P. How can you tell if a reaction is endothermic or exothermic? How does the thermal energy of the reactants compare to that of the products? How does the temperature change observed in the surroundings relate to the endo- or exothermic nature of a reaction? Q. What is a formation equation? Be able to write the formation equation for any substance using only its molecular formula. Be able to examine a chemical equation and identify whether or not it represents a formation equation. R. Under what circumstances is the value of AHrxn the same as the AHf for the product formed by the reaction? S. Why are heats of formation always in kJ/mol while the heat of reaction that's attached to a chemical equation is usually expressed in kJ? T. Which changes to the state of matter are endothermic? Which changes to the state of matter are exothermic? U. Be able to explain why a substance's temperature does not change during a change to its state of matter. For example, when a sample of liquid water reaches 100C, it begins to boil. As the water continues to absorb energy it evaporates, but its temperature remains at 100C until the evaporation is complete. What is the molecular basis for this phenomenon? V. Be able to explain by being burned by water vapor at 100C causes a worse injury than being scalded with liquid water at 100C.