Ask A Scientist Chemistry Archive

name         Emily M.
status       student
age          14

Question -  What is the difference between an endo and exothermic
reaction.  What are some examples?  Like where does the energy for each
come from?  I am told the energy is in the chemical bonds, but I am also
told that a bond is at low potential.  It cannot be at high and low
energy at the same time.  Please help!
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An endothermic reaction absorbs heat from its surroundings; an exothermic
reaction releases heat into its surroundings.  If a reaction running in one
direction is endothermic, the reverse direction is exothermic.  Often, the
"spontaneous" direction of a reaction, that is, the direction it will go if
it can go any direction at all, is the exothermic direction.

An example is dissolving sodium hydroxide in water.  Sodium hydroxide
spontaneously dissolves in water and releases heat in the process.  A
solution of sodium hydroxide in water does not readily separate into pure
water and solid sodium hydroxide.  Another example of an exothermic reaction
is the combustion of hydrogen gas with oxygen to produce water and heat.

An example of a spontaneous endothermic reaction is dissolving ammonium
nitrate in water.  Just as with sodium hydroxide, ammonium nitrate is very
soluble in water. In contrast to sodium hydroxide, when ammonium nitrate
dissolves the solution gets colder, that is, it absorbs heat.  Another
endothermic reaction is the reaction of baking soda and vinegar to make
carbon dioxide and sodium acetate.

"Where does the energy come from" is a simple question, but the answer is
somewhat complex.  I suspect that the answer will also be a lot longer than
you would like it to be.  It turns out that it is not actually energy that
drives a chemical reaction, but instead a quantity called "entropy."

I find it hard to understand a rigorous definition of entropy, but I can
give you an example.  Let us say that a swimming pool is filled with plain
water.  I place a balloon filled with blue dye into the pool and pop the
balloon.  What happens to the blue dye?  It slowly mixes with the rest of
the water in the pool.  Why?  The dye molecules are constantly moving
around, bouncing off each other and off the water molecules.  They can
bounce back into where the balloon used to be, or they can move into places
where they were not before.  If they have no reason to go in one direction
instead of another, where will they go?  They can move to any place in the
pool.  There is no preference for any one place over another.  So, they will
eventually spread out to color the entire pool, not because they are acting
together, but because they each individually move in any direction.

Will the dye molecules ever spontaneously move back to where they started,
that is, where the balloon used to be?  Yes, actually, each individual dye
molecule will, if given enough time, at some time occupy every location in
the pool.  Will all the dye molecules ever spontaneously move back to where
the balloon used to be, all at the same time?  Doing this would amount to
running the reaction backward: all the dye molecules unmix from the pool
water and go back into one spot.  You probably know that this will not happen,
but why will it not?  Here is why.  The volume of the balloon is only a very
small fraction of the total volume of the pool.  Let's call this fraction b.
This fraction b is less than 1 and greater than zero.  At any time, the
probability that any given dye molecule is where the balloon used to be is
b.  For the reaction to run backwards, all of the dye molecules would need
to be in that region.  IF there are N dye molecules, the probability that
this will happen is b^N, that is, b times itself N times.  In chemical
systems of any significant size, N is a very large number, larger than
billions and larger than trillions.  So b^N is an infinitesimally small
probability, so close to zero that as far as we can observe it IS zero.
Thus we can say that dye mixed throughout the water in a pool will NOT
spontaneously unmix and concentrate in a small volume.

The reason I went through that long example was to explain the principle
behind entropy.  "Entropy" is a way to measure the tendency of systems to
arrange themselves so that making many small deviations will not change the
arrangement much.  In the swimming pool example, moving the dye molecules
around when they are already dispersed throughout the water will not make 
things
look very different.  When they are close together in a small region, though,
moving them around will have a bigger relative effect.

There are two basic ways that entropy can increase in a reaction:
  * The reaction can give off heat.  Heat means that molecules are moving
faster, enabling them to move into many different arrangements rapidly.
  * The reaction can transfer molecules from a restricted to a free state.
This is what happens when solids dissolve in liquids, or gases are produced.

In the swimming pool example, the dye molecules go from a restricted state
(inside to balloon) to a freer state (dispersed throughout the pool).  Heat
transfer is not important here.

When sodium hydroxide dissolves in water, heat is given off AND solid sodium
hydroxide becomes freely mobile in the water.  This is a very favorable
reaction.

When ammonium nitrate dissolves in water, the solid ammonium nitrate becomes
more mobile, but the solution absorbs heat.  These two processes oppose each
other, and the winner is the increased mobility of the ammonium nitrate.

The same battle of tendencies occurs in the baking soda + vinegar reaction.
Carbon dioxide gas is produced, which is much more mobile than the baking
soda or vinegar.  This increased mobility overcomes the unfavorable
influence of absorbing heat.

I am almost done answering your questions.  You last question concerned
whether the energy of reactions comes from breaking chemical bonds.  That
question was answered in great detail by a number of scientists (including
me) earlier.  The link to the question and its replies is at
http://146.139.100.40/webpages/askasci/gen99/gen99928.htm

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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