The first lines of his first paper are: “Fruitful as the miscibility of the gases has been in interesting speculations, the experimental information we possess on the subject amounts to little more than the well established fact, that gases of different nature, when brought into contact, do not arrange themselves according to their density, the heaviest undermost, and the lightest uppermost, but they spontaneously diffuse, mutually and equally, through each other, and so remain in the intimate state of mixture for any length of time.” (Philibert, Jean, 2005).

Inspired by the polished experiments on diffusion in gases and of salt in water executed by the Scotsman Thomas Graham, Adolf Fick also introduced the quantitative laws of diffusion. (Philibert, Jean, 2005). The Brownian motion, recognized for the first time by the Scottish botanist Robert Brown, was elucidated decades later by Albert Einstein, the famous German-Jewish physicist. (Mehrer and Stolwijk, 2009). On April 30, 1905 Albert Einstein finally – after a couple of hopeless attempts – rendered successfully to the University of Zurich his doctoral thesis on the assessment of molecular dimensions and Avogadro’s number from the difference of viscosity through dissolving sugar in water. After a few days, on May 11, 1905, Annalen der Physik in Leipzig accepted a paper by Albert Einstein titled “Uber die von der molekulartheoretischen Theorie der Warme geforderte Bewegung von in ruhenden Flussigkeiten suspendierten Teilchen”. This paper on the irregular motion of tiny particles drove on by molecules or atoms in liquids was in quest of a different approach to know molecular dimensions and Avogadro’s number than that Einstein had followed in his thesis. Certainly Einstein had no knowledge of the wide literature on Brownian motion, but identified only finally that the phenomenon that he had deliberated already was recognized to exist and had been widely studied. He uses a new courageous argument by saying that molecules and suspended particles at equal amount will produce the same osmotic pressure. (Vogl, Gero, 2005). The diffusion across a membrane with no energy stake is called passive transport. (Campbell et.al., 2009)

According to Libranza (2012), there are considerable factors which may influence the rate of diffusion of a substance. These factors encompasses the proportion of the particle or the molecular weight of the substance, temperature or availability of energy in the system, discrepancy in concentrations inside the system, diffusion distance, and if the system associates a membrane or barrier, the surface area of the barrier, and the barrier’s permeability. The greater the amount or concentration of a substance in an area of a system necessitate that the frequency of particles striking each other is higher, causing the particles to drive each other at a more rapid rate. These concussions are due to the high molecular velocities identified with the thermal energy invigorating the particles. (Nave, 2008). A smaller particle can be driven or pushed faster than a larger particle. Thus, the hypothesis of the study is that if molecular weight affects rate of diffusion, then, the size of the particle is inversely proportional to the rate of diffusion. This means that the higher the molecular weight, the slower the rate of diffusion and vice versa.

The idea that the molecular weight has an effect on its rate of diffusion was deduced from the glass tube experiment. The tip of two identical-sized cotton balls were dipped in Hydrochloric acid (HCl) and Ammonium hydroxide (NH4OH) which were simultaneously situated in each end of the glass tubing. The molecular weight of NH4OH (35.05 g/mole) is relatively smaller compared to the value of the molecular weight of HCl (36.46 g/mole) and both recurred to form Ammonium chloride (NH4Cl), a white smoke product that may form inside the glass tubing, which make the two substance relevant for contrast on which substance diffuses faster inside the glass tube.

Table 4.1. The distance of hydrochloric acid and ammonium hydroxide from the tips of the glass tube to the smoke ring

Trial

Distance (cm)

(d)

Total distance

(D)

Ratio

dHCl

dNH3

dHCl

D

dNH3

D

NH3

HCl

1

19.2

18.7

37.9

0.51

0.49

0.97

2

15

20.5

35.5

0.42

0.58

1.37

3

16.8

20

36.8

0.46

0.54

1.19

4

17.5

18.5

36.0

0.49

0.51

1.06

Average Ratio

1.15

Table 4.1 shows the distances of the hydrochloric acid and ammonium hydroxide from the tips of the glass tube to the smoke ring. It was observed that the distance of the smoke ring was relatively closer to the side of HCl than to the side of the ammonium hydroxide except for the data gathered at Trial 1 which ranges from 15 to 19.2 centimeters as conflicted to the distance measured from the side of ammonium hydroxide which ranges from 18.5 to 20.5 centimeters.

Hydrochloric acid with the highest molecular weight diffused at a slower rate compared to ammonium hydroxide. Since NH4OH diffused at a more rapid rate, it arrived the HCl side faster than HCl arriving the NH4OH side of the glass tube. At a point nearer the side of the HCl, the smoke ring as the first indication that the NH4OH molecules have met and reacted with the HCl molecules coming from the adjacent side of the tube was formed.

