2019 Year 12 IB Extended Essays

Classic Liesegang reactions occur when one reactant is homogeneously dispersed as another reactant diffuses into it. This type of reaction can also be called a reaction-diffusion process (Büki, 2003). Yet these reactions are still unexplained as several theories of the formation of the periodic precipitation are possible, such theories include diffusion, nucleation, and crystal growth. Many models and investigation of Liesegang rings use Ostwald’s pre-nucleation theory to explain formation, specifically it explains “the periodic precipitation as the consequence of a supersaturation condition necessary for nucleation that is followed by a ring deposition” (Ramírez-Álvarez, et al., 2016). However, there is still a knowledge gap about the variety of complex patterning that cannot be filled with the proposed theories. E XISTING T HEORIES First introduced in Ostwald’s 1987 periodical, Lehrbuch der Algemeine Chemie (when translated means “General Chemistry”), the Theory of Supersaturation is used in most formularisations to explain the Liesegang phenomenon. The basis of Ostwald’s theory can be summarised as follows:

“. . . In some situations, nucleation of crystals can be kinetically hindered, and precipitation can start only at a certain level of supersaturation” (Büki, 2007). Under these parameters, the system within the gel remains supersaturated until the initial crystal is formed. Once this initial formation occurs, a large expansion occurs primarily because the surrounding zones are

FIGURE 2 (SCHAFFER & HERMAN, 2019)

further from the equilibrium” (Büki, 2007). This theory is substantiated when conducting experiments on the formation of Liesegang rings with silver-nitrate and potassium dichromate. When the two reactants interact with each other, the silver-nitrate remains supersaturated. Once the saturation value overcomes its critical value, precipitation starts to

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