Endothermic Reaction Demonstration

Barium hydroxide and ammonium thiocyanate cold reaction

Thermochemistry Intermediate (High School) 20 min ~$12.00

Objective

Demonstrate a dramatically endothermic reaction where the mixture becomes cold enough to freeze water on the outside of the flask.

Background

When barium hydroxide octahydrate reacts with ammonium thiocyanate, the temperature drops to approximately -20°C. This endothermic reaction absorbs so much heat from the surroundings that water on the outside of the flask freezes, effectively gluing the flask to a wet board.

Safety Warnings

  • Barium compounds are toxic if ingested
  • Ammonia gas is released — work in fume hood or ventilated area
  • Wear gloves when handling barium hydroxide
  • Do not inhale the ammonia fumes

Required PPE

goggles gloves lab_coat

Materials

  • Barium hydroxide octahydrate (32 g)
    Toxic
  • Ammonium thiocyanate (16 g)
  • Water (5 mL)
    On the board

Equipment

250 mL Erlenmeyer flask Wooden board Thermometer Stirring rod

Procedure

1

Pour a few drops of water onto the wooden board to create a wet spot.

1 min
2

Add 32 g barium hydroxide octahydrate to the flask.

2 min Wear gloves — barium is toxic
3

Add 16 g ammonium thiocyanate to the flask. Stir the mixture with a glass rod.

2 min Ammonia released — ventilation
4

Place the flask on the wet spot on the board. Insert the thermometer.

1 min
5

Observe the temperature dropping dramatically. Record the minimum temperature.

5 min Do not inhale fumes
6

After 5 minutes, try to lift the flask. It should be frozen to the board.

2 min
7

Explain that the reaction absorbed heat from the surroundings, freezing the water beneath.

2 min

Expected Results

The temperature should drop to approximately -20°C within minutes. The flask should freeze to the wet board. A strong smell of ammonia confirms the reaction is proceeding.

Cleanup

Allow to warm to room temperature. Dispose of barium waste in heavy metal waste container. Rinse flask and ventilate area thoroughly.

Details

Category
Thermochemistry
Difficulty
Intermediate (High School)
Duration
20 min
Est. Cost
$12.00
Steps
7
Materials
3

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Frequently Asked Questions

What is the objective of Endothermic Reaction Demonstration?
Demonstrate a dramatically endothermic reaction where the mixture becomes cold enough to freeze water on the outside of the flask.
How difficult is Endothermic Reaction Demonstration?
This experiment is rated as Intermediate (High School). It takes approximately 20 minutes to complete.
What safety precautions are needed for Endothermic Reaction Demonstration?
Key safety precautions include: Barium compounds are toxic if ingested; Ammonia gas is released — work in fume hood or ventilated area; Wear gloves when handling barium hydroxide.
What materials are needed for Endothermic Reaction Demonstration?
The main materials required are: Barium hydroxide octahydrate, Ammonium thiocyanate, Water.
What results should I expect from Endothermic Reaction Demonstration?
The temperature should drop to approximately -20°C within minutes. The flask should freeze to the wet board. A strong smell of ammonia confirms the reaction is proceeding.