Which of the following statements are true?
(A) Decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction.
(B) The rate of a reaction increases with the passage of time.
(C) A catalyst does not change the equilibrium of a reaction.
(D) The molecularity of a reaction can be zero, fraction and negative.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (C) only
The rate of a reaction depends on the concentrations of the reactants, as given by its rate law. The order is the sum of the powers in the experimental rate law and can be zero or a fraction. The molecularity is the number of species that collide in one elementary step, so it is always a small whole number. A catalyst changes only the speed with which a reaction reaches equilibrium.
(A) Decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction. — Correct. At high pressure the metal surface is fully covered by ammonia molecules, so a rise in pressure does not change the amount of ammonia reacting on the surface:
\( \displaystyle \ce{2NH3(g)} \xrightarrow[\text{1130 K}]{\ce{Pt}\text{ catalyst}} \ce{N2(g) + 3H2(g)} \qquad \text{Rate} = k[\ce{NH3}]^{0} = k \)
(B) The rate of a reaction increases with the passage of time. — Incorrect. The reactants are used up as the reaction proceeds, so their concentrations fall and the rate decreases with time.
(C) A catalyst does not change the equilibrium of a reaction. — Correct. It lowers the activation energy of the forward and the backward reaction by the same amount, so both become faster and equilibrium is reached sooner, but the equilibrium constant is unchanged.
(D) The molecularity of a reaction can be zero, fraction and negative. — Incorrect. Molecularity counts colliding species, so it can only be 1, 2 or 3. Zero and fractional values belong to the order of a reaction.