Which of the following statements are correct?
A. Order and the molecularity of a reaction can be fractional.
B. Catalyst affects the activation energy of reaction but not rate of reaction.
C. For complex reactions, order of reaction is determined by the slowest step.
D. The decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → C and D only
The order of a reaction is the sum of the powers of the concentrations in the experimental rate law, so it can be 0, 1, 2, 3 or a fraction. The molecularity is the number of species that collide in one elementary step, so it can only be a whole number (1, 2 or 3). A catalyst gives the reaction a new path with lower activation energy.
A. Order and the molecularity of a reaction can be fractional. — Incorrect. Only the order can be fractional. Molecularity counts colliding molecules, so it cannot be zero or a fraction.
B. Catalyst affects the activation energy of reaction but not rate of reaction. — Incorrect. By lowering the activation energy, a catalyst lets more molecules react in a given time, so it increases the rate. It does not change ΔG or the equilibrium constant.
C. For complex reactions, order of reaction is determined by the slowest step. — Correct. A complex reaction goes through several elementary steps. The slowest step is the rate determining step, so the rate law and the order come from it.
D. The decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction. — Correct. At high pressure the platinum surface is fully covered by \( \ce{NH3} \) molecules. Adding more ammonia cannot speed up the reaction, so the rate is constant.
\( \displaystyle \ce{2NH3} \xrightarrow{\text{1130 K, Pt}} \ce{N2 + 3H2},\quad \text{Rate}=k[\ce{NH3}]^0=k \)