Solid state reaction energy

The slope of the Arrhenius plot has units (temperature) 1 but activation energies are usually expressed as an energy (kJ mol 1), since the measured slope is divided by the gas constant. There is a difficulty, however, in assigning a meaning to the term mole in solid state reactions. In certain reversible reactions, the enthalpy (AH) = E, since E for the reverse reaction is small or approaching zero. Therefore, if an independently measured AH value is available (from DSC or DTA data), and is referred to a mole of reactant, an estimation of the mole of activated complex can be made. 

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Gomes, W. (1961). "Definition of Rate Constant and Activation Energy in Solid State Reactions," Nature (London) 192, 965. An article discussing the difficulties associated with interpreting activation energies for reactions in solids. 

The solid-state polymerization of diacetylenes is an example of a lattice-controlled solid-state reaction. Polydiacetylenes are synthesized via a 1,4-addition reaction of monomer crystals of the form R-C=C-CeC-R. The polymer backbone has a planar, fully conjugated structure. The electronic structure is essentially one dimensional with a lowest-energy optical transition of typically 16 000 cm-l. The polydiacetylenes are unique among organic polymers in that they may be obtained as large-dimension single crystals. 

Monitoring solid state reactions that play a role in catalyst activation forms a useful application of XRD. The example discussed above concerns a catalyst with large iron oxide particles as is used in the water gas shift reaction, and represents a particularly favorable system for XRD analysis. Similar studies with small particles are certainly also feasible, although it may be advisable to use laboratory X-ray sources of higher energy, such as Mo Ka, or a synchrotron [13]. 

The solid state reactions are extremely complex due to intervention of many physical parameters and it becomes often necessary to make some generalizations in the complex reactions. The rate in solid state reactions cannot be defined in the same way as that for a homogeneous reactions because the concept of concentration in solid state reactions has no significance. The energy of activation in a solid state reaction has also no significance, except in some rare cases. 

Care has to be taken when extrapolating kinetic parameters measured under melt-phase conditions for describing the solid-state reaction. The available kinetic data are not free from mass-transfer influences and the effects of proton and metal catalysis are not thoroughly separated. Therefore, the adaptation of kinetic parameters is often carried out by fixing the activation energies and adjusting the pre-exponential factors to the experimental data. 

Unfortunately the authors argue that they were performing mechanochemical reactions with mechanical energy input for the salt formation or complexation to occur, rather than just creating the required contacts between reacting crystals. Furthermore, they did not exclude moisture, reported intermediate liquid phases in various cases, and did not separate out any real solid-state reactions that might have been achieved. It is therefore not possible to discuss the results in more detail here. 

For the solid-state reaction to take place, the reactants have to be vigorously mixed by applying external mechanical energy. Thus, the sohd-state reaction may be called mechanochemistry. Mechanochemistry has been used mostly for inorganic solids (for example, for alloying, interaction of soft metals with ceramics, activation of minerals for catalysis, and so on) [1,2] and for inor-... 

The amount and uniformity of the solid state reaction of halogen with TTF was probed by the electron microprobe technique. In this analytical method, low energy electron irradiation of a sample provides X-ray core level emissions, characteristic of the element and its relative concentration. Our initial analyses indicated a dramatic dependence of the halogen concentration with the energy of the electron beam. To probe this phenomenon further,... 

Solid state reactions up to 2300 K are usually carried out in furnaces which use resistance heating the resistance of a metal element results in electrical energy being... 

In heterogeneous solid state reactions, the phase boundaries move under the action of chemical (electrochemical) potential gradients. If the Gibbs energy of reaction is dissipated mainly at the interface, the reaction is named an interface controlled chemical reaction. Sometimes a thermodynamic pressure (AG/AK) is invoked to formalize the movement of the phase boundaries during heterogeneous reactions. This force, however, is a virtual thermodynamic force and must not be confused with mechanical (electrical) forces. 

Boundaries between solids transmit shear stress, particularly if they are coherent or semicoherent. Therefore, the strain energy density near boundaries changes over the course of solid state reactions. Misfit dislocation networks connected with moving boundaries also change with time. They alter the transport properties at and near the interface. Even if we neglect all this, boundaries between heterogeneous phases are sites of a discontinuous structural change, which may occur cooperatively or by individual thermally activated steps. 

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