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PCAT Chemical Processes – Rate Constant Units

A chemical reaction has a rate law in the format of rate = k[X][Y]2. What are the units of the rate constant k for this reaction?

A. M⋅s−1
B. s−1
C. M−1⋅s−1
D. M−2⋅s−1

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D is correct. The exponents for the reactants included in the rate law sum to 3, so this is a third-order reaction. The units for the rate constant for such a reaction are M−2⋅s−1. Even if you did not know this, you can reason it out by plugging in the units for the other components of the rate law, remembering that the unit for concentration is molarity (M), while the units for rate are molarity per second (M/s).

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