An environmental chemistry group investigates the dissolution of CaCl2 (s) in seawater using a high-precision calorimeter whose heat capacity is unknown. To determine this constant, the group first performs a calibration run by dissolving 2.500g of CaCl2 (s) of known enthalpy of solution ΔHsoln=−81.0kJmol−1 in 150.0g of seawater at 18.25∘C. The maximum temperature recorded is 20.93∘C. The specific heat capacity of seawater in the range studied is 3.94Jg−1K−1, and its density is 1.025gmL−1.
In a second experiment using the same calorimeter, 1.750g of MgCl2 (s) is dissolved in 150.0g of seawater, also initially at 18.25∘C. The maximum temperature observed is 19.06∘C. Assume no heat is exchanged with the external surroundings other than through the calorimeter walls.
(1) Determine the heat capacity of the calorimeter from the calibration run.
(2) Calculate the total quantity of heat released or absorbed when MgCl2 dissolves in the second experiment.
(3) Compute the molar enthalpy of solution for MgCl2 based on the data provided.
(4) Taking into account the ionic strength and composition differences between seawater and pure water, critically evaluate whether the enthalpy value obtained in part (c) would be expected to differ if the dissolution were performed in pure water. Provide chemical reasoning to support your evaluation.