The Effect of Molecular Weight and Time on the Diffusion Rate of Potassium Permanganate, Potassium Dichromate, and Methylene Blue.1
Arantxa Alex Carpio
Group 1 Sec. X – 4L
March 24, 2015
The effect of molecular weight and time on the rate of diffusion was determined using the agar-water gel test. A petri dish of agar-water gel with three wells was prepared and a prepared solution of each substance was dropped on each well; one with potassium permanganate (KMnO4), the other with potassium dichromate (K2Cr2O7), and the last one with methylene blue. The average diameter of potassium permanganate (11.64 mm) was bigger than potassium dichromate (9.18 mm) and methylene blue (7.91 mm). Note that the molecular weight of potassium permanganate is 158g/mole, potassium dichromate is 294 g/mole and methylene blue is 374 g/mole. Thus, the lighter the molecular weight, the faster the rate of diffusion.
Diffusion is the process in which the molecules merge as an outcome of their spontaneous movement or randomized motion of kinetic energy. It is when two chemical species react together at every encounter (combining or merging of the two species for a number of subsequent collisions in solution) and chemical change take place as fast as the reactants can diffuse together (Reichardt, 1988). It may occur in solids, liquids, or gasses. It depends on many factors like temperature, molecular weight, concentration of molecules in the surrounding gas and the size of the space in which the gas diffuses (Abrash and Hardcastle, 1981). According to Crank and Park (1968), another factor that affects the diffusion in gas is the nature of the gas.
Diffusion offers the clearest evidence for molecular migration in a liquid because it shows the freedom of molecules to move in liquid. Liquid diffusion is slower than gas diffusion because gas molecules are far apart and collisions between them are rare, causing them to diffuse rapidly. Liquid molecules, on the other hand, are always in contact with other molecules and have frequent collisions between them, making the process of diffusion slower (Abrash and Hardcastle, 1981).
It can be derived from the glass-tube experiment that heavier molecules diffuse more slowly than lighter molecules. The glass-tube experiment was done by placing a cotton ball dipped in hydrochloric acid on one end of the glass tube, and placing another cotton ball dipped in ammonium hydroxide on the other end of the glass tube. This was done simultaneously. After a few minutes, it can be observed that a white smoke was formed inside the glass tube. The white smoke was closer to the cotton ball dipped in hydrochloric acid than to the cotton call dipped in ammonium hydroxide. Note that hydrochloric acid has a molecular weight of 36 g/mole and ammonia has a molecular weight of 17 g/mole.
This study aimed to determine the effect of molecular weight and time on the rate of diffusion through the agar-water gel test. The objectives were to describe the effect of molecular weight and time on the rate of diffusion on potassium permanganate, potassium dichromate, and methylene blue; and to explain the relationship of molecular weight and time on the rate of diffusion.
MATERIALS AND METHODS
In finding the effect of molecular weight and time to the diffusion rate, two tests were used. The glass-tube test and the agar-water gel test. In the glass tube test, a two feet glass tube was fastened horizontally to a ring stand. With the use of forceps, two cotton balls were dipped; one cotton ball on hydrochloric acid and the other cotton ball on ammonium hydroxide. One end of the tube was plugged with the wet cotton ball dipped in hydrochloric acid, and the other end of the tube was plugged with the wet cotton ball dipped in ammonium hydroxide, as seen on Figure 1. This was done simultaneously. After a few minutes, the distance of the white...