Reaction Mechanics. Explain the concept of a reaction mechanism. Include: rate determining step, potential energy diagrams Additional KEY Terms Bimolecular.

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Presentation transcript:

Reaction Mechanics

Explain the concept of a reaction mechanism. Include: rate determining step, potential energy diagrams Additional KEY Terms Bimolecular Intermediate

2 NO (g) + O 2 (g) 2 NO 2 (g) For this reaction to occur in one step, 3 particles must collide - at exactly the same time with the correct orientation and enough energy. Three particle collisions are quite rare.

Reactions take place in steps. Each step involves a collision between two particles, or bimolecular. Simple reaction - take place in one elementary step. Multiple steps – complex reactions. 2 NO (g) + O 2 (g) 2 NO 2 (g)

N 2 O 2 - is called a reaction intermediate. (product in one step then reactant in another) Do not confuse with Activated Complex

The steps in which a reaction occurs is called the reaction's mechanism. Sum of the steps of a mechanism equals the total or net equation.

1: Cl 2 + O 3 ClO + O 2 2: O 3 O 2 + O 3: ClO + O Cl 2 + O 2 Decomposition of ozone, with a chlorine catalyst. Cl 2 – catalyst ClO and O – intermediates Neither is included in the Net Reaction. 2 O 3 (g) 3 O 2 (g)

Rate Determining Step Not all steps in a mechanism have the same rate. The step with the slowest rate is called the rate determining step (RDS). RDS usually has the greatest E A. The RDS determines the overall reaction rate.

RDS has the greatest affect on the overall rate. Changes to the reactants in the other steps will have very little effect on the overall rate.

exothermic endothermic -ΔH overall

1. B + B Efast endothermic 2. E + A Cslow endothermic A + 2B C PE (kJ) + ΔH

P + QX + T (slow, endo) X + PY + R (fast, exo) Y + ST (moderate, exo) g) What would a PE graph look like for this reaction? overall - ΔH a) 2P + Q + S  2T + R a) What is the net reaction? b) What are the reaction intermediates? c) Which is the rate determining step? d) What would be the effect of increasing [P]? e) What would be the effect of decreasing [Q]? f) What would be the effect of increasing [S]? b) X + Y c) First Step d)Increases the overall rate e) Decreases the overall rate f) No significant overall change in rate

PE (kJ) P + QX + T (slow, endo) X + PY + R (fast, exo) Y + ST (moderate, exo) overall - ΔH g) What would a PE graph look like for this reaction?

Most reactions occur in several steps. Each step is usually bimolecular. Sum of these steps must equal the net equation. Mechanism for a reaction can only be determined experimentally. Rate determining step is the slowest step and affects the rate of the reaction the most.

CAN YOU / HAVE YOU? Explain the concept of a reaction mechanism. Include: rate determining step, potential energy diagrams Additional KEY Terms Bimolecular Intermediate