Purpose
To demonstrate that heavier molecules move more slowly at a given temperature by comparing the diffusion rates of hydrochloric acid (HCl) and ammonia (NH₃). Refer to Shakashiri 5.6 for more information.
Materials
- Apparatus for measuring the speed of smell
- Ammonia (e.g., Windex)
- Hydrochloric acid
- Black background for visibility
- Cotton balls (wrung out)
- Indicator paper (optional)
Procedure
- Equipment Introduction:
- Introduce the apparatus as a device to measure the speed of smell.
- Explain the experiment’s purpose, emphasizing that Graham’s law of effusion is not applicable here.
- Chemical Smells:
- Introduce ammonia as the smell of Windex and hydrochloric acid as resembling vomit (relatable for students in dorms).
- Observation:
- Wait for the ammonium chloride ring to form in the apparatus.
- Ensure a black background is used for optimal visibility of the ring.
- Improving Visibility:
- While indicator paper can enhance visibility, it may interfere with the diffusion ratio. Use wrung-out cotton balls instead to avoid liquid running along the edges.
Conclusion
This experiment effectively illustrates the concept of molecular weight and diffusion rates, demonstrating that heavier molecules, such as HCl, diffuse more slowly than lighter molecules like NH₃.