㰀琀椀琀氀攀㸀䄀甀琀漀戀椀漀最爀愀瀀栀礀 ⸀ 吀栀攀 一漀戀攀氀 倀爀椀稀攀 椀渀 䌀栀攀洀椀猀琀爀礀 㰀䴀䔀吀䄀 栀琀琀瀀ⴀ攀焀甀椀瘀㴀䌀漀渀琀攀渀琀ⴀ匀琀礀氀攀ⴀ吀礀瀀攀 挀漀渀琀攀渀琀㴀琀攀砀琀⼀挀猀猀㸀㰀䰀䤀一䬀 栀爀攀昀㴀∀猀琀礀氀攀⸀挀猀猀∀ 琀礀瀀攀㴀琀攀砀琀⼀挀猀猀 爀攀氀㴀猀琀礀氀攀猀栀攀攀琀㸀㰀栀攀愀搀㸀㰀⼀栀攀愀搀㸀
㰀椀洀最 猀爀挀㴀∀⸀⸀⼀昀椀最⼀爀攀愀挀琀椀漀渀⸀最椀昀∀ 眀椀搀琀栀㴀 栀攀椀最栀琀㴀㜀㔀 愀氀椀最渀㴀∀爀椀最栀琀∀㸀 A mechanism for a reaction is a collection of elementary瀀爀漀挀攀猀猀攀猀 ⠀愀氀猀漀 挀愀氀氀攀搀 攀氀攀洀攀渀琀愀爀礀 猀琀攀瀀猀 漀爀 攀氀攀洀攀渀琀愀爀礀 爀攀愀挀琀椀漀渀猀⤀ that explains how the overall reaction proceeds.㰀倀㸀 A mechanism is a proposal from which you can work out a rate law琀栀愀琀 愀最爀攀攀猀 眀椀琀栀 琀栀攀 漀戀猀攀爀瘀攀搀 爀愀琀攀 氀愀眀猀⸀ 吀栀攀 昀愀挀琀 琀栀愀琀 愀 洀攀挀栀愀渀椀猀洀 explains the experimental results is not a proof that the mechanism is挀漀爀爀攀挀琀⸀ 䄀 洀攀挀栀愀渀椀猀洀 椀猀 漀甀爀 爀愀琀椀漀渀愀氀椀稀愀琀椀漀渀 漀昀 愀 挀栀攀洀椀挀愀氀 爀攀愀挀琀椀漀渀Ⰰ and devising mechanism is an excelent academic exercise.㰀倀㸀 The animation here shows an elementary step of two molecules coliding眀椀琀栀 攀愀挀栀 漀琀栀攀爀 愀渀搀 攀砀挀栀愀渀最攀 愀 栀礀搀爀漀最攀渀 愀琀漀洀 椀渀 琀栀攀 瀀爀漀挀攀猀猀⸀ Since elementary processes are the language of mechanism, let us first搀攀昀椀渀攀 攀氀攀洀攀渀琀愀爀礀 瀀爀漀挀攀猀猀攀猀 漀爀 猀琀攀瀀猀⸀
圀栀攀渀 愀 洀漀氀攀挀甀氀攀 漀爀 椀漀渀 搀攀挀漀洀瀀漀猀攀猀 戀礀 椀琀猀攀氀昀Ⰰ 猀甀挀栀 愀渀 攀氀攀洀攀渀琀愀爀礀 猀琀攀瀀 椀猀 called a unimolecular step (or process).䄀 甀渀椀洀漀氀攀挀甀氀愀爀 猀琀攀瀀 椀猀 愀氀眀愀礀猀 愀 昀椀爀猀琀 漀爀搀攀爀 爀攀愀挀琀椀漀渀⸀ 吀栀攀 昀漀氀氀漀眀椀渀最 examples are given to illustrate this point:㰀倀㸀
䄀 㰀䈀㸀戀椀洀漀氀攀挀甀氀愀爀 瀀爀漀挀攀猀猀 㰀⼀䈀㸀椀渀瘀漀氀瘀攀猀 琀眀漀 爀攀愀挀琀椀渀最 洀漀氀攀挀甀氀攀猀 漀爀 椀漀渀猀⸀
The rates for these steps are 2nd order, and some examples are given琀漀 椀氀氀甀猀琀爀愀琀攀 栀漀眀 礀漀甀 猀栀漀甀氀搀 最椀瘀攀 琀栀攀 爀愀琀攀 攀砀瀀爀攀猀猀椀漀渀⸀
The simulation illustrates a bimolecular process.㰀搀椀爀㸀
NO + O3 = NO2 +O2, Rate = k [NO] [O3]䔀氀攀洀攀渀琀愀爀礀 瀀爀漀挀攀猀猀攀猀 愀爀攀 眀爀椀琀琀攀渀 琀漀 猀栀漀眀 栀漀眀 愀 挀栀攀洀椀挀愀氀 爀攀愀挀琀椀漀渀 progresses leading to an overall reaction. Such a collection椀猀 挀愀氀氀攀搀 愀 㰀䈀㸀爀攀愀挀琀椀漀渀 洀攀挀栀愀渀椀猀洀㰀⼀䈀㸀⸀ In a mechanism, elementary steps proceed at various speeds. The slowest猀琀攀瀀 椀猀 琀栀攀 爀愀琀攀ⴀ搀攀琀攀爀洀椀渀椀渀最 猀琀攀瀀⸀ 吀栀攀 漀爀搀攀爀 昀漀爀 琀栀愀琀 攀氀攀洀攀渀琀愀爀礀 瀀爀漀挀攀猀猀 is the order of a reaction, but the concentrations of reactants in that step洀甀猀琀 戀攀 攀砀瀀爀攀猀猀攀搀 椀渀 琀攀爀洀猀 漀昀 琀栀攀 挀漀渀挀攀渀琀爀愀琀椀漀渀猀 漀昀 琀栀攀 爀攀愀挀琀愀渀琀猀⸀
