Ch15_Kinetics_3 - Theories about Reaction Rates Reaction...

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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
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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 
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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?
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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
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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
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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 don’t have this minimum amount, reaction will  not occur even when correctly oriented. Major reason all collisions do not lead to reaction
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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 
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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
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9 Potential Energy Diagram (Exothermic)
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