Should the melting of ice be represented as a solid state reaction?

Ice cubes, held in a flow of water maintained at a constant temperature, melt at a rate that is well expressed by the contracting volume rate equation, familiar from kinetic studies of solid state reactions. The apparent activation energy, 28.5 +/- 3.0 kJ mol(-1) between 276.2 and 303.4 K, is close to the strength of the hydrogen bond in ice. From these observations, it appears that this physical change exhibits a pattern of kinetic features that is superficially identical with behavior characteristic of the chemical steps occurring during thermal decompositions of solids. However, careful examination of the rate data at the temperatures closest to the melting point of ice shows that here rates are much slower than is consistent with expectation from the Arrhenius line. It is concluded, therefore. that rate constant measurements are more satisfactorily represented overall by a rate of interface advance, during fusion, that is directly proportional to heat how: this is directly proportional to the difference between the temperatures of the ice surface and of the flowing water. It follows that the melting rate is most satisfactorily represented as being controlled by heat transfer across a boundary layer of moving liquid, close to the ice surface. These alternative analyses of the same data are presented to emphasize that mechanistic and kinetic interpretations of rate processes involving solids must be based on realistic assessments of conditions within the zone of change. This demonstration that a kinetic expression that is characteristic of solid state reactions satisfactorily describes the data together with an activation energy that correlates with a known bond strength in the reactant does not necessarily prove that a solid state, activated reaction is occurring here. Aspects of the mechanistic interpretations of the kinetic characteristics of many solid state decompositions remain difficult to understand and are incompletely resolved. (C) 2001 Elsevier Science B.V. All rights reserved.