Atomic Interactions & Chemical Energy

1. Atomic Interactions & Chemical Energy

2. A note on usage: The clicker slides in this booklet are meant to be used as stimuli to encourage class discussion. They are intended for use in a class that attempts to help students develop a coherent and sophisticated understanding of scientific thinking. They are NOT intended as items to test whether students are “right or wrong” or “know” the correct answerby one-step recall if enough cues are given. This has a number of instructional implications that are reviewed in general on the next four slides. The individual slides also contain annotations discussing their intended use.

3. Usage: 1 • Feedback One of the most important values of a clicker-response system is to provide instructors with some understanding of what students are thinking. Good clicker questions can be highly revealing (and surprising). But the critical fact is not that the students make mistakes but to use those mistakes to probe their thinking and find out why. This raises the importance of a rich subsequent discussion well above “letting the students know what the right answer is.”

4. Usage 2: • Student-student interactions The critical value for student learning occurs in what happens after a clicker question has obtained a mixed response from the students. The standard next cue is, “Find someone who disagreed with the answer you chose and see if you can convince them.” After a minute or two of discussion, a second click may show students having moved dramatically towards the correct answer. A brief call for who changed their answer and why can lead to a useful exchange. When they have not moved significantly, more discussion is called for.

5. Usage: 3 • Incompletely specified questions Some items have questions that are simple if idealized assumptions are made, subtler if they are not. Part of the discussion of these items are intended to include issues of modeling, idealizations, and hidden assumptions. • Questions where answers are not provided. In these items, the intent is to have students come up with potential answers and have the instructor collect them and write them on the board. Occasionally, especially at the beginning of a class, it may take some time before students are willing to contribute answers. It can help if you have some prepared answers ready, walk around the class, and put up the answers as if they came from the students. This can help students get more comfortable with contributing.

6. Usage: 4 • Cluster questions Some questions are meant to be used as part of a group of questions. In this case, resolving the answers to individual questions is better left until the entire group is completed. The value of the questions are often in the comparison of the different items and in having students think about what changes lead to what differences and why. • Problem solving items In these items (indicated by a pencil cluster logo), the intent is to have students work together to solve some small problem. After a few minutes, ask the groups to share their answers, vote on the different answers obtained, and have a discussion.

7. This potential energy diagram represents the interaction between two atoms. It is negative at its lowest point. This means… • An atom can never be at this location, since energy can’t be < 0. • When atoms are separated, energy is released. • When atoms start far and move closer, energy is released. • Energy must be added to get atoms to separate. • Energy must be added to get atoms to move closer together. • The fact that the potential energy is negative tells us nothing; this is just a result of choosing an arbitrary zero point. • More than one of these • None of these • There is not enough information to answer

8. The figure shows the potential energyof two interacting atoms. The point with the minimum value is r0 and the point where the curve crosses 0 is r1. Where is the force between the two atoms the largest? • At r0. • At r1. • At fairly large values of r. NEXUS/Physics Clicker Questions

9. The figure shows the potential energyof two interacting atoms. The point with the minimum value is r0 and the point where the curve crosses 0 is r1. Where is the force between the two atoms repulsive? • Between r1 and r0. • Between r1 and ∞. • Nowhere. NEXUS/Physics Clicker Questions

10. The figure shows the potential energyof two interacting atoms. The point with the minimum value is r0 and the point where the curve crosses 0 is r1. Where is the force between the two atoms attractive? • Between r1 and r0. • Between r1 and ∞. • Nowhere. NEXUS/Physics Clicker Questions

11. You know that two atoms that are far apart are barely interacting. How is this represented visually in the PE diagram? • The potential energy approaches zero as r gets large. • The PE curve is close to horizontal as r gets large. • The PE curve is close to vertical as r gets small. • The potential energy has a minimum. • More than one of these • The PE diagram doesn’t demonstrate this information • None of these

