Negative Activation Energy
In some cases, rates of reaction decrease with increasing temperature. When following an approximately exponential relationship so the rate constant can still be fit to an Arrhenius expression, this results in a negative value of Ea. Elementary reactions exhibiting these negative activation energies are typically barrierless reactions, in which the reaction proceeding relies on the capture of the molecules in a potential well. Increasing the temperature leads to a reduced probability of the colliding molecules capturing one another (with more glancing collisions not leading to reaction as the higher momentum carries the colliding particles out of the potential well), expressed as a reaction cross section that decreases with increasing temperature. Such a situation no longer leads itself to direct interpretations as the height of a potential spot.
Read more about this topic: Activation Energy
Famous quotes containing the words negative and/or energy:
“The idealist’s programme of political or economic reform may be impracticable, absurd, demonstrably ridiculous; but it can never be successfully opposed merely by pointing out that this is the case. A negative opposition cannot be wholly effectual: there must be a competing idealism; something must be offered that is not only less objectionable but more desirable.”
—Charles Horton Cooley (1864–1929)
“The scholar may be sure that he writes the tougher truth for the calluses on his palms. They give firmness to the sentence. Indeed, the mind never makes a great and successful effort, without a corresponding energy of the body.”
—Henry David Thoreau (1817–1862)