Partial Rate of Diffusion of Potassium Dichromate

ABSTRACT

Diffusion is the process where molecules spread into spaces. It was observed that the formation of ammonium chloride was near the hydrochloric acid meaning the ammonia diffused faster. One factor that affects the rate of it is the molecular weight of a substance. If molecular weight affects the rate of diffusion, then, the higher or lower the molecular weight the slower or faster the rate of diffusion. To test this factor, a drop of potassium dichromate, potassium permanganate, and methylene blue were placed on each wells of the petri dish containing agar water gel. After thirty minutes, the partial rate and average rate of diffusion are calculated and the results showed that the substance with the lowest molecular weight, potassium permanganate, diffuses fastest. The molecular weight affects the rate of diffusion, the lighter the molecular weight the slower the rate of diffusion and vice versa. The molecular weight of the substance is inversely proportional to the rate of diffusion.

INTRODUCTION

Diffusion is the process where the molecules of solid, liquid or gas spread spontaneously to occupy a space. Gas molecules are the fastest to diffuse since their molecules are scattered from one another. The molecules of diffusion moves randomly and they spread into available space (Mendoza E, 2003).

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Hydrochloric acid is formed by mixture of hydrogen ion and chlorine ion. According to the Lewis’ theory of acids and bases, hydrochloric acid is an acid because an element (chlorine) was paired to a hydrogen ion and when it is dissolved in water, it produces hydrogen ion (Ebbing and Gammon, 2011). A 0.1M of hydrochloric acid has a pH level of 1 meaning it is strong enough to dissolved iron nails (Mendoza E, 2003). Hydrochloric acid has a molecular weight of about 36.46 g/mol and can be found inside our stomach because it helps on the digestion of the food we eat (Reece et al, 2005).

Ammonium hydroxide is a base because it produces hydroxide ion when dissolves in water. The ammonium hydroxide decomposes forming ammonia and water. In a decomposition reaction, a compound turns into simpler substances or elements. The ammonia has a molecular weight of about 17.031 g/mol and water has about 18g/mol (Mendoza E, 2003).

When ammonium hydroxide breaks down into ammonia and water, since hydrochloric acid is an acid, when it is dissolved in water, it will produce hydrogen ion. On the other hand, the chlorine ion(C ) will react with ammonia (N). This reaction is called synthesis reaction (HCl + N O → NCl + +O) (Mendoza E, 2003).

From the reaction, ammonium chloride is produced by the synthesis of ammonia and chlorine ion. The ammonia and hydrochloric acid spread to available space until they meet. After they met, a reaction of white powder is formed. The ring is near to the hydrochloric acid compare to the ammonia (France C. 2014)

There are many factors that affect the rate of diffusion. When heat is applied, the molecules moves quicker, making the solute dissolves faster. The diffusion rate also increased by stirring where molecules move faster between the molecules of water. Diffusion is also affected by the density of the solvent where diffusion is slower when it has a higher density (Lozano & Sandico, 2003). Molecular weight also affects the rate of diffusion. Lighter particles diffuse faster than heavy particles It has an inverse proportionality where the smaller the size of the particles, the faster the rate of diffusion (Tro, 2008). The difference in concentration can affect the rate of diffusion too. The distance of diffusion also affects the rate of it where it takes time to diffuse a particle for farther place. Permeability is also a factor of diffusion where if the substance does not permit a substance to pass through it, thus, there will be no diffusion (Meyertholen E., 2014)

Based on the observation, the ammonium hydroxide decomposed into ammonia (N) and water (O), while the hydrochloric acid decomposed into hydrogen ion() and chlorine ion (C). A single displacement had occurred from the ammonia and hydrochloric acid, forming ammonium chloride (NCl). This substance appeared in the tube in the presence of a white smoke. With this, data were obtained to know how molecular weight affects the diffusion of a certain substance. As seen in table 4.1, comparing the ratio of the distances from the two substances up to the smoke over the total distance, the ratio of the ammonia are bigger than the ratio of the hydrochloric acid. But on the first trial, the ratio of hydrochloric acid over total distance is bigger by 0.2cm compared to the ammonia. Using the formula, the ratio of the ammonia over the ratio of hydrochloric acid is used to get the average ratio. With ammonia having a molecular weight of about 17.031 g/mol and the hydrochloric acid having a molecular weight of about 36.46 g/mol, the lighter the molecular weight of the substance, the faster the diffusion is. However, the molecules of the substances on the observation cannot be seen.

Table 4.1. The ratios of the distances from the hydrochloric acid over total distance, from the ammonia over total distance, and ammonia over hydrochloric acid.

