Purpose
To illustrate an endothermic reaction involving reactants in the solid state.
Materials
- Barium hydroxide 8-hydrate, Ba(OH)2·8H2O
- Ammonium thiocyanate, NH4SCN
- 500 mL Erlenmeyer flask with rubber stopper
- Wood board
- Electronic balance
- Read the MSDS/WHMIS sheets for all chemicals before using them
- Ammonia gas is produced, avoid inhalation (You may wish to do this demonstration in a fume hood)
- Wear chemical safety glasses, gloves, and lab coat
- Firmly hold the rubber stopper to prevent it from popping off
Procedure
- Pour a small amount of water on a square wooden board. (Enough to ensure good contact with the base of the Erlenmeyer flask).
- Place 50 g of barium hydroxide octahydrate and 25 g ammonium thiocyanate into the Erlenmeyer flask. Place a rubber stopper in the flask. Swirl to mix.
- Place the flask in the puddle of water on the board.
- Lift the flask after a few minutes.
Results
The reaction vessel gets cold enough to freeze the water between the board and the flask.
Follow-up Teaching Notes
- Energy absorbed by the reaction freezes the water so that the board will be attached to the flask.
- Reaction is:
Ba(OH)2 · 8H2O(s) + 2 NH4SCN(s) à Ba(SCN)2(aq) + 2 NH3 (g) + 10 H2O(l) - Ammonium nitrate can be a substitute for ammonium thiocyanate.
Connections
Enthalpy and heat of reaction, reaction between two solids.
Extensions
- Demonstration of an endothermic reaction: dissolve ammonium chloride in water.
- Demonstration of an exothermic reaction: dissolve calcium chloride in water.
Disposal/Clean-up
- Open the flask in a fume hood and allow the ammonia to evaporate overnight.
- Remaining contents can be placed in the science department’s heavy metal waste container for proper disposal.
