A hydrogel is composed of crosslinked polymer chains, which are highly water absorbent. The gels are mostly (mostly>90%) water. Liesegang line patterns are precipitation patterns that appear in a gel when a chemical reaction takes place when a diffusing chemical (reactant A) in a gel reaches a concentration that is just enough to react with the reactant B (already present and homogeneously distributed in the gel) to form a precipitate. This precipitate appears as a colored line in the gel. Once the precipitate forms, reactant A is consumed all – however, diffusion of A does not stop. A ions propagate deeper into the gel and form another line of the precipitate when the A concentration again reaches the threshold concentration, just enough to react with the reactant B. In the system shown on the right, A=Cu2+ ions, B is CrO42-. Cu2+
ions diffuse from the solution at the top into the yellow-colored
gel, which has CrO42- solution. The Cu2+ ions react with CrO42-
ions to form the CuCrO4 precipitate. There are no Cu2+ ions
present in the gel before the diffusion. Assuming that the
concentration of Cu2+ does not change at the solution/gel interface
and is 4.0 mol/L, and it is 0 mol/L at the bottom of the gel for the
duration of the precipitate formation,
1) find the depth of the first precipitate line that would appear in
the gel if the threshold concentration for the line formation is 0.05
mol/L and the time for the line to appear is 5 hrs. The temperature
is 25oC. D25= 1.2x10-8 m2/s.
2) at which temperature the line would appear at the same distance from the start, but in 75 min? Qd= 40 kJ/mol