The Effect of Molecular Weight on the Diffusion Rate of Substances

The Effect of Molecular Weight on the Diffusion Rate of Substances

Jesselyn P. Rodriguez
Group 4 Section X-2L

August 13, 2013

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ABSTRACT
The effect of molecular weight on the rate of diffusion of substances is tested using two tests: the glass tube test and the agar-water gel test. In the glass tube test, two cotton balls were soaked in two different substances, one in hydrochloric acid (HCl) and the other one in ammonium hydroxide (NH4OH). The cotton balls were inserted into the two ends of the tube at the same time. After some time, a white smoke was observed to evolve nearer the cotton ball soaked in HCl. This indicated that NH4OH, which is lighter, diffused faster than HCl. The next test was the agar-water gel test. It was composed of a petri dish of agar-water gel with three wells. A drop of potassium permanganate, potassium dichromate, and methylene blue were introduced to the three wells simultaneously. The diameters of the three substances were measured after a regular 3 minute interval for 30 minutes. Results showed that potassium permanganate, which is the lightest, have the fastest diffusion rate while methylene blue, the heaviest, have the slowest diffusion rate. Thus, the higher the molecular weight of a substance, the slower its rate of diffusion.

INTRODUCTION
All substances, whether solid, liquid or gas, are composed of molecules which are in constant motion. Because of this constant motion, they produce an energy known as thermal energy (heat) (Campbell, ). Moreover, due to this constant movement, molecules frequently collide with each other and to the sides of the container. After some time, this movement results to even distribution of molecules throughout the system. When there are no other forces acting on the system, a substance will diffuse from where it is more concentrated to where it is more concentrated (Campbell, 2011). According to Callister (2007), there are several factors affecting the rate of diffusion of substances. Temperature has the greatest effect on the diffusion rate and the easiest to change. It is directly proportional to diffusion rate. When you increase the temperature, you increase the energy of the particles of the substance, thus it will move faster or have a higher diffusion rate. The difference in concentration between the substances also affect diffusion rate. When the difference in concentration between two different substances is very high, it is also observed that the diffusion rate is very fast. Similarly, when the difference in the concentration is low, then the diffusion rate is also low. Another factor affecting the rate of diffusion is the diffusion distance. Smaller distances result in faster diffusion rate and larger distances result in slower diffusion rate. Lastly, the size of particles of the substances also affects the diffusion rate. The hypothesis which is “the molecular weight affects the diffusion rate of the substance” was arrived at using the glass tube test. In this test, two cotton balls were soaked in two different substances, one in hydrochloric acid, the other one in ammonium hydroxide are plugged in simultaneously at the two ends of the glass tube. Though the two substances are liquid, their product, NH4Cl, is a white smoke which made it possible to measure and to compare their diffusion rates. The study aimed to assess the effect of molecular weight on the rate of diffusion of potassium permanganate (KMnO4), potassium dichromate (KCr2O7), and methylene blue using the agar-water gel test. Specifically, the paper aimed to explain the relationship of molecular weight of a substance to its diffusion rate. The study was conducted at one of the Biology laboratory rooms in Institute of Biological Sciences Wing C, specifically at C-107, University of the Philippines Los Banos on August 6, 2013 under the supervision of Dr. Daniel E. M. Husana.
MATERIALS AND METHOD I. Glass tube test
A two feet glass tube was position vertically on an iron stand held by iron clamp. Two cotton balls, one soaked in hydrochloric acid and the other soaked in ammonium hydroxide, were plugged simultaneously at the ends of the glass tube. The mixture of HCl and NH4OH produced a white smoke which is actually NH4Cl. The distance of the white smoke to each cotton ball was measured and recorded. All three groups in the class performed the experiment and each performance served as a trial since the experiment needed three trials. II. Agar-water gel test
A petri dish of agar-water gel with three wells was prepared. Three different solutions were used, namely potassium permanganate (KMnO4), potassium dichromate (KCr2O7), and methylene blue. A drop of each substance was place in each of the three wells. The diameters (mm) of the three solutions were measured and recorded under diameter at t=0min. The diameters of the three substances were measured and recorded every three minutes for 30 minutes. The average rate of diffusion was computed by first computing the partial rate of diffusion at each interval with the formula: The computed values were recorded and the mean of the partial rate of diffusion of each substance was computed and recorded.

