The equation A + 2 B = C + D describes an elementary reaction, which takes place in a single step. Thus, the rate law must be?

My answer is k(A)(B)^2

Is this correct?

To determine the rate law for a reaction, you can rely on the coefficients of the reactants in the balanced chemical equation. In this case, the balanced equation is A + 2B = C + D.

If the reaction occurs in a single step, we can assume that the rate-determining step is the overall reaction itself. In other words, the coefficients in the balanced equation directly correspond to the powers to which the concentrations of the reactants are raised in the rate law.

From the balanced equation, we can see that the stoichiometric coefficients of A, B, C, and D are all 1, 2, 1, and 1, respectively. Therefore, the rate law for this reaction is:

Rate = k[A]^1[B]^2[C]^1[D]^1

Simplifying this expression, the rate law for the given reaction is:

Rate = k[A][B]^2[C][D]

So, the rate law is not exactly what you suggested. Instead of k(A)(B)^2, it should be k[A][B]^2[C][D].