Lab Report Gas Diffusion

Topics: Molecular diffusion, Diffusion, Transport phenomena Pages: 11 (3047 words) Published: March 1, 2013
S5E1 Gaseous Diffusion


In this experiment, our objective is to find out the gas diffusion coefficient, D of acetone in the air. This experiment is conducted at a temperature of 50ºC and atmospheric pressure. The method that is applied to conduct this experiment is called the Winkleman method where the level of acetone (Z) is determined every 15 minutes by using a microscope. With the level of acetone being determined, a graph of t/L+Lo (min /mm) vs. L-Lo (mm) is plotted and the gradient s of the graph is being calculated. With the gradient of the graph s, we calculated the diffusion coefficient, D of the experiment by applying Fick’s Law with mathematical derivation. From the experiment, the diffusion coefficient, D is 0.83 x 10-5 m2/s. Several errors are made in this experiment which causes the value of diffusion coefficient to deviate from the handbook value which will be discussed later.

Keywords: Acetone, Diffusion Coefficient, Fick’s Law, Temperature, Pressure


The transport of one constituent from region of higher concentration to that of a lower concentration is called mass transfer. A lump of sugar added to a cup of black coffee eventually dissolves and then diffuses uniformly throughout the coffee. Perfume presents a pleasant fragrance which is imparted throughout the surrounding atmosphere. These are examples of mass transfer. Mass transfer plays a very important role in many industrial processes: the removal of pollutants from plant discharge streams by absorption, the stripping of gases from wastewater, neutron diffusion within nuclear reactors, the diffusion of adsorbed substances within the pores of activated carbon, the rate of catalyzed chemical and biological reactions, and air conditioning are typical examples. . Mass transfer takes place in either gas phase or liquid phase or in both cases simultaneously. When a liquid evaporates into a still gas, vapour is transferred from the surface to the bulk of gas as a result of the concentration gradient. This process continues until the gas is saturated and the concentration gradient is reduced to zero. In a still fluid or in a fluid flowing under streamline conditions in a direction of right angles to the concentration gradient, the transfer is affected by random motion of the molecules. Molecular diffusion or molecular transport can be defined as the transfer or movement of individual molecules through a fluid by means of the random, individual movements of the molecules. Whenever a particular molecule of this mixture diffuses, it must diffuse through other molecules; consequently, in almost every practical example there are at least two components present and possibly more. The molecular diffusion process is shown schematically in the below figure. A random path that molecule A might take in diffusing through B molecules from point (1) to (2) is shown. If there are a greater number of A molecules near point (1) than at (2), then, since molecules diffuse randomly in direction, more A molecules will diffuse from (1) to (2) than from (2) to (1). The net diffusion of A is from high-to-low concentration regions.


Molecule A
Molecule B

Diffusion is explained in this experiment through the First Fick’s Law. The first Fick’s Law states that the molar diffusional flux of A in B at a certain direction (say Z), is proportional to the negative of the concertration gradient of A in that direction: [pic]

Molar diffusional flux is defined as the molar diffusion flow rate (nAB) per cross sectional area unit of diffusion (L): [pic]
First Fick’s law is turned to an equation by introducing a coefficient named diffusion coefficient of A in B or diffusivity of A in B, DAB: [pic] (1)
where [pic] is the molar flux of A in the z direction,[pic] is the diffusion coefficient of A in B. Its dimension is L2T-1 and thus the unit is m2/s,...
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