This study aimed to determine how molecular weight affects the rate of diffusion. The specific objectives are:

1. To explain the relationship between molecular weight and the rate of diffusion of a certain substance.
2. To prove if molecular weight affects the rate of diffusion, then, the higher or lower the molecular weight the slower or faster the rate of diffusion.

MATERIALS AND METHODS

To find out if the molecular weight of a substance affects the rate of diffusion, a petri dish of agar water gel with three wells, potassium dichromate (C), potassium permanganate (KMn), and methylene blue were used.

The diameter of each well is measured in millimetre using a ruler. Then, a drop of potassium dichromate, potassium permanganate, and methylene blue were dropped on each wells of the petri dish containing agar water gel. It was immediately covered to prevent it from drying. At a three-minute interval for thirty minutes, by lifting the petri dish, the diameter of the coloured area is measured. Using the collected data in the setup, a line graph was used to compare the rate of each substance’s diffusion.

RESULTS AND DISCUSSION

On the experiment, potassium dichromate, potassium permanganate, and methylene blue in agar-water gel inside the petri dish were used to determine the partial rate and the average rate of diffusion. After 30 minutes of observing, the diameter of the potassium permanganate has the biggest among the rest which is 14, compared to the potassium dichromate which is 12 and to methylene blue which is 11. The average rate of diffusion is calculated by the formula:

Where: df= final diameter

do= initial diameter

t= total time

While the partial diffusion rate is calculated by the formula

Where: dx=diameter at a given time

dx-1= diameter immediately before dx

tx= time when dx measured

tx-1= time immediately before tx

Table 4.2. Rate of diffusion of potassium dichromate, potassium permanganate, and methylene blue in agar-water gel inside the petri dish for 30 minutes.

Table 4.3. Partial rate of diffusion of potassium dichromate, potassium permanganate, and methylene blue in agar-water gel inside the petri dish for 30 minutes.

Figure 4.3. A bar graph comparing the average rate of diffusion of potassium dichromate, potassium permanganate, and methylene blue in agar-water gel inside the petri dish.

Figure 4.4. A bar graph comparing the partial rate of diffusion of potassium dichromate, potassium permanganate, and methylene blue in agar-water gel inside the petri dish.

SUMMARY AND CONCLUSION

To test the hypothesis, an experiment was performed to test if the molecular weight affects the rate of diffusion, then, the higher or lower the molecular weight the slower or faster the rate of diffusion. An observation using a glass rod to measure the distance on how far the particles of the hydrochloric acid and ammonia travelled until both of them had a chemical reaction producing ammonium chloride. Ammonia that has 17.031 g/mol travelled faster than the hydrochloric acid that has 36.46 g/mol.

The experiment was done to support the observation because it is more clearly and visible to the naked eye because the diffusion can be observe in this setup. The potassium permanganate having a diameter of 14 is the biggest among them. The average rate of diffusion of each substances was calculated and the result is potassium permanganate has the fastest rate of 0.367mm/min. potassium permanganate has the lightest molecular weight of 158g/mol.

Based from the setup that was performed, the substance that has the lightest molecular weight has the fastest rate of diffusion. This study confirmed the hypothesis where the molecular weight affects the rate of diffusion, the lighter the molecular weight the slower the rate of diffusion and vice versa. The molecular weight of the substance is inversely proportional to the rate of diffusion. However, some errors were made during the experiment like the unequal amounts of the substances placed in the petri dish, methylene blue was spilled from the well, and the three substances are not measured at exactly every three minutes. This study needs further research because there are also other factors other than the molecular weight that could affect the rate of diffusion. It is recommended to research and study other factors that can affect the rate of diffusion.

LITERATURE CITED

Cain, M.L., Jackson, R.B., Minorsky, P.V., Reece, J.B., Urry L.A., and Wasserman, S.A.2011. Campbell Biology 9^th Edition. USA: Pearson Education Inc. p. 53.

Ebbing,D. and Gammon, S. D. 2009. General Chemistry Enhanced Edition. USA: Cengage Learning Inc. p. 143-144.

France C. 2014. Elements, Compounds and Mixtures: Information retrieval. <http://gcsescience.com/e17- ammonium-chloride-reversible.htm> Accessed October 12, 2014.

Lozano L.F. and Sandico P.M. C.2003. Science and Technology for the Future II. Makati City: Diwa Learning System Inc. p. 110.

Mendoza, E.E.2003. Phoenix Science Series Chemistry. Quezon City: Phoenix Publishing House, Inc. p. 163, 244, 299, 305.

Meyertholen E. 2014. Diffusion II: Information retrieval. < http://austincc.edu/emeyerth/ diffuse2.htm> Accessed October 13, 2014.

Tro, N. J.2008. A Molecular Approach.Oregon, USA: GEX Publishing Services. p. 229.