Single Speed for Reactions
Date: November 2006
Are spontaneous chemical reactions always fast?
First of all, we need to be more specific in our use of the
technical/scientific term: "spontaneous". It is not like the normal
English word. In normal English, spontaneous we mean a change that
tends to proceed without apparent external influence, but in
science, we mean a process in which the combined states of the
initial system is more energetic (has less disorganization) than the
combined final state.
Thus, while the normal English usage of the word spontaneous can
apply in a scientific context, what we really mean in science is
that something is spontaneous if a system starts with more energy,
less disorganization, than the resulting products.
This suggests that "spontaneous" is a function of starting and
ending conditions only AND has nothing to do with HOW the system
proceeds from initial to final state.
So, with regard to your question, there should be NO expected
correlation between the spontaneity of a chemical reaction and its
speed. Spontaneity is about the initial and final states, speed (the
technical term is kinetics) is about process, or how the initial
becomes the final state.
Greg (Roberto Gregorius)
There are only two things that control reactions. Thermodynamics
and Kinetics. Thermodynamics is the formation of the most
energetically favorable reaction products, while Kinetics is the
formation of the products that happen the quickest. To answer your
question directly, spontaneous chemical reactions are not always
fast. The best chemical transformation to explain this question is
diamond into carbon. Carbon has three allotropes, which is the
exhibition of an element to form different crystals--these three
allotropes are Diamond, Graphite and Bucky Balls. Without going
into Bucky Balls, Graphite and Diamond are both made up of the same
carbon atoms, but the atoms are in different arrangements. Graphite
is the most stable form, meaning that it is most thermodynamically
stable (lowest energy). Diamond is not thermodynamically favored
under STP conditions. But it stands to exist under Standard
Temperature and Pressure as a diamond. Over the course of many of
hundreds (or maybe thousands--I have not looked up the number) of
years, diamond will eventually become graphite. This is because
graphite is the most thermodynamically stable allotrope of carbon,
yet the activation energy between diamonds and graphite is so large,
that the time that it takes diamonds to turn to graphite is much
longer than our lifetimes.
Kinetics is all about speed. In organic chemistry, five-membered
rings form faster than six-membered rings. Five-membered rings are
kinetically favored (quicker to form), though six-membered rings are
thermodynamically favored (more stable in the long run). You can
run reactions under either thermodynamically favored conditions or
under kinetically favored conditions. Thermodynamic conditions tend
to be very low temperatures to slow things down, while kinetically
favored conditions tend to be warmer conditions to favor speed.
While spontaneous reactions will always go towards products, they may do so
in microseconds or milliseconds to millions of years or more (think about
how fast an explosion happens versus how fast a nail rusts). Scientists
think that molecules need to get kicked over an "activation barrier" before
they can break bonds and form new ones. Since molecules have a range of
energy, the percentage of molecules that have an energy higher than the
energy barrier, and therefore can react, depends on how big that energy
barrier is. If the barrier is large, the probability of molecules having
energy higher than that barrier is low, so the reaction will take a long
time (and vice-versa).
An example of an important spontaneous reaction that is very slow is the
hydrolysis (breaking of a bond by adding water to it) of the peptide bonds
that make up the proteins in your body. While this reaction is
spontaneous, there is a large energy barrier to this reaction so it is very
slow and takes years. That is why the proteins in your body are not instantly
falling apart even though this process is favored.
Since the energy barrier to a reaction is what determines its rate,
molecules known as catalysts can increase the rate of a reaction by
lowering the activation barrier. To break down the proteins in food, your
body uses enzymes such as trypsin to catalyze the hydrolysis of the bonds
in the proteins that you eat, otherwise it would take years for you to
digest your Thanksgiving turkey!
Stanford Department of Chemistry
The "short" answer is NO. A spontaneous reaction refers to the
extent to which a reaction will occur, without regard to "how fast"
that reaction might occur. The combustion of carbon in the form of
a lump of charcoal is approximately as spontaneous as the
combustion of hydrogen gas. However, charcoal burns very slowly on
a grill, but hydrogen gas usually explodes.
Your question raises the important distinction between how
spontaneous a reaction is, and how fast it occurs. Many reactions
are favored from the standpoint of the stability of the reactants
compared to the products, and the speed at which possible reactions occur.
The two concepts are very different.
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Update: June 2012