12. These two atoms can exist in a stable bound state. How is this represented visually in the PE diagram? • The potential energy approaches zero as r gets large. • The PE curve is close to horizontal as r gets large. • The PE curve is close to vertical as r gets small. • The potential energy has a minimum. • More than one of these • The PE diagram doesn’t demonstrate this information • None of these

13. Two atoms interact with a potential energy between them that varies as a function of their separation as shown in the graph at the right. We take the zero of energy to be when they are very far apart and at rest. They have a total energy E1 as shown on the figure. Which of the following statements are true about them? • They are in a bound state. • The total energy of the molecule is positive. • The total energy of the molecule is negative. • The total energy of the molecule is zero. Only A A and B A and C A and D Only B Only C Only D NEXUS/Physics Clicker Questions

14. If the atoms were in the state labeled E1, which of the statements below are true? • To pull them apart, you would have to put in an energy E1. • To pull them apart, you would have to put in an energy -E1. • By pulling them apart, you would gain an energy E1that you could use elsewhere. • By pulling them apart, you would gain an energy -E1that you could use elsewhere.

15. While wandering around their environment, the molecule collides with another (fast moving) molecule and winds up being shifted to the state with a total energy E2 as shown on the figure. Which of the following statements are now true about these atoms? • They are no longer in a bound state. • They are more tightly bound than they were before. • There are less tightly bound than they were before. • Their binding energy has not changed. • More than one is true. • None are true.

16. While wandering around their environment, the molecule collides with another (fast moving) molecule and winds up being shifted to the state with a total energy E2 as shown on the figure. After the collision it winds up with a total energy E2 as shown on the figure. The molecule that struck them does not change its internal (chemical) energy in the collision. Which of the following statements is true about the molecule that struck them? • It has less kinetic energy after the collision than it had before. • It has the same kinetic energy after the collision as it had before. • It has more kinetic energy after the collision than it had before. • There is not sufficient information to decide.

17. What can you say about the average separation between the two atoms when they are in the states shown on the diagram? • When the atoms in the molecule have an energy E1 the atoms are on the average closer together than when they have an energy E2. • When the atoms in the molecule have an energy E2 the atoms are on the average closer together than when they have an energy E1. • In both cases, the atoms are, on the average, the same distance apart.

18. The figure below shows the interaction potential between two molecules (along a particular orientation of the two molecules). The units are in nm (r) and eV (U).When the molecules are separated by 7 nm the force between them is • Attractive • Repulsive • Zero • Cannot be determined from the figure.

19. The figure below shows the interaction potential between two molecules (along a particular orientation of the two molecules). The units are in nm (r) and eV (U).When the molecules are separated by 2 nm the force between them is • Attractive • Repulsive • Zero • Cannot be determined from the figure.

20. The figure below shows the interaction potential between two molecules (along a particular orientation of the two molecules). The units are in nm (r) and eV (U).When the molecules are separated by 0.5 nm the force between them is • Attractive • Repulsive • Zero • Cannot be determined from the figure.

21. The Gauss gun Spheres numbered 1, 2, and 3 all “stick” when added one at a time. Which is more tightly bound? • Sphere 1 (when 2 and 3 are NOT there) • Sphere 3 (when 1 and 2 ARE there) • They will be the same. NEXUS/Physics Clicker Questions

22. The Gauss gun When sphere 0 is released it is attracted to the magnet and begins to speed up. What do you think will happen when it hits the magnet? • Sphere 0 will stick. Nothing else will happen. • Sphere 3 will be kicked off at the same speed that sphere 0 hit with and will slow down to a stop – reversing what 0 did as it approached. • Something else will happen. (What?) NEXUS/Physics Clicker Questions

23. Which describe(s) the difference between a strong and a weak chemical bond between two atoms? • The strong bond stores more energy than the weak bond. • More energy is needed to separate strongly bonded atoms than weakly bonded atoms. • More energy is released tothe environment when two atoms become strongly bonded than when two atoms become weakly bonded. I only II only III only II and III only I, II, and III None