Reaction Kinetics 2

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Reaction Kinetics (2)
Physical Chemistry

• Xuan Cheng
• Xiamen University

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Physical Chemistry
Reaction Kinetics
Simple Order Reactions
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Physical Chemistry
Reaction Kinetics
Typical Complex Reactions
Reversible reaction
Opposing reaction
(1)
Consecutive reaction
(2)
Competing reaction
Parallel reaction
(3)
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Physical Chemistry
Reaction Kinetics
Reversible First-Order Reactions
Consider the reverse reaction (opposing reaction)
the first order in both forward and back
directions
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Physical Chemistry
Reaction Kinetics
Reversible First-Order Reactions
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Physical Chemistry
Reaction Kinetics
Reversible First-Order Reactions
At equilibrium,
and
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Physical Chemistry
Reaction Kinetics
Reversible First-Order Reactions
When Aeq0, kb0
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Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
Consider two consecutive irreversible first-order
reactions
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Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
Let only A be present in the system at t 0
(17.37)
Integration
?
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Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
At all times (conservation of matter)
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Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
The intermediates concentration rises to a
maximum, and then falls to zero
(the position of maximum k1/k2)
?
The concentration of the product C rises from
zero and reaches Ao
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Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
Suppose that in an industrial batch process a
substance A produces the desired product B which
goes to decay to a worthless product C, each
stage of the reaction being first-order. At what
time will product B be present in greatest
concentration?
The time dependence of B
A maximum should occur at
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Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
Since Ao ? 0, k1 ? 0, so
The maximum concentration of B
?
For a given value of k1 gt k2, as k2 increases,
both the time at which B is a maximum and the
yield of B increase.
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Physical Chemistry
Reaction Kinetics
Competing First-Order Reactions
Consider two competing irreversible first-order
reactions
(17.42)
Parallel reactions
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Physical Chemistry
Reaction Kinetics
Competing First-Order Reactions
Compare with
For C
?
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Physical Chemistry
Reaction Kinetics
Competing First-Order Reactions
similarly
Division of (17.44) and (17.45)
At any time during the reaction
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Physical Chemistry
Reaction Kinetics
Competing First-Order Reactions
Consider the reverse reactions
Moreover
At equilibrium
Thermodynamic control
When any reverse reactions or interconversion of
C and D can be neglected
Kinetic control
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Physical Chemistry
Reaction Kinetics
Homework
Supplemental Material
Page 591
Page 145, Prob. 16
Prob. 17.13
Page 149, Prob. 30
Page 158, Prob. 68