Chapter 13 Chemical Kinetics

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Chapter 13Chemical Kinetics

• Factors that affect reaction rates
• How to express reaction rates
• Rate Laws
• Effects of temperature reaction rates
• Mechanisms of reactions
• Catalysis

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Kinetics

• Studies the rate at which a chemical process
  occurs.
• Besides information about the speed at which
  reactions occur, kinetics also sheds light on the
  reaction mechanism (exactly how the reaction
  occurs).

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Factors That Affect Reaction Rates

• Physical State of the Reactants
• In order to react, molecules must come in contact
  with each other. The more rapidly reactant
  collide with each other, the more rapidly they
  react
• Most of reactions we consider are homogenous,
  involving either gases or liquid solutions

• Concentration of Reactants
• -As the concentration of reactants increases, so
  does the likelihood that reactant molecules will
  collide.

• Temperature
• At higher temperatures, reactant molecules have
  more kinetic energy, move faster, and collide
  more often and with greater energy.

• Presence of a Catalyst
• Catalysts speed up reactions by changing the
  mechanism of the reaction.
• Catalysts are not consumed during the course of
  the reaction.

On the molecular level, reaction rates depend on
the frequency of collisions between molecules.
The higher the frequency of collision, the faster
the reaction
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Reaction Rates

• Reaction rate is the change in concentration of
  reactants or products per unit time. Usually
  expressed in units of M/s.

Let us consider a hypothetical reaction where

• Rates of reactions can be determined by
  monitoring the change in concentration of either
  reactants or products as a function of time.

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An Example
For the time interval between t1 0 sec and t2
20 sec
For the time interval between t1 20 sec and t2
40 sec
Practice Exercise calculate the average rate of
the above reaction over the time interval between
0 and 40 second.
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Change of Rate with Time
Consider the following actual reaction,
C4H9Cl(aq) H2O(l) ??? C4H9OH(aq) HCl(aq)
Butyl chloride
Butyl alcohol
In this reaction, the concentration of butyl
chloride, C4H9Cl, was measured at various times.

• The average rate of the reaction over each
  interval is the change in concentration divided
  by the change in time

Average rate -
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Change of Rate with Time
C4H9Cl(aq) H2O(l) ??? C4H9OH(aq) HCl(aq)

• Note that the average rate decreases as the
  reaction proceeds.

• This is because as the reaction goes forward,
  concentrations of reactants decrease and
  therefore there are fewer collisions between
  reactant molecules.

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Change of Rate with Time
C4H9Cl(aq) H2O(l) ??? C4H9OH(aq) HCl(aq)

• The slope of a line tangent to the curve at any
  point is the instantaneous rate at that time.

• All reactions slow down over time. Therefore,
  the best indicator of the rate of a reaction is
  the instantaneous rate at the beginning, i.e. the
  initial rate.

What is the instantaneous rate at t 600 s?
In what follows, the term rate means
instantaneous rate, unless indicated otherwise
What is the instantaneous rate at t 0 s?
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Reaction Rates and Stoichiometry
C4H9Cl(aq) H2O(l) ??? C4H9OH(aq) HCl(aq)

• In this reaction, the stoichiometric ratio of
  C4H9Cl to C4H9OH is 11.
• Thus, the rate of disappearance of C4H9Cl is the
  same as the rate of appearance of C4H9OH.

What if the stoichiometric ratio is not 11?
2 HI(g) ??? H2(g) I2(g)
For example
2 moles
1 mole
1 mole
Rate of disappearance of HI twice the rate of
production of I2 or H2
Therefore,
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Reaction Rates and Stoichiometry

• To generalize, then, for the reaction

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SAMPLE EXERCISE Relating Rates at Which Products
Appear and Reactants Disappear
(a) How is the rate at which ozone disappears
related to the rate at which oxygen appears in
the reaction
(b) If the rate at which O2 appears, ?O2??t, is
6.0 ? 105 M/s at a particular instant, at what
rate is O3 disappearing at this same time,
?O3??t?
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PRACTICE EXERCISE The decomposition of N2O5
proceeds according to the following equation
If the rate of decomposition of N2O5 at a
particular instant in a reaction vessel is 4.2 ?
107 M/s, what is the rate of appearance of (a)
NO2, (b) O2?
Answers (a) 8.4 ? 10 7 M/s, (b) 2.1 ? 10 7 M/s