Question

Reactions of which order show rate independent of concentration of reactant? Give one example.

Hint:

Zero-order: Rate $= k$ (constant). Units of $k$ are mol L$^{-1}$s$^{-1}$.

Answer 1

Step 1: Define zero-order reactions.
In a zero-order reaction, rate $= k[A]^0 = k$ (constant). The rate does not change even if the concentration of the reactant changes.
Step 2: Why this happens.
Zero-order kinetics typically occur in heterogeneous catalysis when the catalyst surface is completely saturated with reactant molecules. Adding more reactant has no effect on rate since all surface sites are occupied.
Step 3: Example.
Decomposition of ammonia on a hot platinum surface: $2NH_3 \xrightarrow{Pt,\,\Delta} N_2 + 3H_2$. The rate is constant regardless of $[NH_3]$.
Step 4: Conclusion.
Zero-order reactions show rate independent of concentration. Example: decomposition of $NH_3$ on a hot platinum surface.