Gas phase reaction help

The gas-phase reaction between trichloromethane, CHCl3, and chlorine, Cl2, has time-independent stoichiometry and can be represented as follows:

Equation 1 CHCl3(g) + Cl2(g) = CCl4(g) + HCl(g)

Under certain experimental conditions, the experimental rate equation was found to be:

Equation 2 J = kR[CHCl3][Cl2]^1/2

A three-step mechanism has been proposed for Reaction 1:

Equation 3 k1 Cl2 ↔ 2Cl• k-1

Equation 4 k2 CHCl3 + Cl• → HCl + CCl3•

Equation 5 k3 CCl3 + Cl• → CCl4

(i) Explain why the form of the experimental rate equation indicates that Reaction 1 cannot be an elementary reaction.

(ii) With reference to the three-step mechanism (Equations 3–5 ), and assuming that the second step (Equation 4) is rate-limiting, derive the chemical rate equation for this mechanism and then compare it with the experimental rate equation given in Equation 2.

(iii)The activation energy for the forward reaction, Ef , of step 1 (Equation 3) is 243.4 kJ mol^-1. Given this information, determine the activation energy for the reverse reaction, Er, and comment on the significance of the value (one sentence only).

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24 August 2012
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