For an elementary reaction A + B -> products, what is the form of its rate law and its overall order?

• A. rate = k[A]
• B. rate = k[A][B]
• C. rate = k
• D. rate = k[A]^2

Answer: (B)

For an elementary bimolecular reaction, the rate reflects how often the two reactant molecules collide. This collision frequency depends on both concentrations, so the rate law is rate = k[A][B]. The exponents add to give the overall order, which is 1 + 1 = 2, so the reaction is second order overall. This form directly corresponds to the mechanism: a single collision between A and B drives the reaction. If the two reactants were the same, you’d have rate = k[A]^2, which is still second order, but for distinct A and B the product form is the correct one. The other forms would describe different mechanisms or conditions and do not match the elementary A + B step.