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Chemistry Tutorial

Chemistry Tutorial - Tutorial 1 Sulfonation of benzene has...

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Tutorial 1. Sulfonation of benzene has the following mechanism: 1. 2. 3. 4. a. Write an overall equation for the reaction. b. Write the overall rate law in terms of the initial rate of the reaction. Plan: The overall reaction can be obtained by adding the three steps together. The overall rate law for the mechanism is determined from the slowest step (the rate-determining step). An overall rate law can only include reactants and products; intermediates cannot be included in the rate law. Express [intermediate] in terms of [reactant]. Solution: Add the steps together and cancel: H 2 SO 4 + C 6 H 6 àฏ C 6 H 5 SO 3 H + H 2 O For the slow step: Rate = k[SO 3 ][C 6 H 6 ] SO 3 is an intermediate and cannot be included in the overall rate law. SO 3 is produced in step 1 and its concentration is dependent on k 1 and [H 2 SO 4 ]: [SO 3 ] = k 1 [H 2 SO 4 ] 2 Substituting for [SO 3 ] in the rate law from the slow step: Rate = k 2 { k 1 [H 2 SO 4 ] 2 }[C 6 H 6 ] Rate = k [H 2 SO 4 ] 2 [C 6 H 6 ] 2. Even when a mechanism is consistent with the rate law, later work may show it to be incorrect. For example, the reaction between hydrogen and iodine has this rate law: Rate = k [H 2 ][I 2 ]. The long-accepted mechanism had a single bimolecular step; that is, the overall reaction was thought to be elementary:
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In the 1960s, however, spectroscopic evidence showed the presence of free iodine atoms during the reaction. Kineticists have since proposed a three-step mechanism: Show that this mechanism is consistent with the rate law.
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