Match the LIST-I with LIST-II.
| LIST-I (Condition/ Reaction) | LIST-II (Possibility) |
| A. Molecularity of reaction | I. Can have fractional value |
| B. Thermal decomposition of HI on gold surface | II. Not meaningful for complex reaction. |
| C. Order of reaction | III. First order reaction |
| D. Radioactive decay | IV. Zero order reaction |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A-II, B-IV, C-I, D-III
Order and molecularity are often confused. Order is an experimental quantity: the sum of the powers of the concentration terms in the rate law, so it can be zero, a whole number or a fraction. Molecularity is a theoretical quantity: the number of reacting species that collide in one elementary step, so it is always a whole number (1, 2 or 3).
A. Molecularity of reaction — This matches II. Not meaningful for complex reaction. A complex reaction takes place in several elementary steps, each with its own molecularity, so the reaction as a whole has no single molecularity. Its rate is decided by the slowest step.
B. Thermal decomposition of HI on gold surface — This matches IV. Zero order reaction. At high pressure the gold surface is fully covered with HI molecules, so adding more HI does not change the rate: the rate is independent of concentration.
\( \displaystyle \ce{2HI(g)} \xrightarrow{\ce{Au}} \ce{H2(g) + I2(g)},\quad \text{Rate}=k[\ce{HI}]^{0}=k \)
C. Order of reaction — This matches I. Can have fractional value. Order comes from the experimental rate law, and the powers in it can be fractions (for example, Rate = k[CH3CHO]3/2).
D. Radioactive decay — This matches III. First order reaction. The rate of decay is proportional to the number of nuclei present, which is why every radioactive element has a constant half-life.
\( \displaystyle \text{Rate}=\lambda N,\quad t_{1/2}=\frac{0.693}{\lambda} \)
Shortcut: The pair C–I appears in Options (1) and (2), and A–II then leaves only Option (1).