Ask A Scientist Chemistry Archive

name         Erin J.
status       educator
age          20s

Question -   Can you please explain, since you claim that activation
energy is inherently positive, how there are many reactions that as
temperature is increased, the rate decreases.
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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.

Vince Calder
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Erin-
  I have not read all of our comments.  Which of us claimed that?  I do believe it is 
  either positive or zero, though.
Frankly, I do not remember any reactions which slow down as temperature increases.
They change their favorite ending-point, but they almost never slow down going there.
I need an example.

It might help to distinguish clearly between:
 a) net energy gain of the reaction, which, yes, can be either positive or negative, 
 and
 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.

Jim Swenson
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