SimuLab 12: Intermolecular Forces

                        

Your objective is to: Recognize the forces that acts between particles of gas and relate these forces to the macroscopic behavior of the substance. You will be able to: Explore the microscopic interactions of two noble gas atoms. Study how the forces acting between atoms of noble gases depend on the distance between the two atoms. Explore the relation between the internal potential energy and the interatomic forces. Test if the law of conservation of energy is satisfied. Relate temperature and strength of intermolecular forces to the phase changes.

Make two copies of the following table. Do not fill data in the column Distance in the second copy.

No. of	Distance	Particle's	Kinetic	Potential	Total	Final
Experiment		Behavior	Energy	Energy	Energy	State
1	3.5
2	2.0
3	0.9
4	1.2

Using Fig. 3.1 make your predictions, in the following experiments, concerning two particles, whose centers are separated by:

1) more than 3 particle diameters
2) approximately 2 particle diameters
3) less than 1 particle diameter
4) 1.2 particle diameters

 Fill your predictions in the first copy of the table using the following possible choices:



Particle's Behavior: Stay in place (A), Attract (B) or Repulse (C)
Kinetic Energy: Stays constant (A), Increases (B) or Decreases (C)



Potential Energy : Stays constant (A), Decreases (B) or Increases (C)



Total Energy : Stays constant (A), Decreases (B) or Increases (C)



Final State : Particles stay apart (A), Oscilate near equlibrium position (B), Move freely in the entire box (C) or Stay close to each other in fixed position (D)

  Now you will test your predictions by performing the SimuLab and recording your observations in the second copy of the table.

1. Open SMD, select file Intermolecular in the Energy folder.

You see a single particle in the center of the particle screen.

2. Press Start

The particle is completely motionless. Watch the graph of the energies at the right part of the simulation window. It shows kinetic, potential, and total energy of the particle as functions of time by the red, blue, and black lines, respectively. If the red or blue lines are invisible, it means that they exactly coincide with the black line.

3. Press Pause.

The potential energy of a single particle is defined to be zero, because our particle does not interact with any other particles or its surroundings.

                        

Q3.2: What is the kinetic energy of the particle? Explain your answer.

                        

Q3.3: What is the total energy of the system?

4. Reset Experiment from the File menu.

You will be placing another particle at various distances from the existing atom and study their interaction.

5. Select Particles from the Edit menu. The dialog box Edit Particles appears. Select Add Particle G and click mouse inside the particle box near the edge.

A new green particle will appear.

6. Press Start, wait approximately 5 time units and press Pause. Using the graph, record your observations in the table and compare them with your prediction.

Make your observation of the particle behavior and the behavior of the kinetic, potential, and total energies.

7. Reset Experiment from the File menu. Select Particles from the Edit menu. The dialog box Edit Particles appears. Select Add Particle G and place the center of the second particle at the distance of approximately 2 particle diameters from the center of the first one. Repeat Step 6.

8. Reset Experiment from the File menu. Select Particles from the Edit menu. The dialog box Edit Particles appears. Select Add Particle G and place the center of the second particle at the distance of approximately 0.9 particle diameters from the center of the first one. Repeat Step 6.

The edges of the particles should touch each other. Our computer model does not allow particles to be less then 0.8 particle diameters from each other. So try to click the mouse several times, slowly moving the cursor away from the center of the existing particle until you succeed in placing the new particle at the desired distance.

9. Reset Experiment from the File menu. Select Particles from the Edit menu. The dialog box Edit Particles appears. Select Add Particle G and place the center of the second particle at the distance of approximately 1.2 particle diameters from the center of the first one.Repeat Step 6.

There should be a small gap between the edges of the particles.

                        

Q3.4: Explain the observed changes in potential and kinetic energies.

                        

Q3.5: Relate the behavior of the particles to the behavior of the particles in a crystal.

10. Press Pause. Using Temperature scrollbar, increase the value of Temperature to 0.4. Press Start.

You will study the effect of temperature on the particle behavior. The temperature scroll bar indicates the average kinetic energy of the particles at given time. By increasing this value, you are increasing the kinetic energies of both particles by the same amount. The graph of energies indicates the average kinetic energy, average potential energy, and average total energy of each particle. You can precisely determine the values by pressing the mouse at a given position on the graph.

                        

Q3.6: This case corresponds to a crystal melting. Describe the particle behavior in terms of potential energy graph. Clue: See Fig. 3.2.

11. Press Pause. Using the Temperature scrollbar, increase the value of Temperature to 0.5. Press Start.

Predict the particle behavior. Watch the behavior of the particles and determine if your prediction is accurate.

                        

Q3.7: This case corresponds to evaporation. Describe the particle behavior in terms of potential energy graph. Clue: See Fig. 3.2.