Solid water at 0 °C is converted to liquid water at 0 °C under constant atmospheric pressure. The molar enthalpy of fusion for water is 6.01 kJ mol−1. Assume exactly one mole of water undergoes the phase change.
(a) Calculate ΔH for this physical process and state its sign. Briefly explain the physical meaning of both the magnitude and the sign.
(b) Sketch a qualitative energy diagram for the phase change, labeling the axes, initial state, final state, and the numerical value of ΔH you calculated. No numerical scale for the reaction coordinate is required, but relative spacing should be chemically reasonable.
(c) Describe the direction of heat flow between the system and the surroundings during melting. Relate your explanation to the features of your energy diagram.
(d) If the liquid water re-freezes to ice under the same conditions, describe how the energy diagram would differ from the one you sketched in part (b), and classify the freezing process as endothermic or exothermic.