Date: Fri Sep 15 09:39:42 2006
Posted By: Calvin Cole, Faculty, Engineering Physics, Northeastern State University
Area of science: Chemistry
The basis for the answers to your specific questions is rather lengthy.
The answers themselves are at the end of this answer. I don’t know any short way
to really answer this so here goes.
Any mixture of chemicals will react spontaneously. The tendency to
produce one species at the expense of another (the direction of the
reaction) depends on their concentrations relative to those at
equilibrium. Please note: From a molecular point of view the reaction
doesn’t stop running at equilibrium. It just runs as fast one way as it
does the other. All chemical equilibria are dynamic not static.
Using a match (raising the temperature) to “start” a reaction can involve
the interplay of several things, so first the players and then the interplay.
The players: 1. The sign of the change in Gibbs Free Energy (delta G)
determines the net direction a mix of chemicals at some concentrations and
conditions (temperature, pressure, etc.) will run to try and reach
equilibrium. Negative indicates it will run in the direction written and
positive the reverse. It is zero at equilibrium. This free energy change
does not tell us how fast the reaction will run. It also, by itself, does
not tell us whether the reaction is exothermic or endothermic. 2. The
change in Enthalpy (Delta H does that. 3. The activation energy can be
thought of as a speed bump along the reaction direction. 4. Entropy,
which has to do with how randomly the energy available is distributed among
the various places the molecules can store it. 5. The absolute
temperature, which is proportional to the random kinetic energy present.
The interplay: Gibbs Free Energy depends on Enthalpy, Temperature, and
Entropy. Delta G = Delta H - T(Delta S) Please note this has nothing to
do with activation energy. This is because the change terms in this
equation are net changes from beginning to end for the reaction involved.
This means that activation energy has nothing to do with whether the
reaction is spontaneous or not for the direction written. Raising the
temperature however can cause a variety of things to change.
Answers: This last interplay (4 just above) is probably closest to the
case for burning wood. If you get wood hot enough you create a wide
variety of chemicals, some gaseous, some liquid, and some solid that react
nicely with oxygen and can release enough energy to sustain or even
increase the initial temperature (about 240C) required to start the complex
process we call burning. Many of the reactions that occur are spontaneous
and occur very slowly unless the temperature is raised. As discussed above
the need for the match does not make the burning non-spontaneous in the
strict chemical sense but it can determine if the reaction cascade is
initiated or not. If that is what is meant by a more everyday use of
spontaneous then yes you need the match. The questions about activation
energy should be addressed above this answer section.
- Even if changing T does not change Delta H significantly (though it
sometimes can, especially for higher temperatures) it will cause the
T(Delta S) term to change. If the change in entropy (Delta S) is positive
(this is the usual case, entropy increases) then the whole term becomes
even more negative. If the reaction is already exothermic (Delta H is
negative) then it is spontaneous at any temperature. If the reaction is
endothermic (Delta H is positive) then there will be some temperature above
which the reaction becomes spontaneous in the direction written. Below
this temperature it runs spontaneously the other way.
- Raising the temperature increases the number of molecules present whose
kinetic energy exceeds any activation energy that may exist and so
increases the rate at which the reaction reaches equilibrium.
- If the reaction releases energy as it runs and this energy is not
removed quickly enough it can begin to raise its own temperature which in
turn raises the reaction rate and so on in a sort of thermal runaway.
- It is also possible that raising the temperature enough can cause a
cascade of reactions. In other words at some temperature one or more of
the components of the mixture react to produce some chemical that wasn’t
there before but which once produced is extremely reactive with the rest of
the mixture. This situation gives the appearance of a sort of spontaneity
requiring an energy input but is really two or more sequential reactions.
References: You can find all of this in principle in any college level
Physical Chemistry text but…. It may be spread over several chapters and
far from clear. I hope this helps tie it together a bit for you. Thanks
for a really good question.
(Sorry about all the "Delta" stuff, I couldn't get the Greek symbol to
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