Bio Lab 1st Sci Paper

The Effect of Molecular Weight on the Rate of Diffusion 1

Cynara Alger Group 1 Sec. U - 7L

 August 16, 16, 2013

 ______________  _____________________ _________ __ 1

 A scientific scientific paper submitted in partial fulfillment of the requirement requirements s in General General Biology I laboratory under Prof. Riaflor Alcabedos, 1st sem., 2013-2014.

 ABSTRACT The effect of molecular weight on the rate of diffusion was verified using two kinds of test; the glass tube test and the agarwater gel test. Two cottons were moistened with two different substances, HCl and ammonium hydroxide NH 4OH, and then were simultaneously placed at the end of the glass tube set-up. The ammonium (NH3(g)) diffused at a faster rate compared to hydrochloric acid (HCl (g)) then the reaction between substances formed a white ring smoke, ammonium chloride (NH 4Cl(s)), that is near the hydrochloric acid (HCl). In the agar-water gel test, 3 kinds of different substances, namely KMnO4, K2Cr2O7 and methylene blue, were dropped into three wells with the same amount. Methylene blue, having the largest molecular weight between the three, exhibited the smallest diameter meaning it had the slowest rate of diffusion. Thus, when a substance has higher molecular weight it will have a slower rate compared to other substances.

INTRODUCTION Molecules dissolved in a solution are in a constant random motion due to their kinetic energy. One result of this motion is that dissolved molecules become evenly distributed throughout the solution. This tendency of molecules to spread out is an example of diffusion. It is the movement of molecules from higher concentration area to areas of lower concentration (McKinley, 2006). It is caused by random molecular motion that leads to complete mixing. It can be a slow process. In gasses, diffusion progress at rate of about 10 cm per minute; in liquids, its rate is about 0.05 cm per minute; in solids, only about 0.0001 cm per minute (Cussler, 2009). There are several factors affecting the rate of diffusion of substances, the size of the particle or the molecular weight of the substances that will be diffused, temperature wherein the substance is placed, concentration difference of the two compartments

where the substance is diffused, diffusion distance of the two compartments where the substance is diffused, surface area of the two compartments and the permeability of the intersection of the compartments. The force of the heat does not move the molecule in a particular direction but pushes it randomly. The atoms (and molecules and other microscopic particles) move every which way, constantly bumping into each other and changing direction. It may help to think of the atoms as balls rolling randomly around, always proceeding in a straight line until it hits another ball (or a wall) and bounces in another direction. The actual movement of the particle is random and we cannot predict where the heat energy will push any particular particle. (Meyertholen, 2007). Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. If the partition is removed as in the lower illustration, the gases will mix because of the random velocities of their molecules. In time a uniform mixture of A and B molecules will be produced in the container. The tendency toward diffusion is very strong even at room temperature because of the high molecular velocities associated with the thermal energy of the particles. (Nave, 2013) The lighter molecule will be pushed by the heavier molecule with greater force so the distance of the lighter molecule will diffuse at a fast rate. The effect of molecular weight on the rate of diffusion can be seen from the experiment of hydrochloric acid (HCl) and the ammonium hydroxide (NH 3) that were placed simultaneously in the glass tube. The experiment illustrated that the lighter substance, ammonium hydroxide (NH 3), with a molecular weight of 17 g/mol resulted to a faster rate of diffusion compared to the heavier substance, HCl, with a molecular weight of 36 g/mol then a formation of a white smoke, ammonium chloride (NH 4Cl), was

a product of the reaction. The experiment was a basis for the comparison of the effect of the molecular weight in the diffusion of substances. The agar-water gel test was used to test whether the hypothesis in the first experiment was correct and to prove that the molecular weight is one factor of the diffusion of substance. Equal amount of potassium permanganate (KMnO 4), potassium dichromate (K2Cr 2O7) and methylene blue were placed at different wells in the agarwater gel and were observed for their diameter of diffusion within 30 minutes.

The study aimed to determine the control of the molecular weight on the rate of diffusion of the three substances namely potassium permanganate (KMnO 4), potassium dichromate (K2Cr 2O7) and methylene blue via agar-water gel test in a given time. It aims to 1. to identify which would be likely a factor of the rate of diffusion of substance; and 2. to explain what is the effect of the molecular weight on the rate of diffusion of the substances

The study was conducted at the Institute of Biological Sciences Laboratory, University of the Philippines Los Banos Campus, Los Banos, Laguna on August 8, 2013.

MATERIALS AND METHODS

In the first experiment, a glass tube that was placed, equal in both sides, in an iron stand was used as a medium between the two cottons that were moistened with 2 different reagents, ammonium (NH 3) and hydrochloric acid (HCl) and was placed at the end of the glass tube. Afterwards, a white smoke ring appeared shown in Figure 1. The place where the cottons were placed and the smoke formation were marked. The distances (in cm) between the smoke formation and the hydrochloric acid (HCl) or from the ammonium (NH3) were recorded. The other trials were also recorded for the average distance of each substance with the smoke formation and the average ratio of the measurements. The measurements were tallied in a table for comparison.

