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Wednesday, 28 November 2018

Diffusion





Example 1. If we place a crystal potassium permanganate in a beaker of water and leave it to stand, the purple colour of the permanganate starts to spread outwards from the crystal. Eventually, the colour spreads evenly through out the water medium so that the water has become the same shade of purple.

Example 2 if we place a few drops of liquid bromine in a gas jar, cover  it and leave it to stand, the bromine evaporates to form brown bromine vapour which starts to spread upwards. Eventually, the brown color spreads evenly throughout the gas jar.

    In exams 1, at the start, the potassium and permanganate ions ate concentrated in the crystal , while the surrounding water does not have any of these ions. Slowly, the potassium and permanganate ions break away from the crystal structure and move into the spaces between the water molecules, I.e.  the crystal dissolves. The potassium and permanganate ions (solute) keep moving from where they are crowded to where they are less crowded. In the process, they collide with one another and the water molecules and intermingle until they are evenly distributed in the space between water molecules (solvent). This final change is indicated by the unchanging purple share of the final solution.

Potassium permanganate + water (solvent) ---> potassium permanganate solution (solution)

      In example 2, the bromine molecules move in the spaces between the molecules in the gas jar. They move away from the button of the jar where they are crowded together, and spread out until they are evenly distributed throughout the species between the air molecules in the jar.

    The thermal process by which the potassium and permanganate ions and the bromines molecules is known as diffusion. By diffusion, ions or molecules of a substance move from a region where they are concentrated to a legion where they are less concentrated. The difference in the concentration  of the substance in the two region before diffusion is known ad the concentration gradient or diffusion gradient.

   Note :   Diffusion is actually a thermal motion of molecules and is affected by temperature change and not concentration.

     Diffusion is rapid and concentration is large. It slows down the difference in the concentration becomes less. When the molecules (solute)  are evenly distributed in the medium (solvent), we say a dynamic equilibrium has been reached. At this stage, although all the molecules continue to move about (I.e dynamic), there is no net diffusion of the solute molecules in any direction. Therefore, the concentration of the solution remains unchanged.

     Diffusion takes place quite rapidly in gases because the molecules of a gas can move freely. Liquids, on the other hand, diffuse slowly, while solids, with very restricted molecular movement, diffuse extremely slowly.

      Let us see what happen when equal volumes of a very weak (dilute) sugar solution and a strong (concentrated) sugar solution are separated by a permeable membrane.

    Note : A permeable membrane allows l molecules to pass through it.

    A concentration gradient exists between

• the sugar molecules, and 

• the water molecules

In the weak and strong sugar solutions. Sugar molecules will diffuse from the strong sugar solution through the permeable membrane to the weak one. At the same time, water molecules will defuse from the weak sugar Solution (which has a high concentration of water molecules), to the strong one ( where concentration of water molecules is low). This process with continue until the strength or concentration of the sugar solution is the same on both sides of the membrane.

Here,  we see that in a given system, it is possible for the diffusion of different types of molecules or ions to be in different directions at the same time. Each type of molecule or ion moves at its own rate, according to its concentration gradient.

     A sugar Solution is made by dissolving sugar in water

Sugar (solute) + water (solvent ) ---> sugar Solution (solution)

To make a weak sugar  solution, add a spoon of sugar to a beaker of water; to make a strong one, add sugar to a beaker of water until no more will dissolve.

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