Ch15_Kinetics_3 - 1 Theories about Reaction Rates Reaction...

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Unformatted text preview: 1 Theories about Reaction Rates Reaction rate depends on [reactants] & T Collision Theory Based on Kinetic Molecular Theory Accounts for both effects on molecular level Central Idea Molecules must collide to react Greater # collision/sec = greater reaction rate 2 Theories about Reaction Rates Reaction rate depends on [reactants] & T Collision Theory As [reactants] # Collisions Reaction rate As T Molecular speed Molecules collide with more force (energy) Reaction rate . 3 Collision Theory Rate of reaction proportional to # effective collisions/sec among reactant molecules Effective collision 1 that give rise to product Ex. At Room Temperature & Pressure H 2 & I 2 molecules undergoing 10 10 collisions/sec Yet reaction takes a long time Not all collisions lead to reaction Only very small % of all collisions leads to net change. Why? 4 1. Molecular Orientation Molecules must be oriented in certain way during collisions for reaction to occur Ex. NO 2 Cl + Cl NO 2 + Cl 2 Cl must come in pointing directly at another Cl atom for Cl 2 to form Wrong Orientation Correct Orientation 5 2. Temperature Greatly Affects Rates Over moderate T range, E a unchanged As T , More molecules have E a So more molecules undergo reaction Reaction rate as T Shaded area is Proportional to fraction of collisions with energy E act Minimum KE for reaction 6 3. Activation Energy, E a Molecules must possess certain amount of kinetic energy (KE) in order to react. Activation Energy, E a Minimum KE needed for reaction to occur Get energy from collision with other molecules Upon collision, KE converted to potential energy (PE) used to stretch, bend & break bonds leading to chemical reaction If molecules move too slowly, too little KE, just bounce off each other If dont have this minimum amount, reaction will not occur even when correctly oriented. Major reason all collisions do not lead to reaction 7 Transition State Theory Used to explain details of reactions What happens when reactant molecules collide Potential Energy Diagram To visualize what actually happens during successful collision Relationship between E a & developing Total PE 8 Potential Energy Diagram Reaction Coordinate (progress of reaction) Potential Energy Activation energy (E a ) = hill or barrier between reactants & products heat of reaction ( H) = difference in PE between products & reactants H reaction = H products H reactants 9 Potential Energy Diagram (Exothermic)...
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Ch15_Kinetics_3 - 1 Theories about Reaction Rates Reaction...

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