PURPOSE: To aid a discussion of entropy.

DESCRIPTION: 31.5 grams of barium hydroxide and 15.2 grams of ammonium thyocyanate, 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.


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

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

SETUP TIME: 5 min.

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