Q.(a)

(a) Crystalline and amorphous solids differ fundamentally in their internal order and melting behaviour; and in any multistep reaction mechanism, it's the single slowest step that sets the overall observed rate. OR (b) applying the law of mass action to a weak electrolyte's dissociation equilibrium, and simplifying for small $\alpha$, yields Ostwald's dilution law relating degree of dissociation to concentration.

(a)(i) Crystalline vs amorphous solids:

(a)(ii) Rate-determining step, with an example: in a reaction that proceeds via several sequential elementary steps (a mechanism), the overall observed rate of the reaction is governed entirely by the slowest of these steps — this is called the rate-determining step (RDS), since the reaction as a whole cannot proceed faster than its slowest step, regardless of how fast the other steps are. Example: the iodide-ion-catalysed decomposition of hydrogen peroxide proceeds via two steps: Step 1 (slow, RDS): $H_2O_2 + I^- \rightarrow H_2O + IO^-$; Step 2 (fast): $H_2O_2 + IO^- \rightarrow H_2O + O_2 + I^-$. Since Step 1 is much slower, the overall rate law is determined entirely by it: $\text{Rate}=k[H_2O_2][I^-]$, even though the overall stoichiometric equation is $2H_2O_2 \rightarrow 2H_2O + O_2$.

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