Controlling factors and reaction laws

The rates of heterogeneous reactions do not usually follow rate laws of homogeneous reactions instead, they involve multisteps and are controlled by the 

Component exchange between phases is controlled by mass transfer. Between solid phases, mass transfer is through diffusion where the exchange of components may be used as a geospeedometer (Lasaga, 1983). Convection rather than diffusion may play a dominant role if fluid phases are involved. In reactions between solid and fluid phases, diffusion in the solid phase is usually the slowest step. However, dissolution and reprecipitation may occur and may accomplish the exchange more rapidly than diffusion through the solid phase. 

Because interface reaction and mass/heat transfer are sequential steps, crystal growth rate is controlled hy the slowest step of interface reactions and mass/heat transfer. For a crystal growing from its own melt, the growth rate may be controlled either by interface reaction or heat transfer because mass transfer is not necessary. For a crystal growing from a melt or an aqueous solution of different composition, the growth rate may be controlled either by interface reaction or mass transfer because heat transfer is much more rapid than mass transfer. Different controls lead to different consequences, including the following cases  

Mixed controi PbS04 (anglesite) 1 x 10 4 MgCl2 6H2O (bischofite ) 5 2 

Sometimes crystal growth, dissolution, or oxidation is said to follow a linear growth law or a parabolic growth law. The linear law means that the thickness of the crystal depends linearly on time. 

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