5: Chemical Kinetics Flashcards Preview

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Flashcards in 5: Chemical Kinetics Deck (15):

collision theory of chemical kinetics

rate of a reaction is proportional to the number of collisions per second between the reacting molecules



activation energy/energy barrier

minimum energy of collision necessary for a reaction to take place


Arrhenius equation


  • k is the rate constant of a reaction
  • rate of reaction increases with teperature 
  • rate of reaction increases as activation energy decreases
  • A is frequency factor which increases by increasing number of molecules in a vessel


transition state

theoretical structures that cannot be isolated with higher energy that reactants and products 


free energy

  • - ΔGrxn indicates an exergonic reactin
  • +ΔGrxn ​indicates an endergonic reaction



substances that increase reaction rate without themselves being consumed in the reaction

  • decrease activation energy, but doesn't affect free energy change, or Keq
  • may increase frequency of collisions between reactants, change relative orientation of the reactants, making higher percentage of collisions effective, reduce intramolecular bonding within reactant molecules, or donate electron dnesity to the reactants
  • homogenous catalysis: catalyst is in the same phase as the reactants
  • hetergenous catalysis: catalyst is in a distinct phase


transition state theory

when molecules collide with energy equal to or greather than the activation energy, they form a transition state in which the old bonds are weakened and the new bonds begin to form


definition of rate

for aA + bB --> cC + dD

rate = -Δ[A]/aΔt = -Δ[B]/bΔt = -Δ[C]/cΔt


rate law

rate= k[A]x[B]y

  • values of x and y are NOT same as stoichiometric coefficients
  • only includes concentrations of reactants and indicates how quickly the reaction will get to equilibrium
  • NOT same as equilibrium expression, which includes concentrations of products and reactants and indicates where reaction equilibrium lies
  • order of reaction = x+y
    • must be determined experimentally 
    • stoichiometric coefficients on reactant side of rate-determining step are equal to orders of reaction


experimental determination of rate law

  • for x & y... identify a pair of trials in which concentration of one of the reactants is changed while concentrations of all other reactant remain constant 
  • for k... plug in actual values from any of the trials 


zero order reaction

rate of formation of product is independent of changes of any reactants

rate=k (M/s)

  • temperature and addition of a catalysts are the only factors that can change rate of a zero-order raction


first-order reaction

rate is directly proportional to only one reactant

rate = k[A] or rate = k[B]

  • ex. radioactive decay


second-order reaction

rate is proportional to either concentration of 2 reactants or to the square of concentrations of a single reactant

rate=k[A][B] or rate=k[A]2



mixed-order reactions

reactions with rate orders that vary over the course of the reaction

ex. rate=k1[C][A]2/k2+k3[A]


broken-order reactions

fractional orders