2. How can work be converted into potential energy? How does a water mill use potential energy, and what does it convert it into? Why does a water mill convert some of this energy into heat, and where does the heat appear?

3. Give an example of the conversion of heat into work, and of work into heat.

4. If the burning process gives off real heat, why do chemists say that the heat of reaction is negative?

5. Why are energy, E, and enthalpy, H, different? Which quantity represents the heat of a reaction carried out in a constant-pressure situation such as the open air of a laboratory?

6. How might you carry out an experiment such that the energy, E, rather than H, represented the heat of reaction?

7. What is the standard heat of formation of a substance?

8. Why is the standard heat of formation of a liquid always more negative than that of the corresponding gas? Illustrate with an energy-level diagram.