What would be the mathematical sign for the energy change (positive for endothermic or negative for exothermic) for each of the following processes? Explain the reasoning for each choice:
a. Solid dry ice sublimes at room temperature (about 22.0 ºC).
b. Two hydrogen atoms combine to make a hydrogen molecule.
c. Molten (liquid) sodium cools to form solid sodium.
d. Burning of gasoline in a car engine.