Consider the reaction in which HCl adds across the double bond of ethene: HCl + H₂C=CH₂ → H₃C-CH₂Cl The following mechanism, with the accompanying energy diagram, has been suggested for this reaction:
Step 1 HCl + H₂C=CH₂ → H₃C=CH₂⁺ + Cl^-
Step 2 H₃C=CH₂⁺ + Cl^- → H₃C-CH₂Cl
a. Based on the energy diagram, determine which step is rate limiting.

Verified step by step guidance

1. Identify the rate-limiting step as the one with the highest energy barrier in the energy diagram.
2. Examine the energy diagram to find the step with the largest activation energy, which is the energy difference between the reactants and the transition state.
3. In the given mechanism, compare the energy levels of the transition states for Step 1 and Step 2.
4. The step with the higher transition state energy compared to its starting materials is the rate-limiting step.
5. Conclude that the step with the highest energy barrier is the rate-limiting step, as it determines the overall reaction rate.

Key Concepts

Reaction Mechanism

A reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. In this case, the mechanism involves two steps: the formation of a carbocation intermediate and its subsequent reaction with a chloride ion. Understanding the mechanism is crucial for analyzing how reactants transform into products and identifying the rate-limiting step.

Rate Limiting Step

The rate limiting step in a reaction mechanism is the slowest step that determines the overall rate of the reaction. It is often characterized by a higher activation energy compared to other steps. In the provided mechanism, identifying which step is rate limiting involves examining the energy diagram to see where the largest energy barrier occurs, indicating the step that will take the longest to proceed.

Energy Diagram

An energy diagram visually represents the energy changes during a chemical reaction, showing the energy of reactants, products, and intermediates. It typically includes activation energies for each step and the overall energy change. By analyzing the energy diagram associated with the reaction, one can determine the relative energies of the transition states and identify the rate limiting step based on the highest peak in the diagram.