Download presentation
Presentation is loading. Please wait.
Published byAugust Green Modified over 9 years ago
1
Reaction Mechanisms Chapter 12, Section 6
2
Reaction Mechanisms The sequence of events that describes the actual process by which reactants become products is called the reaction mechanism. Proposed Mechanism: Step 1: NO 2 + NO 2 NO 3 + NO Step 2: NO 3 + CO NO 2 + CO 2 NO 2 + CO NO + CO 2 Rate = k[NO 2 ] 2
3
Reaction Mechanisms Reactions may occur all at once or through several discrete steps. Each of these processes is known as an elementary reaction or elementary process.
4
Reaction Mechanisms The molecularity of a process tells how many molecules are involved in the process. Termolecular elementary reactions seldom occur.
5
Multistep Mechanisms In a multistep process, one of the steps will be slower than all others. The overall reaction cannot occur faster than this slowest, rate-determining step.
6
Reaction Mechanisms must satisfy 2 requirements: 1.The sum of the elementary steps must give the overall balanced equation. 2.The mechanism must agree with the experimentally determined rate law.
7
Slow Initial Step The rate law for this reaction is found experimentally to be Rate = k [NO 2 ] 2 CO is necessary for this reaction to occur, but the rate of the reaction does not depend on its concentration. This suggests the reaction occurs in two steps. NO 2 (g) + CO (g) NO (g) + CO 2 (g)
8
Slow Initial Step A proposed mechanism for this reaction is Step 1: NO 2 + NO 2 NO 3 + NO (slow) Step 2: NO 3 + CO NO 2 + CO 2 (fast) Does it make sense? What is the intermediate in this reaction?
9
Fast Initial Step The rate law for this reaction is found to be Rate = k [NO] 2 [Br 2 ] Because termolecular processes are rare, this rate law suggests a two-step mechanism. 2 NO (g) + Br 2 (g) 2 NOBr (g)
10
Fast Initial Step A proposed mechanism is Step 2: NOBr 2 + NO 2 NOBr (slow) Step 1 includes the forward and reverse reactions. Step 1: NO + Br 2 NOBr 2 (fast)
11
Fast Initial Step The rate of the overall reaction depends upon the rate of the slow step. The rate law for that step would be Rate = k 2 [NOBr 2 ] [NO] But how can we find [NOBr 2 ]?
12
Fast Initial Step NOBr 2 can react two ways: –With NO to form NOBr –By decomposition to reform NO and Br 2 The reactants and products of the first step are in equilibrium with each other. Therefore, Rate f = Rate r
13
Fast Initial Step Because Rate f = Rate r, k 1 [NO] [Br 2 ] = k −1 [NOBr 2 ] Solving for [NOBr 2 ] gives us k1k−1k1k−1 [NO] [Br 2 ] = [NOBr 2 ]
14
Fast Initial Step Substituting this expression for [NOBr 2 ] in the rate law for the rate-determining step gives k2k1k−1k2k1k−1 Rate =[NO] [Br 2 ] [NO] = k [NO] 2 [Br 2 ]
15
Catalysts A catalyst speeds up a reaction without being consumed. Identify the catalyst in the reaction mechanism below: Cl (g) + O 3(g) ClO (g) + O 2(g) O (g) + ClO (g) Cl (g) + O 2(g) How is it different than an intermediate?
Similar presentations
© 2024 SlidePlayer.com Inc.
All rights reserved.