Chemistry Reaction Kinetics MDCAT 2019
PMDC Verified Question 56 of 80
The decomposition of phosphorus pentachloride in the presence of moisture takes place by the following mechanism,
\( \text{PCl}_{5(s)} + \text{H}_{2}\text{O}_{(l)} \longrightarrow \text{POCl}_{3(l)} + 2\text{HCl}_{(aq)} \) (Slow Step)
\( \text{POCl}_{3(l)} + 3\text{H}_{2}\text{O}_{(l)} \longrightarrow \text{H}_{3}\text{PO}_{4(aq)} + 3\text{HCl}_{(aq)} \) (Fast Step)
Overall: \( \text{PCl}_{5(s)} + 4\text{H}_{2}\text{O}_{(l)} \longrightarrow \text{H}_{3}\text{PO}_{4(aq)} + 5\text{HCl}_{(aq)} \)
The rate equation for this reaction will be:
A
Rate = \( k[\text{PCl}_{5}][\text{H}_{2}\text{O}]^{4} \)
B
Rate = \( k[\text{POCl}_{3}][\text{H}_{2}\text{O}]^{3} \)
C
Rate = \( k[\text{PCl}_{5}][\text{H}_{2}\text{O}] \)
D
Rate = \( [\text{PCl}_{5}][\text{H}_{2}\text{O}] \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: Rate = \( k[\text{PCl}_{5}][\text{H}_{2}\text{O}] \)
Concept:

In a multi-step reaction mechanism, the overall rate of the reaction is strictly governed by the slowest step, known as the Rate-Determining Step (RDS).

Solution:

  • Identify the slow step from the mechanism: \( \text{PCl}_{5} + \text{H}_{2}\text{O} \longrightarrow \text{POCl}_{3} + 2\text{HCl} \).
  • The rate law is derived directly from the molecularity of this slow step.
  • The reactants in the slow step are one molecule of \( \text{PCl}_{5} \) and one molecule of \( \text{H}_{2}\text{O} \).
  • Therefore, the rate equation is: \( \text{Rate} = k[\text{PCl}_{5}][\text{H}_{2}\text{O}] \).


Why other options are incorrect:

Option A incorrectly uses the stoichiometry of the overall reaction. Option B uses the fast step. Option D is missing the essential rate constant (\( k \)).

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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