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Safety: Save waste for special disposal

  • I5-41: ENDOTHERMIC REACTION - ENTROPY

    I5-41
    Aid a discussion of entropy.
    31.5 grams of barium hydroxide and 15.2 grams of ammonium thiocyanate, both solid powders initially at room temperature, are mixed. As they are stirred using the digital thermometer probe a chemical reaction occurs, producing water with barium cyanate and ammonium, both of which are soluble in the water. The solution becomes very cold, and can freeze a wettened wooden block to the bottom of the beaker.

    This experiment can be described in terms of entropy using two approaches: (1) As the mixture cools, it must become a liquid, increasing its disorder so that entropy will increase, or (2) The tendency toward disorder drives the reaction, creating the liquid reaction product.

    Note: PLEASE CALL THE LECTURE DEMONSTRATION GROUP AT 405-5995 NOT LATER THAN THE MORNING BEFORE YOUR REQUEST SO THAT WE HAVE THE TIME TO PREPARE THE NEEDED CHEMICALS.

  • I6-24: DIFFUSION VELOCITY

    I6-24
    Show that the diffusion velocity is proportional to the RMS molecular velocity.
    Cotton swabs dipped into HCl and NH4OH respectively are inserted into the ends of the tube. HCl gas (molecular weight 36.5) and NH3 or ammonia gas (molecular weight 17) begin to diffuse inward. In less than 25 minutes they meet, forming a ring of ammonium chloride (NH4Cl). The diffusion velocity is proportional inversely to the square root of the molecular mass, because according to equipartition of energy the average molecular speed is inversely proportional to the square root of the molecular mass. This means that the distances traveled by each vapor before they meet are in the proportion:

    d(NH3):d(HCl)=SQRT(36.5/17)=3:2 approximately.

    Use caution when handling chemicals. Always wear protective gloves. Please do not throw used swabs in garbage; place in included water beaker instead for proper disposal.

  • M7-31 TYNDALL'S EXPERIMENT - COLLOIDAL SUNSET

    M7-31
    Colloidal sunset demonstration
    The collimated white light from the bright point source passes through the empty tank and hits a nearby screen. Chemicals previously prepared are then mixed in the tank: 2.5 ml sodium thiosulfate solution in 100 ml water, and 2.5 ml concentrated HCl 1:4 dilution in 650 ml water. When the chemicals mix they begin to form a suspension of sulfur particles which act as scattering centers for the light, especially blue light at first. This leaves the light on the screen with a yellowish tint. As time passes the sulfur particles grow larger and scatter more light and light of a longer wavelength, changing the light on the screen to a bright red. Ultimately most light is scattered, leaving no light on the screen.
    M7, LS1, OM1