PURPOSE: To aid a discussion of entropy.

DESCRIPTION: 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.

SUGGESTIONS: 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.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for further information on this reaction.

EQUIPMENT: Prepared bottles of barium hydroxide andd ammonium thiocyanate, beaker, digital thermometer, and wet wooden block.

SETUP TIME: 2 hours (NOTE: This demonstration requires the preparation and packaging of chemicals before use.)