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sagar bansal bansal Grade: 12

can we have any reaction having activation energy zero

7 years ago

Answers : (1)

askIITianexpert IITDelhi
8 Points

We can have  reaction with zero activation energy,at least theoretically it can even be negative......!!!!

It might help to distinguish clearly between:
a) net energy gain of the reaction, which, yes, can be either positive or negative,
b) activation energy, which is a little road-bump, or wall, or barrier, in between the
before-reaction situation and the after-reaction situation.

     So a negative reaction energy would be like a deep trench between a floor and a
small step.  (The step could be up or down.)  A trench might stop walking people,
but it does not usually stop atoms.  They would more likely roll down into the
trench,gain momentum, and roll right back out with just as much energy as they had before.  
i.e. no barrier, same effect as zero activation energy.
    A barrier which sticks up, like a wall sticks out of the ground, would stop
randomly rolling balls if they were not rolling fast.  Even if the other side of
the wall is lower.
Likewise, a positive activation energy stops atoms from reacting if they are cold. 
Even if reacting would gain energy.

You must ask yourself, why does it take a match to start a fire?
Why does air not just burn all paper on contact?
That is what activation energy is about.

But here is anothe catch.....!!!!!!

The expression for the activation energy, Ea,: d[ln (k)]/dT = Ea/RT^2 is
an argument by analogy to the analogous thermodynamic expression for the
enthalpy of reaction and the temperature dependence of  the equilibrium
constant. It does not apply to reactions that decrease in rate as the
temperature increases. It does not apply to a lot of chemical reactions.
Reaction rates can increase, decrease, remain constant, increase then
decrease, decrease then increase, or even be essentially discontinuous with
an increase in temperature. The concept of an activation energy should not
be taken too literally. Reaction kinetics are much more complicated.

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. 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.

Having gone through all of the above arguments i believe that Ea could be either zero or positive but for ideal case of Arrhenious Equation only.For all practicality it is always positive.

I hope above will help you in your query.Any further clarifications if required by you will be most certainly entertained here.

7 years ago
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