A Nanoscale View: Elementary Reactions A Nanoscale View: Elementary Reactions Most reactions occur through a series of simple steps or elementary reactions.

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A Nanoscale View: Elementary Reactions A Nanoscale View: Elementary Reactions Most reactions occur through a series of simple steps or elementary reactions. Elementary reactions could be unimolecular - rearrangement of a molecule bimolecular - reaction involving the collision of two molecules or particles termolecular - reaction involving the collision of three molecules or particles

2NO 2 (g) + F 2 (g) 2NO 2 F (g) If the reaction took place in a single step the rate law would be: rate = k [NO 2 ] 2 [F 2 ] Observed: rate = k 1 [NO 2 ] [ F 2 ] If the observed rate law is not the same as if the reaction took place in a single step that more than one step must be involved Elementary Reactions and Mechanism

Elementary Reactions A possible reaction mechanism might be: Step oneNO 2 + F 2 NO 2 F + F (slow) Step twoNO 2 + F NO 2 F (fast) Overall 2NO 2 + F 2 2NO 2 F slowest step in a multi-step mechanism the step which determines the overall rate of the reaction rate = k 1 [NO 2 ] [ F 2 ] rate = k 1 [NO 2 ] [ F 2 ] Rate Determining Step

T his type of plot shows the energy changes during a reaction. T his type of plot shows the energy changes during a reaction. Reaction profile of rate determining step HH activation energy Potential Energy Reaction coordinate

What Potential Energy Curves Show What Potential Energy Curves Show Exothermic Reactions Endothermic Reactions Activation Energy (E a ) of reactant or the minimum energy required to start a reaction Effect of catalysts Effect of temperature

Exothermic reaction Endothermic reaction Examples of reaction profiles

High activation energy (kinetic) Low heat of reaction (thermodynamic) Low activation energy (kinetic) High heat of reaction (thermodynamic) Examples of reaction profiles

Catalysts Lowers E a

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