Reaction Kinetics - MCAT Chemical and Physical Foundations of Biological Systems

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Question

A kinetics experiment is constructed, and the following rates of product formation are observed after adding various substrates. The results are recorded below. What is the equation for the rate law?

ABCInitial Rate (M/s)
0.1M0.2M0.2M1.5
0.2M0.2M0.2M3
0.2M0.2M0.4M3
0.2M0.4M0.2M12

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Answer

We can look at the differences between trial 1 and trial 2 to see that the reaction is first order with respect to \[A\], since the rate doubles when the reactant concentration doubles. Reactants are considered first order when this 1:1 relationship between concentration and rate is observed.

We can look at trials 2 and 3 to see that \[C\] has no effect on the reaction, making it a zeroth order reactant and simply equal to 1 in the rate law.

Trials 3 and 4 show that when \[B\] doubles, the rate quadruples. This means it is second order with respect to \[B\], increasing the rate by the square of the increase in concentration. Since we already know that reactant C does not affect the rate, it is irrelevant that its concentration is halved.

This gives us Rate = k\[A\]\[B\]2.

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