Rate Laws More Complex Reactions

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Presentation transcript:

Rate Laws More Complex Reactions The collision theory describes the collision possibilities of molecules as follows: Most reactions – involve collisions of 2 particles H2 + Cl2 2HCl Uncommon situations – collision of 3 particles at the same time 2SO2(g) + O2(g) 2SO3(g) Impossible situations – simultaneous collision of 4 or more particles 2H2(g) + 2NO(g) N2(g) + 2H2O Such a reaction is believed to occur through a # of steps or smaller collisions called “elementary processes” in which only 2 or 3 particles collide at once.

REACTION MECHANISM Refers to the series of steps according to which reaction occurs Ex. 2H2(g) + 2NO(g) N2(g) + 2H2O The steps in the reaction mechanism are: (1) H2 + 2NO N2O + H2O (2) H2 + N2O N2+ H2O Overall: 2H2 + 2NO N2 + 2H2O

But the actual, observed rate law is found to be R = k [H2][NO]2 The possibility that reactions may actually consist of a number of sequential steps rather than a single large step also helps explain rate laws of complex reactions. For example, in the previous slide, the predicted rate law is R = k [H2]2[NO]2 But the actual, observed rate law is found to be R = k [H2][NO]2 which appears to be the rate law for just ONE of the steps in the reaction mechanism (the slowest step). Explain the dishwashing analogy. You have stackers (3 people) 40 dishes/min, washer (1) 10 dishes/min & dryers (2) 30 dishes/min. What is the overall rate of dish washing? 10 dishes/min…….slowest step

So……. rate law of the slowest step = rate law of overall reaction The only species whose concentration will affect the rate of reaction, & thus the only species whose concentration appear in the rate law, are those which are involved in the slowest step called “rate determining step” (RDS)

SAMPLE QUESTION: Reaction: 4HBr + O2 2H2O + 2Br2 Steps: 1) HBr + O2 HOOBr (slow) 2) HOOBr + HBr 2HOBr (fast) 3) 2HOBr + 2HBr 2H2O + 2Br2 (fast) 4HBr + O2 2H2O + 2Br2 Write the rate law for the overall reaction.

Finding Rate Laws from Collected Data REACTION: A + A B TRIAL Concentration of A Rate of Reaction 1 0.05 M 3 x 10-4 M/s 2 0.10 M 12 x 10-4 M/s 3 0.20 M 48 x 10-4 M/s a) Find the rate law. b) Find the value of “k”.

2) REACTION: I- + HSO4- TRIAL [I-] (M) [HSO4-] Rate of reaction (M/min) 1 1.0 5 2 2.0 10 3 3.0 15 4 20 6 45 7 4.0 80

3) REACTION: A+B+C D + E Find the rate law & “k” Trial [A] [B] [C] Rate (M/s) 1 2 0.5 6 4 24 3 54 12 5 36 7 8