Results and Discussion Table 1 showed that distance of the formed product (NH4Cl), a white smoke) from the 2 cotton plugs. It is seen in the table that the white smoke was formed closer to the cotton plug soaked in HCl. It is clearly seen in this table that the site of initial reaction is nearer HCl. This behaviour of the two substances can be attributed to their molecular weight. It was mentioned earlier that substances are composed of atoms or molecules that are constantly in motion. However, the movement is affected by weight. The heavier the molecular weight is, the slower the movement of its atoms or molecules. Thus, it is expected that the product of the reaction will be formed in an area nearer the heavier substance, and in this case, the hydrochloric acid. In table 2, it showed the diameter (mm) of the three substances at a regular time interval of three minutes. It can be seen in this table that as the time increases, the diameter also increases. However, diameter is not a factor that affects diffusion. This table only shows that as the time elapsed, the increase in the diameter decreased or did not increased at all. A clearer illustration that supports the hypothesis is shown in Table 3 and Figure 1. In table 3, the partial rates of diffusion of the three substances after the regular time interval of three minutes were shown. The average rate of diffusion (mm/min) was also recorded in the table. You can see in this table that potassium permanganate (KMnO4), which has a molecular weight of 158g/mole, has diffused faster since it has the greatest value of diffusion rate which is 0.35mm/min. The methylene blue, which is the heaviest (weighing 374g/mole), has the lowest value of diffusion rate thus it diffused the slowest.
Figure 1 is a graphical presentation of the data seen in Table 3. The mean diffusion rates of the three substances were also plotted against their molecular weights in Figure 2 so as to present the relationship in a simpler and more straight-forward manner. The positive results strongly supported the hypothesis that molecular weight affects the diffusion rate of a substance. And it is seen in the results that the relationship of molecular weight and diffusion rate is inversely proportional. The lighter the substance is, the greater the value of its diffusion rate or the faster its diffusion rate. Conversely, the heavier the substance, the slower it diffuses.
SUMMARY AND CONCLUSION The effect of molecular weight on the diffusion rate of substances was first tested using the glass tube test. Then to prove further the relationship of molecular weight and diffusion rate, the agar-water gel test was conducted. The results on the first experiment showed that heavier substances travel or diffuse at a slower rate than substances with a lower molecular weight since the product of the reaction of HCl and NH4OH, which is NH4Cl, a white smoke, formed on the area nearer the cotton plug soaked in HCl. The hypothesis was further supported by the results obtained in the second experiment. The results were as follows: KMnO4 diffused at a rate of 0.35mm/min, KCr2O7 diffused at a rate of 0.283mm/min, and methylene blue has a diffusion rate of 0.184mm/min. The lightest substance among the three, KMnO4 has the fastest diffusion rate while the methylene blue has the slowest diffusion rate. Therefore, the lower the molecular weight of a substance, the faster is its diffusion rate, and the greater the molecular weight of the substance, the slower the diffusion rate is. However, note that in this experiment, the only factor considered was the molecular weight of the substance. For future experiments regarding the rate of diffusion, it is recommended to also test other factors that may affect the rate of diffusion.
LITERATURE CITED
Duka, IMA and Diaz, MGQ (n.d.) Biology I Laboratory Manual: An Investigative Approach 9thEdition. UP Los Baῆos, College, Laguna, Philippines. 2009

Callister, William Jr., Materials Science and Engineering: An Introduction 8th Edition, 2010

Cited: Duka, IMA and Diaz, MGQ (n.d.) Biology I Laboratory Manual: An Investigative Approach 9thEdition. UP Los Baῆos, College, Laguna, Philippines. 2009 Callister, William Jr., Materials Science and Engineering: An Introduction 8th Edition, 2010