Diffusion in action | The reaction of ammonia and hydrogen chloride demonstration

Summary

This script revisits the classic diffusion demonstration involving ammonia and hydrogen chloride vapors, resulting in the formation of ammonium chloride. It emphasizes the importance of using fresh reagents and suggests an improved technique with a cotton wool bung to simultaneously insert chemicals and seal the tube. Safety precautions are highlighted, and the script clarifies that Graham's Law does not apply due to air molecules in the tube, which creates beautiful convection currents. The experiment concludes with a method to neutralize remaining chemicals, advocating for revisiting even simple demos for updated practices.

Takeaways

• 🔬 The script discusses the importance of revisiting even simple chemistry demonstrations like the diffusion of ammonia and hydrogen chloride.
• 📚 It highlights updates and common misconceptions about the applicability of Graham's Law in the diffusion demonstration.
• 💭 The basic concept involves vapors of ammonia and hydrogen chloride diffusing through a tube to form a white ring of ammonium chloride where they meet.
• 📏 The script suggests using a tube of moderate length to keep the demonstration quick and engaging, under a minute.
• 🔨 A technique from CLEAPSS involves using an adapted bung with a cotton bud to insert chemicals and seal the tube simultaneously.
• 🥽 Safety precautions are emphasized, including working in a fume hood, wearing goggles, gloves, and using fresh reagents.
• 🧪 Fresh ammonia and hydrochloric acid should be used directly from the reagent bottle for optimal results.
• 📦 Prepare the liquids in sample vials placed in small beakers to prevent spillages and catch drips.
• 👥 The demonstration should be conducted with care, keeping the chemicals at least one meter apart and using a clamp and stand for the tube.
• ⏱ After a minute, a white circle of ammonium chloride forms closer to the hydrogen chloride source, indicating diffusion but not influenced by Graham's Law.
• 🌀 The script notes the formation of convection currents due to the density differences between NH3 vapors and HCl vapors, and the product ammonium chloride.
• 🧼 Post-experiment, the bungs should be safely disposed of in water to dilute and neutralize remaining chemicals.
• 🔄 The importance of revisiting classic demos for updated health and safety guidance is stressed, along with the reminder to use the freshest reagents possible.

Q & A

What is the purpose of revisiting the diffusion demo?

The purpose of revisiting the diffusion demo is to ensure that the demonstration is conducted safely and effectively, incorporating any updates or improvements, and to remind educators of the importance of using fresh reagents for optimal results.

What are the two vapors used in the diffusion demonstration?

The two vapors used in the diffusion demonstration are ammonia and hydrogen chloride.

Why is it suggested to use a shorter tube for the demonstration?

A shorter tube is suggested because it allows the result to appear in under a minute, helping to maintain the class's attention during the demonstration.

What is an 'adapted bung' and how is it used in the demo?

An 'adapted bung' is a modified stopper with a hole drilled in it, into which half a cotton bud is inserted and wrapped with extra cotton wool secured by an elastic band. It is used to simultaneously insert the chemicals and seal the tube in one step.

What safety precautions are recommended when conducting the diffusion demo?

Safety precautions include working in a fume cupboard, wearing splash proof goggles, proper gloves, and using fresh ammonia and hydrochloric acid from reagent bottles.

Why is it important to use fresh reagents for the demonstration?

Using fresh reagents is important because they are more reactive and ensure the demonstration works at its best, producing clear and immediate results.

How should the sample vials be prepared and stored before the demonstration?

The sample vials should be filled with up to 4 cm3 of liquids and placed in small beakers to prevent spillages and catch any drips. They should be kept at least one meter apart before and during the demonstration.

What happens when the bungs are placed into each end of the tube at the same time?

When the bungs are placed into each end of the tube simultaneously, the ammonia and hydrogen chloride vapors are allowed to diffuse through the tube, meeting and forming a white ring of ammonium chloride where they interact.

Why does the white circle of ammonium chloride form closer to the hydrogen chloride source?

The white circle forms closer to the hydrogen chloride source because, despite the presence of air molecules making Graham's Law inapplicable, the heavier particles of hydrogen chloride tend to settle closer to their source.

What phenomenon occurs when ammonia and hydrogen chloride are mixed, and how does it relate to the demonstration?

When ammonia and hydrogen chloride are mixed, convection currents are established. This phenomenon is related to the demonstration as it shows the difference in density between the NH3 vapors and the HCl vapors, with the latter being more dense and contributing to the formation of ammonium chloride rings.

How should the remaining chemicals be handled after the experiment is completed?

The remaining chemicals should be neutralized by dumping the bungs into a beaker of water before washing down the sink, ensuring safe disposal of the chemicals used in the demonstration.

What is the significance of revisiting 'classic' or 'easy' demos and updating health and safety guidance?

Revisiting 'classic' or 'easy' demos and updating health and safety guidance is significant because it ensures that educators are aware of the latest best practices, safety measures, and techniques to conduct experiments effectively and safely.