  Reactant molecules MUST collide to produce a chemical reaction  The concentrations of reactants affect the # of collisions among reactants  For.

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Presentation transcript:

  Reactant molecules MUST collide to produce a chemical reaction  The concentrations of reactants affect the # of collisions among reactants  For reactions occurring in one step—rate of reaction is proportional to product of reactant concentrations  Rate = k[A] [B]  Rate of any reaction step dependent on collision frequency Collision Theory

 1)Collision Rates between reactants 2)% of collisions with reactants arranged in proper orientation to produce reaction. 3)% of collisions with energy energy (activation energy) to produce reaction. Variables Affecting Reaction Rate

  Increase in concentrations of reactants  Temperature increases  WHY? When do collision rates increase?

  Small percentage of collisions actually convert reactants to products. Why? 1)Molecular Orientation  Random orientation  Not all collisions have correct orientation 2)Molecular Energy at Collision  Molecules have different kinetic energies  Collision energy is energy source to get a reaction started Most collisions do NOT result in a chemical reaction!

  The amount of collision energy needed to overcome E a so the reaction can occur  Amount of energy needed for a chemical reaction to happen, energy needed to convert reactants to products. Activation Energy (E a )

 Activation Energy-- Exothermic

 Activation Energy-- Endothermic

 1)MUST have a collision 2)Collision must happen with the correct molecular orientation to generate a reaction 3)Collision energy ≥ E a When will reactions occur?

  HOW? Temperature increases reaction rate.

  Rate constant and reaction rate are temperature dependent.  Enables the activation energy for a reaction to be determined based on the relationship between reaction rate and temperature. Arrhenius Equation

  lnk = -E a ( 1/T ) + lnA R  k = rate constant  E a = activation energy (J)  R = J/mol  K  T = Kelvin  Z = proportionality constant, changes based on reaction Arrhenius Equation

  Different form of equation can be used to observe how temperature changes affect the rate constant (k)  ln (k 1 /k 2 ) = E a (1/T 2 – 1/T 1 ) R Arrhenius Equation

  Calculate activation energy (E a ) for HI decomposition with the following data. Example 1 Temperature (K)Rate Constant (M/s) x x x 10 -2