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.