Gas Laws: Avogadro’s Law

Teacher Notes

Alignment:

• Performance Expectation: HS-PS3-2 (Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles and energy associated with the relative position of particles)
• Science and Engineering Practice: Developing and Using Models
• Disciplinary Core Idea: PS3.A: Definitions of Energy
• Crosscutting Concept: Energy and Matter

Task Overview: Students will use an interactive simulation of Avogadro’s Law to explore how increasing the amount of gas (in moles) increases the volume at a constant temperature and pressure. They will graph their data and explain their findings. The anchoring phenomenon is the expansion of a balloon as more air is blown into it.

Success Criteria (Evidence Statements):

• Students develop a model identifying relevant components (gas particles, volume, amount of gas).
• Students describe relationships between components (more particles lead to greater volume to maintain constant pressure at a given temperature, demonstrating the relationship between macroscopic volume and atomic-level particle counts).
• Students use the model to illustrate that energy at the macroscopic scale is linked to the kinetic energy of freely moving particles.

Part 1: Engage (Anchoring Phenomenon)

Imagine blowing up a balloon. As you blow more air into the balloon, it expands and gets larger.

1. What causes the balloon to expand when you blow more air into it?
2. If you were to measure the pressure and temperature inside the balloon, would they change significantly as you add more air, assuming the room conditions stay the same and the balloon is very elastic?
3. Generate one “need to know” question about the relationship between the amount of gas and the volume it occupies.

Part 2: Explore (Simulation Investigation)

Open the Gas Laws: Avogadro’s Law Simulation. This simulation allows you to change the amount of gas (in moles, $n$) while keeping the temperature and pressure constant.

Instructions:

1. Locate the Amount of Gas (n) slider.
2. Note the initial values:
   • Moles ($n$) = 1.0 mol
   • Volume ($V$) = 22.4 L
3. Gradually increase the amount of gas using the slider.
4. Press the Record Data Point button at 5 different values of moles (e.g., 1.0, 2.0, 3.0, 4.0, 5.0 mol).
5. Observe the Volume vs Moles chart that is generated.

Data Table: | Amount of Gas, $n$ (mol) | Volume, $V$ (L) | Ratio $V/n$ (L/mol) | | :—: | :—: | :—: | | 1.0 | 22.4 | | | 2.0 | | | | 3.0 | | | | 4.0 | | | | 5.0 | | |

Calculate the ratio $V/n$ for each row and record it in the table.

Part 3: Explain (Sensemaking)

1. Describe the relationship between the amount of gas ($n$) and the volume ($V$). Is it a direct or inverse relationship?
2. Look at the $V/n$ ratio you calculated. What do you notice about this value?
3. In terms of gas particles (molecules/atoms), explain why the volume must increase when you add more gas to maintain a constant pressure. What happens to the rate of collisions if the volume didn’t increase?

Part 4: Elaborate/Evaluate (Argumentation & Modeling)

Synthesis Task: Develop a model (using a diagram with captions, or a written explanation referencing the simulation) to illustrate Avogadro’s Law. Your model must:

• Identify the components (gas particles, container volume).
• Explain how the macroscopic volume depends on the microscopic number of particles.
• State how the kinetic energy (related to temperature) and collision frequency (related to pressure) are maintained.
• Use your model to predict the volume if you had 10.0 moles of gas under the same conditions.