An endothermic process or reaction absorbs energy in the form of heat (endergonic processes or reactions absorb energy, not necessarily as heat). Examples of endothermic processes include the melting of ice and the depressurization of a pressurized can. In both processes, heat is absorbed from the environment. You could record the temperature change using a thermometer or by feeling the reaction with your hand. The reaction between citric acid and baking soda is a highly safe example of an endothermic reaction, commonly used as a chemistry demonstration. Do you want a colder reaction? Solid barium hydroxide reacted with solid ammonium thiocyanate produces barium thiocyanate, ammonia gas, and liquid water. This reaction gets down to -20°C or -30°C, which is more than cold enough to freeze water. It's also cold enough to give you frostbite, so be careful! The reaction proceeds according to the following equation:
Ba(OH)2.8H2O (s) + 2 NH4SCN (s) --> Ba(SCN)2 (s) + 10 H2O (l) + 2 NH3 (g)
Here's what you need to use this reaction as a demonstration:
- 32g barium hydroxide octahydrate
- 17g ammonium thiocyanate (or could use ammonium nitrate or ammonium chloride)
- 125-ml flask
- stirring rod
- Pour the barium hydroxide and ammonium thiocyanate into the flask.
- Stir the mixture.
- The odor of ammonia should become evident within about 30 seconds. If you hold a piece of dampened litmus paper over the reaction you can watch a color change showing that the gas produced by the reaction is basic.
- Liquid will be produced, which will freeze into a slush as the reaction proceeds.
- If you set the flask on a damp block of wood or piece of cardboard while performing the reaction you can freeze the bottom of the flask to the wood or paper. You can touch the outside of the flask, but don't hold it in your hand while performing the reaction.
- After the demonstration is completed, the contents of the flask can be washed down the drain with water. Do not drink the contents of the flask. Avoid skin contact. If you get any solution on your skin, rinse it off with water.