![]() When the liquid forms, there is rapid breaking of hydrogen bonds (trillionths of a second) and forming new ones with adjacent molecules. However, a change in the motional energy - not an increase in the quantity of energy - occurs from the transfer of vibrational energy in the ice crystal to the liquid. ![]() Of course, it is released when the temperature of the system drops below the freezing/melting point).The process is isothermal, and therefore there is no energy transferred to the system to increase motional energy. (This potential energy remains unchanged in a substance throughout heating, expansion, mixing, subsequent phase change to a vapor, or mixing. Because melting involves bond-breaking, it is an entropy increase in the potential energy of the substance involved. The rigid tetrahedral structure is no longer present in liquid water but the presence of a large number of hydrogen bonds is shown by the greater density of water than ice due to the even more compact hydrogen-bonded clusters of H 2O.) Quantitatively, the entropy increase in this isothermal dispersal of energy from the surroundings is ΔH Fusion /T. Many, but not all of the hydrogen bonds in crystalline ice are broken. (“To the degree required” has special significance in the melting of ice. To change a solid to a liquid at its melting point requires large amounts of energy to be dispersed from the warmer surroundings to the solid for breaking the intermolecular bonds to the degree required for existence of the liquid at the fusion temperature.
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