CHE 104 Chapter 14 - CHE 104 Chapter 14 Dr Mark Shuman What We Can\/Cant Predict about Rxns We can predict Endothermicity Exothermicity We can not yet

CHE 104 Chapter 14 - CHE 104 Chapter 14 Dr Mark Shuman What...

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CHE 104 Chapter 14 Dr. Mark Shuman
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What We Can/Can’t Predict about Rxn’s We can predict: Endothermicity Exothermicity We can not yet predict: Speed (rate) of the reaction. How to affect the speed (rate) of the reaction. The mechanism of the reaction (what actually happens at the molecular level). The direction of spontaneity of the reaction.
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Kinetics The study of reaction speed (rate) Rate vs mechanism Rate vs concentration Rate vs Activation Rate vs Activation Energy Rate vs temperature Rate vs catalysis
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Reaction Mechanism A reaction mechanism is a series of plausible mini-reactions Adds up to the stoichiometric expression for the overall reaction Yields a rate expression ( rate law ) that matches experiment
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Reaction Mechanism (cont.) A + B C + D expresses the overall results (stoichiometry) of a reaction but says nothing about what occurs at the molecular level. A and B may never actually react (or come into contact) with one another.
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Reaction Mechanism (cont.) A reaction mechanism is the exact molecular pathway that starting materials follow on their way to becoming products. A reaction mechanism is made up of one or more elementary reactions . Actual events that occur at the molecular level. Unimolecular reaction X Y + … Bimolecular reaction X + Y Z + … Termolecular reaction X + Y + Z q + …
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Reaction Mechanism (cont.) Frequently, one can picture more than one possible reaction mechanism. A + B C + D 1) A X 2) B + X C + D or 1) A + B X 2) X C + D or 1) A X 2) B Y 3) X + Y C + D or Etc.
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Reaction Mechanism (cont.) 2 NO 2 2 NO + O 2 1) NO 2 NO + O 2) NO 2 + O NO + O 2 Subtotal) 2 NO 2 + O 2 NO + O + O 2 Total) 2 NO 2 2 NO + O 2 2 NO 2 2 NO + O 2 1) 2 NO 2 NO 3 + NO 2) NO 3 NO + O 2 Subtotal) 2 NO 2 + NO 3 2 NO + NO 3 + O 2 Total) 2 NO 2 2 NO + O 2 These are legitimate since they add up to the overall stoichiometry. We would need to determine experimentally which of these can be ruled out. It is not possible to rule a mechanism in because there may be other possible mechanisms that we haven’t even considered.
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Reaction Mechanism (cont.) Both of these mechanisms produce a chemical species in the first step that is consumed in the second step. This is called a chemical intermediate . In the first mechanism, it is O In the second mechanism it is NO 3 An intermediate is a chemical species produced in an early step of a mechanism and consumed in a later step. Highly reactive Exist only briefly Therefore, concentration is very low Typically can be detected only with great difficulty If we could detect O or NO 3 in the reaction mixture, it would be powerful evidence in favor of one of the mechanisms or the other (once again, not absolutely conclusive).
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Reaction Mechanism (cont.) The speed of a chemical reaction is described by its rate .
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