A chemistry student adds a measured mass of solid sodium hydroxide pellets to a coffee-cup calorimeter containing an aqueous solution originally at 25.0∘C. The temperature of the solution rises rapidly to 37.8∘C before slowly drifting back toward room temperature.
Answer the following questions about the thermal changes that occurred.
(1) Describe the direction of energy transfer between the dissolving solid and the surrounding solution while the temperature was rising.
(2) Identify the sign (positive or negative) of the enthalpy change per mole for the dissolution of sodium hydroxide and justify your answer in terms of the observed temperature change.
(3) Explain why the numerical value of the heat released by the dissolving solid is equal in magnitude but opposite in sign to the heat absorbed by the solution in the calorimeter.