The agar-water gel experiment was used to check and evaluate the effect of the molecular weight on the rate of diffusion of distinctive substances. One drop of Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7), and methylene blue was introduced in three different small wells on a petri dish containing agar-water gel. These substances were used because of their distinctive colors making them easy to be identified and suitable for measurement of the diameter of the drops within a period of 30 minutes.

This study aimed to evaluate the effect of molecular weight on the rate of diffusion of Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7), and methylene blue relative to the time by way of the water-agar gel experiment.

Certainly, the objectives of this study are:

  1. To recognize other factors that may affect the rate of diffusion of substances specifically the time; and
  2. To expound the effect of molecular weight on the rate of diffusion of substances.

The study was administered at Room 117 of Institute of Biological Sciences Lecture Hall, University of the Philippines Los Baños, Laguna on the 8th of October, 2014.

MATERIALS AND METHODS

Glass Tube Experiment

A two feet glass tube was fastened horizontally to a ring stand. Two cotton balls of the same size was carefully and simultaneously moistened using fine forceps, one with hydrochloric acid (HCl) and the other with ammonium hydroxide (NH4OH). One end of the tube was plugged with one wet cotton ball and the other end with the other cotton ball simultaneously. The glass tube was carefully watched for the white smoke inside the glass tube to appear and mark its position.

Agar-Water Gel Experiment

In evaluating the relationship between the molecular weight and rate of diffusion, a petri dish of agar-water gel with three wells was obtained per group. The wells were labeled as follows: Potassium permanganate (KMNO4), Potassium dichromate (K2Cr2O7) and methylene blue. One drop of the prepared solution of each substance was carefully placed into each well. The petri dish was immediately covered and the diameter (in mm.) of the colored area was measured. The diameter measured was recorded for zero minute.

At a regular three-minute interval for thirty minutes, the diameter of the colored area of each substance was measured and recorded.

The data for the hypothesis was evaluated; however, precise relationship between diffusion and molecular weight is not guaranteed by such evaluation. So the average rate of diffusion of each substance was computed for a clearer relationship. The average rate of diffusion (in mm/min.) was obtained by first computing for the partial rate of diffusion at each interval with the following equation:

Partial rate (rp) = di – di-1

ti – ti-1

where: di = diameter of colored area at a given time

di-1 = diameter of colored area immediately before di

ti = time when di was measured

ti-1 = time immediately before ti

""After computing, the average of the computed partial rates of each substance was obtained and this was the average rate of diffusion.

Figure 4.1. The petri dish of agar-water gel with three wells each containing KMNO4, K2Cr2O7, and methylene blue at zero minute.

""

Figure 4.2. The petri dish of agar-water gel with three wells each containing KMNO4, K2Cr2O7, and methylene blue after thirty minutes.

RESULTS AND DISCUSSION

Table 4.2. The diffused distance in diameter of Potassium permanganate, Potassium dichromate, and Methylene Blue at a regular three-minute interval for thirty minutes.

Time

(minute)

Diameter (mm)

Potassium permanganate

(MW 158 g/mole)

Potassium dichromate

(MW 294 g/mole)

Methylene

Blue

(MW 374 g/mole)

0

3

3

3

3

6

5

5

6

8

7

6

9

10

8

7

12

11

9

7

15

12

9

8

18

12

10

9

21

13

10

9

24

13

11

10

27

13

11

10

30

14

12

11

As seen in Table 4.2, results showed that Potassium permanganate with a molecular weight of 158 g/mole diffused the greatest distance of 14 mm in diameter after 30 minutes. Also seen in Table 4.2, Methylene Blue with 374 g/mole molecular weight diffused the least distance of 11 mm in diameter after 30 minutes. Potassium dichromate with a molecular weight of 294 g/mole diffused neither the greatest nor the least distance because of the 12 mm diameter after 30 minutes.

Table 4.3. The rates of diffusion of Potassium permanganate, Potassium dichromate, and Methylene Blue at a regular three-minute interval for thirty minutes.

Time elapsed

(minute)

Partial rates of diffusion (mm/min)

Potassium permanganate

(MW 158 g/mole)

Potassium dichromate

(MW 294 g/mole)

Methylene

Blue

(MW 374 g/mole)

3

1

0.67

0.67

6

0.67

0.67

0.33

9

0.67

0.33

0.33

12

0.33

0.33

0

15

0.33

0

0.33

18

0

0.33

0.33

21

0.33

0

0

24

0

0.33

0.33

27

0

0

0

30

0.33

0.33

0.33

Average rate of diffusion

(mm/min.)

