# MCAT Chemistry Question — Rate Law

The reaction A + B → C proceeds at a rate that varies with the initial concentration of the reactants, as follows:

 Trial [A]t=0 [B]t=0 Rate t=0 (M/s) 1 0.1 M 0.5 M 5 x 10-4 2 0.2 M 1.0 M 1 x 10-3 3 0.1 M 4.0 M 4 x 10-3

What is the rate law for the reaction above?

1. Rate = k[A]
2. Rate = k[B]
3. Rate = k[A][B]
4. Rate = k[A][B]2
##### Click for Explanation

To determine the rate law, you must compare trials to see how different initial reactant concentrations affect the initial rate of the reaction. Comparing trial 1 and trial 3, the initial rate in trial 3 increased by a factor of 8. Since [A] is constant and [B] increases by a factor of 8, the reaction rate is first order with respect to B.

Comparing trial 1 and trial 2, the initial rate of trial 2 increase by a factor 2. Since [B] also increases by a factor of 2 and the reaction rate is first order with respect to B, the increase in the reaction rate is due to [B] alone. Thus, the reaction is zero order with respect to A. Thus, the rate law can be described by answer choice B.

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