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12.6 Reaction Mechanisms
A balanced equation for a chemical reaction shows what is happening, but it doesn't show how the reaction actually happens.
A chemical reaction may not be obvious to an observer.
Elementary reactions can't be broken down into simpler steps.
The mechanism of the reaction does not involve the collision and reaction of two ozone molecules, which is what the equation indicates.
Rather, it involves a molecule of ozone decomposing to an oxygen molecule and an intermediate oxygen atom; the oxygen atom reacts with a second ozone molecule to give two oxygen molecule.
The reaction mechanism shows the two elementary reactions.
There are several elementary reactions in a complex mechanism.
One part of a two-step reaction mechanism is shown in O2 + O.
Some unimolecular reactions may only have a single reaction.
The separation of parts of single reactant molecule into products is all that is required for these unimolecular reactions to occur.
During chemical reactions, chemical bonds do not fall apart.
Energy is needed to break bonds.
The energy for the decomposition of C4H8 is 261 kJ per mole.
A few of the rapidly moving C4H8 molecule collide with other rapidly moving molecule and pick up additional energy.
When the C4H8 molecules gain enough energy, they can transform into an activated complex and form ethylene molecules.
The C4H8 molecule is knocked into the geometry of the activated complexample by a particularly energetic collision.
Only a few molecules can pick up enough energy from a collision.
Concentration is directly proportional to the rate of decomposition.
The amount of C4H8 in a sample is doubled.
The total number of such molecules is twice as great as the fraction with enough energy to react.
The reaction rate is directly proportional to the concentration of the reactant, and the reaction exhibits first-order behavior.
The rate constant is the proportionality constant.
A single bimolecular elementary reaction is one of the mechanisms that make up some chemical reactions.
There is a probable mechanism for the reaction between NO2 and CO.
There are steps in a multistep reaction mechanism.
The probability of three particles colliding at the same time is less than one thousandth of the probability of two particles colliding.
There are some established termolecular elementary reactions.
One step in a multistep reaction mechanism is often slower than the others.
The rate at which the overall reaction occurs will be limited by this step because a reaction cannot proceed faster than its slowest step.
A cattle chute is an example of a rate-determining step.
Cattle can only be moved from one holding pen to another as quickly as one can make its way through the chute.
Ordinary chemical reactions are not the same as this one.
The balanced equations represent the change in a chemical system, and often it is the result of multistep reaction mechanisms.
In every case, we must determine the overall rate law from experimental data and deduce the mechanism from the rate law.
The first order with respect to NO2 and CO is the reaction.
The rate law for the overall reaction is the same as the rate law for the rate-determining step.
When the reaction proceeds in both directions at equal rates, it's an elementary reaction.
Intermediates can be included in an individual elementary reaction of a mechanism, but they cannot be listed as part of the overall rate law expression.
Write the rate law expression for each elementary reaction, identify any intermediates, and determine the overall rate law expression.
The rate-determining step is the third step.
Both of them are intermediates.
No intermediates must remain in the overall rate law expression if the algebraic expressions are used.
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