Chapter 14: Rates of Reaction

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Chapter 14 Rates of Reaction
Chemistry 1062 Principles of Chemistry II Andy
Aspaas, Instructor
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Chemical kinetics

• Study of the rate or speed of reactions
• Rate may be affected by any of the following
• Concentration of reactants
• Presence or concentration of a catalyst
• Temperature at which reaction occurs
• Surface area of solid reactant or catalyst

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Reaction rate

• Reaction rate can be defined in terms of
  appearance of product or disappearance of
  reactant
• Rates are given as a change in molar
  concentration in a certain time interval, unit
  mol/(Ls)
• For the reaction A 2B ? C, the rate of the
  reaction may be expressed 3 ways

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Dependence of rate on concentration

• Reaction rate usually depends on the
  concentrations of reactants and catalysts
• Rate law shows this dependence
• For the reaction a A b B ? d D e E
• (reactants A and B form D and E with catalyst C)
• Rate k AmBnCp
• where m, n, and p are exponents, usually integers
• m, n, and p must be detemined experimentally!

C
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Reaction order

• The reaction order can be given with respect to a
  certain reactant, or overall
• For a certain reactant, its the exponent in the
  rate law
• Overall, its the sum of the exponents
• Ex. 2NO(g) 2H2(g) ? N2(g) 2H2O(g)
• Rate k NO2H2
• The reaction is second order in NO, first order
  in H2, and third order overall.

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Determining the rate law

• The method of initial rates is often used to
  determine the rate law and order of a reaction
• Several experiments are run, varying the
  concentration of individual reactants and
  catalysts
• The exponents in the rate law can be determined
  algebraically
• The rate constant is determined by substituting
  the concentrations of any experiment into the
  rate law

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Method of initial rates
OH

• I(aq) ClO(aq) ? IO(aq) Cl(aq)
• What is the rate law, and what is the rate
  constant, k?
• Concentrations are in mol/L, rates are in
  mol/(Ls)

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Change of concentration writh time

• First order rate law for aA ? products
• Second-order rate law for aA ? products

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Change of concentration with time

• Zero-order rate law for aA ? products
• Half life (t1/2) time at which At
  -(1/2)A0
• (Reactant concentration is at 1/2 its initial
  value
• Radioactive decay, etc.

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Graphing kinetic data

• While the method of initial rates is a quick way
  of determining reaction order, graphing the data
  is more effective
• Concentration of a reactant is measured in
  several time intervals throughout the reaction
• Integrated rate laws can be rearranged if
  necessary to y mx b format for graphing,
  where m is the slope and b is the y-intercept

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Determination of reactant order by graphing

• Graph 3 times for the 3 rate laws, and determine
  which has a straight line
• Zero order A vs. t is linear, slope -k
• 1st order lnA vs. t is linear, slope -k
• 2nd order 1/A vs. t is linear, slope k

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Rate dependence on temperature

• Collision theory rate constant of a reaction is
  a factor of molecular collision frequency,
  activation energy, and the fraction of collisions
  which occur with a constructive orientation
• Activation energy minimum molecular energy
  required in order for a collision to produce a
  reaction
• Transition-state theory reactions must pass
  through an activated complex, an unstable
  grouping of atoms that has an equal chance of
  breaking into reactants or products

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Potential energy diagrams

• Plot of potential energy (kJ/mol) vs. the course
  of a reaction (reactants becoming products by
  passing through an activated complex)
• NO Cl2 ? NOCl2 ? NOCl Cl

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Arrhenius equation

• Rate constant of a chemical reaction is related
  to the activation energy and temperature
• A is a constant, based on collision frequency,
  and proper orientation, etc.

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Elementary reactions

• A chemical reaction may consist of several steps
  in order to get from reactants to products
• Elementary reaction a single molecular event,
  ex. the collision of molecules, or the separation
  of a molecule
• Reaction intermediate species produced during a
  reaction that does not appear in the net equation
  (cancels out when elementary reactions are added)
• The order of a rate law for an elementary
  reaction can be predicted, but without knowledge
  of the mechanism, the rate law for an overall
  reaction cannot be predicted

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Catalysis

• A catalyst increases the rate of a reaction but
  is not consumed
• Must be re-generated stiochiometrically in an
  elementary reaction
• Catalysts do not appear as a reactant or product
  in the overall reaction (shown above arrow)
• Work by reducing activation energy