Home Discussion Forum is it possible to have a negative activation energy (Ea) valve?

is it possible to have a negative activation energy (Ea) valve?

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  1. yes it is possible to have a negative activation energy value when rates of reaction decreases with increase in temperature…..
    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 barrier.
    If a positive activation energy keeps a reaction from occurring
    until that amount of energy is provided from the environment then a
    negative activation energy would imply you could not stop the
    reaction from occurring if you tried. The reactants already has
    plenty of energy for the reaction so the only way to prevent the
    reaction would be to keep the reactants apart………
    For a very small number of reactions, all involving NO, the rate of reaction falls with an increase in temperature. This implies a negative activation energy. The reason for this apparent peculiarity is that although the rate constant k does indeed increase with increasing temperature, the mechanism is such that another constant, the equilibrium constant for one of the mechanistic steps, is also involved in the rate equation. This falls with increasing temperature.
    The reaction of nitrogen monoxide (nitric oxide, nitrogen(II) oxide) with oxygen is:
    2NO(g) + O2(g) → 2NO2(g)
    This is one of only five homogeneous gas reactions known to be third order:
    rate = k[NO]2[O2]
    the others being the reaction of NO with chlorine, bromine, hydrogen and deuterium .
    The probability of a termolecular reaction, where the three species collide simultaneously with the correct energy and the correct orientation in a single step, is very small. The suggested mechanism for the oxidation of NO with O2 involves an initial dimerisation in an equilibrium reaction, followed by reaction of the dimer with the oxygen:
    2NO(g) (NO)2(g) . . . . . . . . . . . . . . . . . . . . . . . . . (1)
    (NO)2 (g) + O2(g) → 2NO2(g) . . . . . . . . . . . . . . . . . . . . .(2)
    The following discussion depends on two things; firstly that the rate of attainment of the equilibrium (1) is very fast compared with reaction (2), and secondly that reaction (2) is the rate limiting step.
    Consider the equilibrium reaction (1). From usual equilibrium considerations
    Kc = [(NO)2]/[NO]2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (3).
    For the rate limiting step (2) we can write
    rate = k'[(NO)2][O2];
    but from (3)
    [(NO)2] = Kc[NO]2
    so rate = k’Kc[NO]2[O2]
    This is the rate equation quoted with k = k’Kc.
    The temperature dependence of the reaction rate.
    In the rate equation
    rate = k [NO]2[O2] = k’Kc [NO]2[O2]
    both k’ and Kc are temperature dependent. The constant k’ increases with increasing temperature; the variation of Kc with changing temperature depends on the thermicity of the equilibrium producing (NO)2. The reaction
    2NO → (NO)2
    involves bond formation and is therefore exothermic. For an equilibrium where the reaction from left to right is exothermic, Kc decreases with an increase in temperature. So; k’ rises with temperature increase, but Kc falls. In this reaction k’ increases less than the fall in Kc, so that the overall value k’Kc also falls with an increase in temperature. So, then, does the reaction rate. The activation energy is only apparently negative; for the rate-limiting step it is, as is usual, positive.

  2. Lancenigo di Villorba (TV), Italy
    No.
    ACTIVATION ENERGY comes from Mathematics Model given by S. A. Arrhenhius (beginnings of XX century, swedish chemist).
    This is a physical parameter meaning an ENERGETICAL THRESHOLD ALLOWING A PHENOMENON RUNS ONCE PARTICLEs GET AN EQUIVALENT OR RICHER Energy’s Distribution.
    By an absurde hypotheses, IF Activation Energy SHOULD BE A NEGATIVE VALUE, THEN REACTION SHOULD SLOWEN WHILST TEMPERATURE GOES UP….impossible! J. J. BERZELIUS REMARKED THAT Temperature FASTENS REACTIONs ALREADY IN XIX CENTURY.
    I hope this helps you.

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