For reaction control in the shrinking core model, the conversion time relation involves
Answer: A
The relation follows from the surface area of the shrinking core, which falls with the square of its radius. Plotting the data in that form gives a straight line if reaction controls. Surface area falls with the square of radius.
Please log in or register to report this question.
State whether the following is true or false.
Plotting the data in the form suggested by each model is the standard way of identifying the controlling resistance.
Answer: B
Whichever plot gives a straight line identifies the mechanism, exactly as with homogeneous rate laws. Mixed control shows as curvature in all three plots. Straight lines identify the mechanism. Each model has its own linearising form.
Please log in or register to report this question.
A particle under reaction control reaches 87.5 per cent conversion. What fraction of the original radius does the core retain?
Answer: C
The unreacted fraction is one eighth by volume, and the cube root of an eighth is a half. This relation between volume conversion and core radius is the heart of the shrinking core model. Volume goes with the cube of radius.
Please log in or register to report this question.
Mixed control, where two resistances are comparable, shows itself as
Answer: D
None of the simple forms fits, so the data must be treated with the full model. Systems commonly shift from reaction to ash diffusion control as the layer thickens. The full model is then needed. Curvature on all three plots is the sign.
Please log in or register to report this question.
Assertion (A): A single fluid solid reaction may change its controlling resistance as it proceeds.
Reason (R): The ash layer thickens with conversion while the reaction area shrinks.
Answer: A
Both are true and the reason gives the mechanism: reaction often controls at the start and ash diffusion by the end, which is precisely why the simple plots curve over the full range. Control shifts as the reaction proceeds.
Please log in or register to report this question.
Consider the following statements about the two models.
1. The shrinking core model suits a dense solid.
2. The progressive conversion model suits a very porous solid.
3. Both models predict a particle that shrinks in size.
Answer: B
The first two are correct. The third is wrong for the progressive conversion model, in which the particle keeps its size while its composition changes throughout. Porosity decides which applies. The two are the extremes of a range.
Please log in or register to report this question.
State whether the following is true or false.
A particle reacting by the progressive conversion model keeps its original size.
Answer: C
The fluid penetrates throughout and the composition changes uniformly rather than a front moving inward. Only the shrinking core picture involves a receding boundary. Composition changes uniformly instead. The size stays constant throughout.
Please log in or register to report this question.
