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Chem 1310: Introduction to physical chemistry Part 2d: rate laws and mechanisms. Unimolecular vs bimolecular reactions. Unimolecular reactions happen within one molecule. The molecule needs to acquire enough energy to overcome the barrier to its reaction.
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Chem 1310: Introduction to physical chemistryPart 2d: rate laws and mechanisms
Unimolecular vs bimolecular reactions Unimolecular reactions happen within one molecule. The molecule needs to acquire enough energy to overcome the barrier to its reaction. Molecules collide all the time and so exchange energy. At any time only a small fraction of them may have enough energy, most will not. Even if they have enough energy, the reaction does not always happen.
To react or not?A game of chance Lower barrier Þ higher probabilityÞ higher rate Higher temperature Þ higher average energyÞ higher rate
Dependence of rate on temperature Arrhenius equation: Interpretation:e-Ea/RT = "energy factor": fraction of molecules having enough energy for the reaction.A = "frequency factor": collision frequency * probability that, given enough energy, the reaction happens.
Arrhenius equationat low temperature Number of moleculeswith sufficient energygrows nearly exponentially. T small -Ea/RT large and negative e-Ea/RT close to 0
Arrhenius equationat high temperature T large -Ea/RT close to 0 e-Ea/RT close to 1 k»A Region ofexponentialgrowth. Every molecule hasenough energy.Frequency factordetermines rate.
Dependence of rate on temperature Plot ln k vs 1/T: straight line with slope -Ea/R and intercept ln A ("Arrhenius plot"). Doing an Arrhenius plot
Unimolecular vs bimolecular reactions Bimolecular reactions happen when two molecules meet. The "encounter complex" also needs to have enough energy to overcome the barrier to its reaction. At any time only a small fraction of the colliding pairs will have enough energy, most will not. Even if they have enough energy, the reaction does not always happen.
Termolecular and higher reactions? "They don't happen." It is just too improbable for three molecules to meet each other simultaneously, with enough energy and the correct alignment. Even bimolecular reactions tend to be slower than unimolecular ones with the same Ea. But two molecules might meet each other, "stick", then meet a third one and finally undergo the "real" reaction.
