An Endothermic Reaction

Purpose:

To demonstrate an endothermic reaction by combining two solids.

Materials:

• Wooden board
• Wooden splint
• 250mL beaker

Reagents:

• Ammonium thiocyanate (NH₄SCN)
• Barium hydroxide octahydrate (Ba(OH)₂⋅8H₂O)

Hazards and PPE:

• Ammonium thiocyanate is an irritant on contact or ingestion. Handle with care."
• Barium hydroxide octahydrate is an acute toxin. Handle with care.
• Wear approved safety goggles and gloves.

Protocol:

Preparation

1. Weigh out 20g BaOH and 10g NH₄SCN (separately).
2. Place the BaOH crystals in 250mL beaker.

Demonstration

1. Add the NH₄SCN to the beaker.
2. Stir the two solids together with a wooden splint.
3. Place a small pool of water on a piece of wood. Place the beaker on the pool of water.
   1. After 2-3 minutes, the beaker will freeze to the board.
4. Lift the beaker to show the freezing has occurred.

Additional Notes:

• Ammonium chloride (7g) or ammonium nitrate (10g) may be used in place of ammonium thiocyanate.

Disposal:

• Excess solutions/solids should be placed in a properly labeled waste container.

Reactions:

• Ba(OH)₂⋅8H₂O_(aq) + 2NH₄SCN_(aq) → Ba(SCN)_2(aq) + 2NH_3(aq) + 10H₂O_(l)

Citations:

1. Summerlin, L., Ealy, J.; Chemical Demonstrations: A Sourcebook for Teachers, 1985.