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Name: Maggie
Status: student
Grade: 9-12
Location: N/A
Country: N/A
Date: November 2006

Question:
Are spontaneous chemical reactions always fast?



Replies:
Maggie,

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)


Maggie,

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.

Matt Voss


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!

Ethan Greenblatt
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.

Vince Calder



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