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PCAT Chemical Processes – Chemical Reactions

  • by Sam
  • Oct 25, 2017
  • PCAT Question of the Day

A researcher constructs a hypothetical reaction that follows a mechanism that uses the two reactions below:

3 Q2 + 2 AX4 → 2AX3Q + 2XQ + 2Q


1. AX3+ Q2→ AX3Q + Q

2. 2 AX4 + Q2 → AX3Q + Q


If the second reaction is experimentally determined to be an order of magnitude slower than the first reaction, the researcher can likely conclude that the rate law followed by the hypothetical reaction would be which of the following?


A) R = k[AX4][Q2]

B) R = k [AX4]2[Q2]

C) R = k [AX4]2[Q2]2

D) R = k [AX4]2[Q2]3


The rate law is determined by the slowest step (the rate-determining step). The problem tells you that the second reaction is the slowest step, so it will determine the rate law. The exponents in the rate law match the stoichiometric coefficients of the reagents in the rate determining step. Here, the AX4 has a coefficient of 2 and the Q2 has a coefficient of 1. That matches the exponents in choice (B).

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