Reaction Kinetics

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Chapter 17

• Reaction Kinetics

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• Reaction Mechanism step-by-step sequence of
  reactions by which the overall chemical change
  occurs.

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I2(g) H2(g) ? 2HI(g) Two possible reaction
mechanisms determined experimentally Step
1 I2 ? 2 I Step 2 2I H2 ? 2HI Net I2
H2 ? 2HI or Step 1 I2 ? 2 I Step 2 I H2
? H2I Step 3 H2I I ? 2HI Net I2 H2 ?
2HI
I H2I do not appear in the net equation
(Intermediates)
Homogeneous Reaction Products and Reactants in
the same phase.
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• Collision Theory describes chemical reactions
  in terms of molecular collisions.
• Collisions must
• Have enough energy
• Have proper orientation

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• Activation Energy the minimum amount of energy
  required to transform reactants into an activated
  complex.
• Activated complex transitional structures that
  result from effective collisions

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Reaction Rate time it takes for reactants to
make products

• Factors that affect Reaction Rates
• Nature of Reactants
• Ionic or covalent
• Phase (gas vs. liquid vs. solid)
• Surface Area
• Temperature
• Concentration
• Catalyst
• Lowers Activation Energy

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Catalyzed Reactions
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Rate Law

• aA bB ? dD eE
• Rate kAnBm
• molar concentration in M (mol/L)
• Rate rate of reaction in M/s
• k rate constant determined experimentally
• A and B concentration of A and B in (M)
• n and m order of the reactants determined
  experimentally

If n2 then A2 The reaction is 2nd order with
respect to A If m1 the B The reaction is 1st
order with respect to B Overall the reaction
is 3rd order
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Three experiments were performed to measure the
initial rate of this reaction 2HI(g) ? H2(g)
I2(g) The results for the three experiments in
which only the HI concentration was varied are as
follows
Write the rate law for the reaction. Find the
value and units of the specific rate constant.
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Three experiments were performed to measure the
initial rate of the reaction A B ? C As the
concentrations were varied the following data was
obtained
Write the rate law for the reaction. Find the
value and units of the specific rate constant.
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Homework

• Pages 581-582
• Numbers 1,11 (2nd order in A, 1st
  order in B),12