When a molten metal is mixed with another substance, there are two mechanisms that can cause an alloy to form: (1) atom exchange or (2) interstitial mechanism. The relative size of each element in the mix plays a primary role in determining which mechanism will occur.

What conditions favor formation of substitutional alloys? Substitutional alloys are formed when the two metallic components have similar atomic radii and chemical bonding characteristics. For example silver and gold.

Substitutional alloys are formed when the metallic components have similar atomic radii and chemical-bonding characteristics. From: Electrons, Atoms, and Molecules in Inorganic Chemistry, 2017.

Substitutional diffusion occurs by the movement of atoms from one atomic site to another. In a perfect lattice, this would require the atoms to “swap places” within the lattice.

A very common type of diffusion mechanism, if not the most dominant, is the vacancy mechanism. Basically in thermal equilibrium, a certain concentration of vacant sites exists in materials. Any atoms adjacent to a vacancy may diffuse by jumping into the vacancy, resulting in an exchange of positions between them.

Several factors affect the rate of diffusion of a solute including the mass of the solute, the temperature of the environment, the solvent density, and the distance traveled.

Diffusion occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration. Diffusion happens when the particles are free to move. This is true in gases and for particles dissolved in solutions – but diffusion does not occur in solids.

The greater the difference in concentration, the quicker the rate of diffusion. The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly. The greater the surface area, the faster the rate of diffusion.

Self-diffusion may be monitored by using radioactive isotopes of the metal being studied. The motion of these isotopic atoms may be monitored by measurement of radioactivity level.

The diffusion coefficient is a physical constant dependent on molecule size and other properties of the diffusing substance as well as on temperature and pressure. Diffusion coefficients of one substance into the other are commonly determined experimentally and presented in reference tables.

Chemical Partitioning and Transport in the Environment The molecular diffusion coefficient is caused by the random thermal motion of molecules in a gas or liquid and depends on the temperature and pressure, molecular properties, such as mass and volume, and the forces between molecules.

Diffusion coefficient, also called Diffusivity, is an important parameter indicative of the diffusion mobility. Diffusion coefficient is generally prescribed for a given pair of species. For a multi-component system, it is prescribed for each pair of species in the system.

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In general, the diffusion coefficient is inversely proportional to pressure. This is also an observed fact: gas production rates from coal seams tend to increase as the reservoir pressure goes down.

It states that ‘the rate of diffusion is proportional to both the surface area and concentration difference and is inversely proportional to the thickness of the membrane’. Fick’s law can be written as: Rate of diffusion ∝ surface area × concentration difference thickness of membrane.

Applications. Equations based on Fick’s law have been commonly used to model transport processes in foods, neurons, biopolymers, pharmaceuticals, porous soils, population dynamics, nuclear materials, plasma physics, and semiconductor doping processes.

Fick’s Law states that, for the diffusion of a gas to be efficient, three conditions should be met. Firstly, the surface area of the diffusion pathway should be large. Secondly, there should be large difference in concentration between the area the gas is diffusion to, and the area it is diffusing from.

Pulmonary edema occurs when the alveoli fill up with excess fluid seeped out of the blood vessels in the lung instead of air. This can cause problems with the exchange of gas (oxygen and carbon dioxide), resulting in breathing difficulty and poor oxygenation of blood.

The alveoli are where the lungs and the blood exchange oxygen and carbon dioxide during the process of breathing in and breathing out. Oxygen breathed in from the air passes through the alveoli and into the blood and travels to the tissues throughout the body.

Gas exchange takes place in the millions of alveoli in the lungs and the capillaries that envelop them. As shown below, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli.

The primary organs of the respiratory system are the lungs, which function to take in oxygen and expel carbon dioxide as we breathe. The gas exchange process is performed by the lungs and respiratory system. Air, a mix of oxygen and other gases, is inhaled. In the throat, the trachea, or windpipe, filters the air.

Terms in this set (5)

• Pulmonary Ventilation. Movement of air in and out of the lungs passage (Thorax and Diaphragm).
• External Respiration. Exchange of gases between air and blood at pulmonary capillaries (Alveoli).
• Transport of gases through blood vessels.
• Internal Respiration.
• Cellular Respiration.

Gas exchange between tissues and the blood is an essential function of the circulatory system. In humans, other mammals, and birds, blood absorbs oxygen and releases carbon dioxide in the lungs. Thus the circulatory and respiratory system, whose function is to obtain oxygen and discharge carbon dioxide, work in tandem.