12.6 Reaction Mechanisms

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.