Thanks by your interest.
What is the difference between physical and chemical processes? One can define scientific terms according to two methodologies: descriptive and systematic. In a descriptive approach, one simply lists the different physical and chemical processes. However, in a systematic approach, we define and use a definition for cataloging the processes of natural word. Nevertheless, let me introduce a bit of history and “marketing” first.
Whereas physicists were more interested in mechanical aspects of matter, alchemists were interested in all properties of matter. Ablation, coloration, dissolution, or evaporation, between other were (al-)chemical processes for a 16th century alchemist. Antoine Lavoisier (1743–1794) is often considered the father of modern chemistry. In his Tableau des substances simples, Lavoisier considered the calorique (heat) and the lumière (light) simple substances participating in a chemical process in the same way that oxygen, hydrogen, zinc, or mercury participates in other processes.
However, the modern tendency of chemistry has been towards an increasing use of the terms “physical” and “physics” instead of “chemical” and “chemistry”. Many physical chemists have guided this wrong philosophy. Two physical chemists wrote the following shocking words: “all subjects treated under chemistry tend to be subject, as time goes on, to treatments at the more advanced degree of sophistication attained under the aspect of the science we call physics
.” What a beautiful piece of chemical “marketing”, perhaps focused to 15-years old students and policymakers! Again, chemistry is underestimated and seen as a cuisine. Attitude like this are the true basis of the current unpleasant status of chemistry.
Actually there is no systematic definition of chemical or physical processes (or properties), and authors use their own criteria according to specific preparation, knowledge, and personal views.
For example, Cram and Hammond’s book on organic chemistry distinguishes chemical processes from other formal processes by the use of “double” or “single” arrows respectively. They symbolize ammonia inversion with a double arrow as in a chemical reaction. However, they state that solubility of a compound is a physical property (i.e. dissolution is a physical process). But what is a chemical reaction? According to Cram and Hammond (CM) chemical reaction is a event in which two molecules collide in such a way as to break one or more of their bonds and make news bond and hence new molecules. Then is ammonia inversion a chemical reaction? Moreover, if participate only atoms in a process, then it appears that is not a chemical reaction, i.e., the redox reaction, Zn + Cu2+
+ Cu, would be a physical process according to above (CM) definition.
According to general chemistry by Chang (C), a chemical reaction is when new substances arise from a process, but as he does not define “substance” then chemical reactions remain undefined. Moreover, Chang opines that by heating a block of ice we do not change water, only its “appearance” and thus is a physical process. I imagine that the ammonia inversion would be a physical process for Chang. It is interesting remark that Chang considers that redox reactions are chemical reactions (note that this is contrary to CH).
Physical chemists McQuarrie and Rock (MR) state that all chemical reactions can be assigned to one of two classes: reactions in which electrons are transferred from one reactant to another and reactions in which electrons are not transferred. This is similar to (C) but different of (CH). Whereas, in a well-known Spanish manual on physical chemistry, the author, Diaz-Peña (DP), states that electrochemistry is outside of “pure” chemistry. For example, the standard electrochemical reaction Mn4+
+ e- ===> Mn3+
is named an electrode reaction, because it is formed of two steps
+ e- ===> Mn2+
===> 2 Mn3+
The first step is named an electron transfer process and the second a chemical reaction, and thus the global reaction (1+2) is considered not a chemical reaction by (DP). Others chemists opine that redox reactions are non-chemical processes because none bond is formed or broken, then I ask they what is a chemical bond and I do not receive systematic reply.
Many chemists state that heat transport is a physical process. However, Karen Timberlake (KT) says, in her manual on chemistry, that evolution or absorption of heat is a chemical change if is not associated with changes of state. (?)
Many inorganic chemists will say you that ammonia inversion is not a chemical reaction but, recently, two theoreticians have claimed that it is a chemical process, because computed how bonds are broken and formed. Philosopher of chemistry Joachim Schummer (JS) opines that the solubility in a certain liquid is an example of chemical property in direct contraposition with (CH) own ideas.
That is, there exits an enormous confusion in theoretical chemistry and this is transferred to current chemical education. Compare by yourself the above criteria and definitions. You will see that there is not unique definition of chemical process or property. Moreover, Demotivator (a Staff member of chemicalforums) has claimed:
I don't think they are chemical or physical. I believe they are biological, deriving behavior from a canonical psychology
I could critique many of the definitions above stated from different views: philosophical, educative, historical, theoretical, experimental, etc. For instance, Chang talks about “physical” processes in water when water changes just its “appearance” in his celebrated manual on general chemistry. However, he fails to define rigorously what water is...
What is water? In a first instance, water “is” H2
O and then in processes like
O(solid) ===> H2
O(liquid) ===> H2
Water “substance” remains “unchanged” according to Chang. However, in a more detailed observation, one observes only internal interactions (chemical bonds) in the gas phase molecule, whereas there is also hydrogen bonding in both ice and liquid water systems. That is, there are extra bonds in condensed phases and the atomic “array” is not the same.
From a quantum view, the wave function of the phase gas molecule of H2
O is not defined in an ice cluster (H2
formed by n
“water” molecules, indicating that the molecular system is not the same as its quantum state is not the same. However, there is more.
In a more accurate level of description, one discovers that Chang calls “pure” water is really a mixture and aggregations of H2
, and OH- species, and that the concentration of all three components varies with temperature. We can measure the changes in the chemical composition of water with temperature. That is, when one studies the process with more molecular detail, then appears that the initial Chang distinction between “physical” and “chemical” processes is somewhat arbitrary.
Due to the current divergence of criteria, I could introduce my own definition of chemical reactions according to my own beliefs but inspired in traditional points of view of ancient chemists. For example, nobody would say to me (once an inorganic chemist did) that, “traditionally heat transport is a physical process” because tradition was changed in 19th and specially 20th century chemistry. However, I prefer talk about generalized chemical reactions with the aim of facilitating the adaptation of “archaic” chemists to the new canonical ideas. I can say that heat transport is a generalized chemical process whereas ammonia synthesis is a standard chemical reaction. Both are chemical processes in a generalized theory.