In the second experiment an agar-water gel was placed in a covered petri dish. There are three wells in the agar-water gel where the drops of the substances will be placed for observation. The substances that were placed in the wells simultaneously were potassium permanganate (KMNO 4) with a molecular weight of 158 g/mole, potassium dichromate (K 2Cr 2O7) with a molecular weight of 294 g/mole, and methylene blue with a molecular weight of 374 g/mole. The solutions differ in color so they are easily observed and compared. The diameter (in mm) at time = 0 minutes was recorded for control. Figure 2 shows the result of the diffusion of the substances in the gel. Then the diameters at a succession of 3 minutes were recorded for comparison and it was tabulated for the calculation of the partial rate of diffusion by using this formula

  ( ) 

   

where di = diameter of colored area at a given time di-t = diameter of colored area immediately before d i ti = time when d i was measured ti-t = time immediately before t i

 All computed answers were tallied and tabulated and the average rate of diffusion (mm/min.) of each substance were computed then placed in a graph. The partial rates of diffusion at a given time was plotted to form a line graph for interpretation and study. RESULTS AND DISCUSSION

Table 1 shows the recorded distances from the trials made, the distances are the distance between the white smoke ring, ammonium chloride (NH 4Cl), with the two substances that were placed at the end of the glass tube. The observations and recorded distances showed that the white smoke ring was closer to the hydrochloric acid (HCl) than where the ammonium hydroxide (NH 4OH) is placed. The distance between the white smoke ring and the hydrochloric acid (HCl) ranges from 8.5 cm  – 15.0 cm while the distance between the white smoke ring and ammonium hydroxide (NH4OH) ranges from 15.4 cm  –  20.5 cm. The formation of the white smoke ring clarifies that the reaction of the substances reacted near the side of the hydrochloric acid (HCl) so the molecules of ammonium hydroxide (NH 4OH) diffused faster than the molecules of hydrochloric acid (HCl). This may prove that molecular weight has an effect on the rate of diffusion of substances because the molecular weight of ammonium (NH3) is 17 g/mol while hydrochloric acid ’s (HCl) molecular weight is 36 g/mol and the

experiment resulted to a faster diffusion of ammonium (NH 3) because it is farther than the site of the reaction of both substances.

 After conducting the experiment of diffusion of different substances using the agar-water gel set up, the following results in table 4.2 were recorded and studied.

In table 4.2 results showed that methylene blue produced the smallest diameter among the three substances that are potassium permanganate (KMnO 4) and potassium dichromate (K2Cr 2O7). The substances’ molecular weights vary from each other, methylene blue having a molecular weight of 374 g/mol, which is the largest molecular weight among the three, potassium dichromate (K 2Cr 2O7) having a molecular weight of 294 g/mol which is less than methylene blue but more than potassium permanganate, and potassium permanganate having a molecular weight of 158 g/mol, which is the smallest among the three. The table shows that at a succession of three minutes the diameter of potassium permanganate (KMnO 4) and potassium dichromate (K 2Cr 2O7) increases but methylene blue’s diameter remains the same until a certain time. The

results also showed that there is a certain time where the rate of diffusion became stabilized and remained constant.

In table 4.3 the datas from table 4.2 were used to distinguished the partial rates of diffusion of the three substances. The recorded results shows that the potassium permanganate (KMnO 4) shows stable rate of diffusion at first but decreased at the

proceeding time, while potassium dichromate (K 2Cr 2O7) shows irregular rates of diffusion same as the methylene blue. The averages of these rates of diffusion showed that potassium permanganate (KMnO 4) had the fastest rate followed by potassium dichromate (K2Cr 2O7) and lastly methylene blue, all of the results are based on their diameters at 30 minutes at an interval of three minutes. Thus, if a substance has higher molecular weight it will have a slower rate of diffusion.

SUMMARY AND CONCLUSION The hypothesis from the glass tube set up was tested if the molecular weight has an effect on the rate of diffusion. In the agar-water gel test equal amounts of potassium permanganate (KMnO 4), potassium dichromate (K 2Cr 2O7) and methylene blue were simultaneously placed at three separate wells in the gel. The diameters of the substances were measured at an interval of three minutes for 30 minutes. The experiments showed that the average rate of diffusion of potassium permanganate (KMnO 4) was 0.23 (mm/min), followed by potassium dichromate (K2Cr 2O7) was 0.20 (mm/min) and lastly methylene blue was 0.07 (mm/min). The experiment clearly showed that the substance with the smallest molecular weight exhibited faster rate than the substance with larger molecular weight. Therefore, the hypothesis can be accepted since it is proven that the molecular weight affects the rate of diffusion of substances, then the higher the molecular weight, the slower the diffusion. It is also recommended to test other volatile substances to know which could also be a factor in the rate of diffusion.

LITERATURE CITED McKinley, Michael and Valerie O'Loughlin. Human Anatomy. McGraw-Hill. 2006. August 9, 2013. http://highered.mcgraw-

hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.ht ml

Cussler, E. L. Diffusion: Mass transfer ass Transfer in Fluid Systems. Cambridge: Cambridge University Press, 2009.x

Meyertholen, E. “Diffusion. ” 2007. August 9, 2013. http://www.austincc.edu/emeyerth/diffuse1.htm

Nave, Carl R. Diffusion. 2013. August 9,2013. http://hyperphysics.phyastr. gsu.edu/hbase/kinetic/diffus.html

Duka, Ivan-Marcelo, et. Al. Biology I Laboratory Manual: An Investigative  Approach. 9th Edition. Los Ba ῆos. 2009