Consider the hypothetical reaction 2 A + B → 2 C + D. The following two-step mechanism is proposed for the reaction: Step 1: A + B → C + X Step 2: A + X → C + D. X is an unstable intermediate. (c) Your result for part (b) might be considered surprising for which of the following reasons: (i) The concentration of a product is in the rate law. (ii) There is a negative reaction order in the rate law. (iii) Both reasons (i) and (ii). (iv) Neither reasons (i) nor (ii).

Verified step by step guidance

1. Step 1: Understand the given reaction and proposed mechanism. The overall reaction is 2 A + B → 2 C + D, and the mechanism consists of two steps: Step 1: A + B → C + X and Step 2: A + X → C + D.
2. Step 2: Identify the intermediate in the mechanism. In this case, X is the intermediate, as it is produced in Step 1 and consumed in Step 2.
3. Step 3: Consider how the rate law is determined from the mechanism. The rate law is typically derived from the slowest step (rate-determining step) of the mechanism.
4. Step 4: Analyze the possible reasons for the surprising result in part (b). Consider if the rate law includes a product concentration or a negative reaction order, which are unusual characteristics.
5. Step 5: Evaluate the options given: (i) The concentration of a product is in the rate law, (ii) There is a negative reaction order in the rate law, (iii) Both reasons (i) and (ii), (iv) Neither reasons (i) nor (ii). Determine which of these reasons could make the result surprising based on typical rate law characteristics.

Key Concepts

Rate Law

The rate law of a reaction expresses the relationship between the rate of a chemical reaction and the concentration of its reactants. It is typically formulated as rate = k[A]^m[B]^n, where k is the rate constant, and m and n are the orders of the reaction with respect to each reactant. Understanding the rate law is crucial for analyzing how changes in concentration affect the speed of the reaction and for determining the overall reaction order.

Reaction Mechanism

A reaction mechanism is a step-by-step description of the pathway by which reactants are converted into products. It includes elementary steps, intermediates, and the transition states involved. In the given question, the proposed two-step mechanism illustrates how the unstable intermediate X is formed and consumed, which can influence the rate law and the observed reaction order.

Negative Reaction Order

Negative reaction order indicates that an increase in the concentration of a reactant leads to a decrease in the reaction rate. This can occur in complex reactions where intermediates or products affect the rate. Understanding negative reaction orders is essential for interpreting unexpected results in rate laws, such as when a product appears in the rate law, which can be counterintuitive.