0.366

0.299

0.265

In Table 4.3, Potassium permanganate having the lowest molecular weight had an average rate of diffusion of 0.366 mm/min. And Methylene Blue with the highest molecular weight had an average rate of diffusion of 0.265 mm/min. Lastly, Potassium dichromate with a molecular weight higher than Potassium permanganate but lower than Methylene Blue had an average rate of diffusion of 0.299 mm/min.

""

Figure 4.3. The average rate of diffusion of a specific molecular weight of substances.

The relationship between molecular weight and average rate of diffusion was determined and clearly presented in the graph (Figure 4.3). The lower the molecular weight of the substance, the faster the average rate of diffusion. And the higher the molecular weight of a substance, the slower the average rate of diffusion. Thus, the molecular weight is inversely proportional to the average rate of diffusion.

""

Figure 4.4. The partial rate of diffusion of substances at specific times.

As shown in the graph (Figure 4.4), at the first 9 minutes the partial rates of diffusion of the three substances were still fast ranging from 0.33 to 1 mm/min. After 30 minutes, the partial rates of diffusion slowed down ranging from 0 to 0.33 mm/min. At the start of the time, the rate of diffusion is fast. But as the time reaches its end, the rate of diffusion slowed down.

This observed effects of molecular weight and time to the rate of diffusion could be explained. If the molecular weight is high, it means that it is harder for the particles to move therefore slower to diffuse. But if the molecular weight is low, the particles could move easily therefore faster to diffuse. At the start of the time, the rate of diffusion is fast because the concentration of substance at the start is very high. But as the time reaches the end, the rate of diffusion slowed down because the concentration of substance is very low because the particles is already scattered. Thus, this may have caused the difference in the diffusion rates of substances

The hypothesis, if molecular weight affects rate of diffusion, then, the size of the particle is inversely proportional to the rate of diffusion is accepted.

Some errors that could have affected the results of the experiments conducted may be due to inaccurate time interval, unequal amount of substances dropped on the wells, inaccurate measurements, other human errors and error in the calculator used for computing.

SUMMARY AND CONCLUSION

The hypothesis of the study is that, if molecular weight affects rate of diffusion, then, the size of the particle is inversely proportional to the rate of diffusion. It was formulated due to the glass tube – cotton ball set-up wherein NH3 diffused faster than HCl.

To test the hypothesis, the water-agar gel set-up was used to determine the effect of molecular weight to the rate of diffusion. Simultaneously, a drop of potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7) and methylene blue were placed on each of the wells on the agar. To know the distance covered by diffusion of the substances for 30 minutes, the diameter of the colored parts were measured and recorded at a regular three minute intervals.

Based on the results, potassium permanganate covered the largest diameter at 14 mm, followed by methylene blue with a diameter of 12 mm. Potassium dichromate covered the smallest diameter at 11 mm. Potassium permanganate diffused the fastest with the highest average rate of diffusion of 0.366 mm/min., followed by potassium dichromate with an average rate of diffusion of 0.299 mm/min. Methylene blue diffused the slowest with the lowest average rate of diffusion of 0.265 mm/min.

The experiment conducted supported the formulated hypothesis. However, the results need to be validated by further experiments. For more accurate and precise results, it is recommended that one should be cautious about the interval of time before measuring the diameter of the diffused distance. One should also learn how to measure accurately.

LITERATURE CITED

Muller, Michael. 2004. Diffusion, Osmosis, and Movement Across a Membrane. <http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect09.htm>. October 11, 2014.

Mehrer, H. and N.A. Stolwijk. 2009. Heroes and Highlights in the History of Diffusion. <http://www.uni-leipzig.de/diffusion/pdf/volume11/diff_fund_11(2009)1.pdf>. October 11, 2014.

Philibert, Jean. 2005. One and a Half Century of Diffusion: Fick, Einstein, before and beyond. <http://www.uni-leipzig.de/diffusion/pdf/volume2/diff_fund_2(2005)1.pdf>. October 11, 2014.

Vogl, Gero. 2005. Diffusion and Brownian Motion Analogies in the Migration of Atoms, Animals, Men and Ideas. Diffusion Fundamentals. pp. 18-23.

Nave, R. 2008. Diffusion. <http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/diffus.html> . October 11,2014.

Libranza, A.K. 2012. The Effect of Molecular Weight on the Rate of Diffusion of Substances. <http://www.academia.edu/1776814/The_Effect_of_Molecular_Weight_on_the_Rate_of_Diffusion_of_Substances> . October 11, 2014.

Campbell and Reece. 2011. Biology. 9th Edition. Boston: Benjamin Cummings: imprint of Pearson.

Thomas M., Smith, Robert Leo. 2012. Elements of Ecology. 8th Edition. Pearson Education Inc., pp 45-46.

Campbell, N., Reece, J.,Taylor, M. 2009. Biology: Concepts and Connections. 6th Edition. Pearson Education, Inc., pp 75-76.

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