Chemical Bonds 4/25/22

Science with Richard Bleil

Somebody once told me that all chemical reactions come down to rearranging chemical bonds.  It’s an interesting concept.  At first, it sounds like an oversimplification, but in reality, it’s very true.  Compounds are really just elements held together by bonds, and when a chemical reaction occurs, those bonds are broken, formed or shifted.  There really is nothing else.  Even shifting chemical bonds can be thought of as breaking a bond to reform it somewhere else. 

Chemical bonds all involve electrons, and a vast majority of time result in pairs of electrons.  Rarely do single electrons result in chemical reactions as they are excessively unstable, and bonds with more than two electrons are unheard-of.  By modern atomic theory, it should be impossible to form bonds with more than two electrons, although there are compounds with double or triple bonds.  However, in these compounds, each bond involves two electrons with single bond exceptions being so rare that we need not discuss them.

There are two types of chemical bonds, those where electrons are physically transferred from one element to another (“ionic” bonds), and those where electrons are shared (“covalent” bonds).  There is a very simple reason that chemical bonds form according to the laws of thermodynamics.  Every time a chemical bond forms, it is more stable (meaning of lower energy) than before the bond forms.  The first law says that natural processes tend towards lower energy, and this is why chemical bonds form.  Keep in mind that sometimes this first law can be overcome in a chemical reaction with an increase in entropy (the second law), but that results from the overall chemical reaction.  The formation of the chemical bond is solely dependent on the first law, and the release of any heat from the reaction is solely due to the formation of the chemical bonds.

Let me restate that.  Every time a chemical bond forms, it releases heat.  When I had the caps cemented to what remained of my two front teeth after my bike accident, they became very hot.  The process of cementing caps on teeth involves the formation of chemical bonds as the cement “cures”, and forming chemical bonds is always exothermic (typically heat releasing).  This must be true, because if a chemical bond resulted in higher energy, then it simply would not form in the first place.  By extension, every time a chemical bond is broken, it requires the input of energy (not necessarily in the form of heat).  But, like I said, a chemical reaction is basically just rearranging these chemical bonds.  If a reaction is exothermic, that means that the heat released by the new bonds is greater than the heat absorbed as the old bonds are broken. 

One side caveat of this is that, in biology, there is common error in what professors teach.  When they speak of adenosine triphosphate (ATP) breaking down to form adenosine diphosphate (ADP), they of course speak of energy released in the process that can be used in other reactions.  Frequently, they often say that breaking the triphosphate bond in ATP is what releases this energy, but that’s impossible.  The triphosphate bond would not form if it resulted in higher energy than if the bond did not exist, and it would simply fall apart before it could be utilized as an energy source.  In fact, breaking the triphosphate bond must require the input of energy.

The story of breaking this bond is often incomplete in classrooms and textbooks.  Breaking down an ATP also requires breaking down a water molecule.  One of the bonds between oxygen and hydrogen in water will break (requiring energy input), as does one of the oxygen and phosphorous bonds in ATP, also requiring the input of energy.  The new bonds that form are between the oxygen in ADP (releasing energy) and phosphorous and oxygen (also releasing energy) as a new phosphate is created.  The energy released comes from the fact that these two new bonds release more energy than that required to break the bonds at the beginning.  Breaking ATP phosphate bonds do not release energy, but the formation of the new bonds in the products (ADP and phosphate) do. 

When I last taught thermodynamics, I had great fun shaking the foundations of knowledge that my (mostly senior) students thought they knew of science.  In reality, although we tend to present pictures of science that look complete and rock solid, there are gaping holes that we tend to leave out.  This question of ATP and ADP is one of the things that I used in challenging my students.  I understand why it is taught as it is, because it’s a lot simpler than explaining the formation of new bonds, but now my students (and you) have a deeper understanding of the way things actually work. 

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