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SOLUTION 18.1b

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The first reaction is faster. The substrate substitution is the same, but iodide is the better leaving group over chloride. I- is larger and the weaker base (more stable anion). In both SN1 and SN2 reactions, the type of leaving group affects the activation energy.

Debatable that they are approximately the same rate. In both cases, the protic solvent is the nucleophile (solvolysis). The identity of the protic solvent is the only difference between the two reactions. Since the strength and concentration of nucleophile do not affect the rate of SN1 reactions, they should proceed at the same rate. A slight difference in rate may come from the solvent effects. Water is the more polar of the two solvents, which could stabilize the carbocation enough MORE than methanol to increase reaction rate in water.
 

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