倀爀漀戀氀攀洀
Solution
椀椀⸀ 一伀㰀匀唀䈀㸀㈀㰀⼀匀唀䈀㸀 ⬀ 䘀 㴀 一伀㰀匀唀䈀㸀㈀㰀⼀匀唀䈀㸀䘀 ⠀昀愀猀琀⤀㰀戀爀㸀
Since step i is the rate-determining step, the rate law is㰀倀爀攀㸀
1 d[NO2] ⴀ ⴀⴀⴀ ⴀⴀⴀⴀⴀⴀ 㴀 㰀䤀㸀欀㰀⼀䤀㸀 嬀一伀㰀匀唀䈀㸀㈀㰀⼀匀唀䈀㸀崀 嬀䘀㰀匀唀䈀㸀㈀㰀⼀匀唀䈀㸀崀
2 dt㰀⼀倀爀攀㸀
Addition of i. and ii. gives the overall reaction.㰀倀㸀㰀䈀㸀㰀䤀㸀䐀椀猀挀甀猀猀椀漀渀㨀㰀⼀䤀㸀㰀⼀䈀㸀㰀䈀刀㸀
This example illustrates that the overall reaction equation has 渀漀琀栀椀渀最 琀漀 搀漀 眀椀琀栀 琀栀攀 漀爀搀攀爀 漀昀 琀栀攀 爀攀愀挀琀椀漀渀⸀ 吀栀攀 攀氀攀洀攀渀琀愀爀礀 瀀爀漀挀攀猀猀 椀渀
the rate-determining step determines the order.㰀倀㸀
Other possible elementary steps in this reaction are:㰀䐀䤀刀㸀
F + F -> F2
䘀 ⬀ 䘀㰀匀唀䈀㸀㈀㰀⼀匀唀䈀㸀 ⴀ㸀 䘀㰀匀唀䈀㸀㈀㰀⼀匀唀䈀㸀 ⬀ 䘀㰀䈀刀㸀
NO2F + F -> F + NO2F
㰀⼀䐀䤀刀㸀
but they do not lead to the formation of products.㰀⼀䐀䤀刀㸀
吀漀 瀀爀漀瀀漀猀攀 愀 洀攀挀栀愀渀椀猀洀 爀攀焀甀椀爀攀猀 琀栀攀 欀渀漀眀氀攀搀最攀 漀昀 挀栀攀洀椀猀琀爀礀 琀漀 最椀瘀攀 plausible elementary processes. A freshman in chemistry will not be愀猀欀攀搀 琀漀 瀀爀漀瀀漀猀攀 洀攀挀栀愀渀椀猀洀猀Ⰰ 戀甀琀 礀漀甀 眀椀氀氀 戀攀 愀猀欀攀搀 琀漀 最椀瘀攀 琀栀攀 爀愀琀攀 laws from a given mechanism.㰀䠀㌀㸀 Summary㰀⼀䠀㌀㸀 The number of particle involved in an elementary step is called the㰀䈀㸀洀漀氀攀挀甀氀愀爀椀琀礀㰀⼀䈀㸀Ⰰ 愀渀搀 椀渀 最攀渀攀爀愀氀Ⰰ 眀攀 挀漀渀猀椀搀攀爀 漀渀氀礀 琀栀攀 洀漀氀攀挀甀氀愀爀椀琀礀 漀昀 1, 2, and 3. Types of elementary steps are summarized below.䤀渀 琀栀攀 琀愀戀氀攀Ⰰ 䄀Ⰰ 䈀Ⰰ 愀渀搀 䌀 爀攀瀀爀攀猀攀渀琀 爀攀愀挀琀愀渀琀猀Ⰰ 椀渀琀攀爀洀攀搀椀愀琀攀猀Ⰰ 漀爀 products in the elementary process. 㰀倀㸀
| 1 | A -> products | rate = k [A]㰀吀刀㸀㰀吀䠀㸀㈀ 㰀吀䐀㸀䄀 ⬀ 䄀 ⴀ㸀 瀀爀漀搀甀挀琀猀㰀䈀刀㸀 䄀 ⬀ 䈀 ⴀ㸀 瀀爀漀搀甀挀琀猀 | rate = k [A]2 rate = k [A] [B]㰀吀刀㸀㰀吀䠀㸀㌀ 㰀吀䐀㸀䄀 ⬀ 䄀 ⬀ 䄀 ⴀ㸀 瀀爀漀搀甀挀琀猀㰀䈀刀㸀 䄀 ⬀ ㈀ 䈀 ⴀ㸀 瀀爀漀搀甀挀琀猀㰀䈀刀㸀䄀 ⬀ 䈀 ⬀ 䌀 ⴀ㸀 瀀爀漀搀甀挀琀猀 | rate = k [A]3 rate = k [A] [B]2 㰀䤀㸀爀愀琀攀 㴀 欀㰀⼀䤀㸀 嬀䄀崀 嬀䈀崀 嬀䌀崀 |
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