Ch_13_III_Mechanisms (1)

Ch_13_III_Mechanisms (1) - Part IIMechanisms of Chemical...

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Part II—Mechanisms of Chemical Reactions First part: we determined the rate of overall reactions This part: we examine elementary steps of reaction [ ] 2 2 from experimental dat Rate a kNO = R e c a l •Why this reaction is second-order with respect to [NO 2 ]. •Why this reaction is zero-order with respect to [CO]. •What the reaction mechanism is, that is, the elementary steps of the reaction? ( ) ( )( ) ( ) 22 NO g CO g NO g CO g +→ + F o r x m p , W i n s we
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Takes place in two elementary steps (or reactions): () ( ) ( ) ( ) 22 3 (Step 1) NO g NO g NO g NO g +→ + ( ) ( ) ( ) 32 2 (Step 2) NO g CO g NO g CO g + Two necessary, but not sufficient , conditions for a correct reaction mechanism: •The sum of the elementary steps = overall reaction •Mechanism predicts correct experimental reaction order ( ) ( ) ( ) ( ) NO g CO g NO g CO g +
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CHECK: the sum of the elementary steps = overall reaction () ( ) ( ) ( ) 22 3 (Step 1) NO g NO g NO g NO g +→ + ( ) ( ) ( ) 32 2 (Step 2) NO g CO g NO g CO g + ( ) ( ) ( ) ( ) NO g CO g NO g CO g + ( ) ( )( ) NO g CO g NO g CO g + NO 3 is called an intermediate species or an intermediate Intermediates are very reactive and have very small concentrations.
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Digression—Elementary Steps (Reactions) Rare
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() ( ) ( ) ( ) 1 22 3 (Step 1) k NO g NO g NO g NO g +⎯ →+ ( ) ( ) ( ) 2 32 2 (Step 2) k NO g CO g NO g CO g [ ][ ][ ] 2 11 2 2 1 2 rate kN O N O O == [ ][ ] 3 rate = O C O
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Ch_13_III_Mechanisms (1) - Part IIMechanisms of Chemical...

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