SN1 reaction undergoes through a carbocation intermediate as follows:

[R = t-Bu, iso-Pr, Et, Me] $(X = C l, B r, I)$
The correct statements are:
I. The decreasing order of rate of SN1reaction is $t - B u X > i s o - P r X > E t X > M e X$
II. The decreasing order of ionization energy is $M e X > E t X > i s o - P r X > t - B u X$
III. The decreasing order of energy of activation is $t - B u X > i s o - P r X > E t X > M e X$

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Solution

I and II are correct.
Explanation:
(1) Formation of the carbocation is Rate determining step, more stable the cation more easily it will form. Hence rate order is $t - B u X > i s o - P r X > E t X > M e X$
(2) Formation of carbocation $C H_{3}$ is most difficult hence ionization energy is highest. Similarly, $E t X$ has lesser ionization energy than $M e X$ and so on.
(3) Order of energy of activation is $M e X > E t X > i s o - P r X > t - B u X$

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