Jurnal Chemistry about Antoine Laurent Lavoisier
Antoine Laurent Lavoisier
Antoine Laurent Lavoisier was, amongst other things, a chemist, economist, and public servant. He is most noted for his discovery of the role oxygen plays in combustion. Lavoisier, the son of a very prosperous lawyer, was born in Paris on August 26, 1743. He was educated at the College des Quatre Nations where he studied a broad range of academics. He was expected to follow in his fathers footsteps and even obtained his licence to practice law in 1764 before turning to a life of science. In particular, he turned to geology. From 1763 to 1767 he studied geology under Jean Etienne Guettard.
In 1765 he wrote and published a paper on how to improve the street lighting in Paris. For this and some works on agriculture, he was elected into the Royal Academy of Science in 1768. Also in this year, he joined the Farmer’s General, a private company that collected taxes and tariffs for the government. In 1771, he married the daughter of a Farmer General, Jaques Paulze. She immediately became her husbands collaborator, learning to read English (which Lavoisier could not do) and becoming a skilled draftsman and engraver.
In 1775, Lavoisier was appointed to the National Gunpowder Commission. He then moved to the Arsenal of Paris where he created a superb laboratory for his growing experimentation. In large part, he was able to afford his experiments and such a laboratory because of his inherited wealth. His new home quickly became a gathering place for scientists and freethinkers.
Lavoisier was politically liberal and shared many of the ideas of the philosophes. He was deeply persuaded of the need for social reform in France. Because of this, he played an active part in the events preceding the French Revolution. He served on a committee concerned with the social conditions of France and proposed tax reforms and new economic strategies. He also served on a committee that explored hospitals and prisons of Paris and then reccommended remedies for their horrible state. During the revolution, he published a report on the state of France’s finances. When the Estate’s General reconvened in 1789, he became an alternate deputy and drew up a code of instructions for the other deputies to follow.
Soon after the revolution began, Jean-Paul Marat and other radical journalists began to slander Lavoisier for being a member of the Farmer’s General. On May 8th, 1794, all of the Farmer’s General were arrested and thrown in prison. In a trial that lasted less than a day, they were all convicted and sentenced to execution. When Lavoisier requested time to complete some scientific work, the presiding judge was said to have answered “The Republic has no need of scientists.” His body was thrown into a common grave. The Phlogiston Theory Chemistry was so underdeveloped at the time Lavoisier gained interest in it that it could hardly be called a science. The prevailing view of combustion was the Phlogiston Theory which involved a weightless or nearly weightless substance known as phlogiston. Metals and fire were considered to be rich in phlogiston and earth was considered oxygen poor. When metal is calcined, or roasted in the presence of air, it turns to a powdery substance called a calx (now known as an oxide). This reduction in weight was explained as a loss of phlogi ton into the air. For the smelting of an ore, the process reversed. Charcoal was believed to be rich in phlogiston and so, when charcoal was burned with this powdery calx, phlogiston supposedly passed from the charcoal to the calx restoring the metal.
The Phlogiston Theory Explained…
- Weight loss when combustibles are burned because they lose phlogiston
- Fire burns out in an enclosed space because it saturates the air with phlogiston
- Charcoal leaves very little residue when burned because it is made mostly of phlogiston
- Animals die in an airtight space because the air becomes saturated with phlogiston
- Some metal calxes turn to metals when heated with charcoal because the phlogiston from the charcoal restores the phlogiston in the metal
The problem was that when some metals were calcined, the resulting calx was heavier than the initial metal. Some proponents of the phlogiston theory tried to explain this phenomena by saying that in some metals, phlogiston has negative weight. Furthermore, it was discovered that mercury could be turned back into a metal simply by heating it, that is without a phlogiston rich source such as charcoal. Rather than except this theory that phlogiston could have positive weight, negative weight, and sometimes no weight at all, Lavoisier suspected and later proved that the weight increase was a result of the metal combining with air. In 1777 Lavoisier presented his theory that combustion and related processes were reactions in which oxygen combines with other elements in a paper entitled Memoir on Combustion in General.
Laviosier changed chemistry from a qualitative to a quantitative science. He freed society from the disillusionment of the phlogiston theory by showing that the mass of the products in a reaction are equal to the mass of the reactants. The following diagram uses the example of mercury calx to show the major difference between the mental model used by Priestly and the one developed by Lavoisier. Therefore, Lavoisier proved that oxygen played the major role in the differences in weight associated with combustion, disproving the accepted view of the Phlogiston Theory.
Lavoisier is now known as the Father of Modern Chemistry. However, when Lavoisier started his experiments on combustion and respiration, chemistry was still in the very early stages of development. There was lots of empirical information but very little theoretical basis and no formal language. Enough characteristics of acids, alkalis, salts and metals were known so that they could usually be distinguished, but gases were hardly known to exist.
Lavoisier was an excellent discoverer because he was quick to see the significance of new findings. He readily confirmed and extended the experimental discoveries of others and formed mental models to organize all of these ideas. He was one of the few chemists at the time to fully appreciate the importance of careful measurements of reactants and products. In order to make such careful measurements he invented a balance which was good to about .0005 grams. He proved the Law of Conservation of Mass, showing that the mass of the reactants had to equal the mass of the products.
Regarding respiration, he showed that oxygen is consumed and carbon dioxide is given off. In 1783 he began heat measuring experiments using a calorimeter and showed that the heat produced by repiration was equal to the heat produced when the same amount of oxygen was used to burn charcoal. He also used a calorimeter to find the specific heats of various substances and measure the heat produced in chemical reactions.
States of Matter
In his school days, Lavoisier was taught that fluid was vaporized by either being transmuted into air or by being dissolved in air like salts dissolve in water. Lavoisier showed that all substances can exist in the three stages of matter. However, he believed that these changes in state were the result of fire combing with matter. This “matter of fire” or “caloric” was weightless and combined with solid to form liquid and combined with liquid to form gas.
Lavoisier and a small group of other scientists created the Method of Chemical Nomenclature in which they classified the distinctions between elements. Lavoisier also clarrified the distinction between elements and compounds
Common Gases : General Info
- Carbon Dioxide (CO2) : Fixed Air
Carbon Dioxide was the first gas prepared and truly characterized as a pure substance. It was studied around 1750 by Joseph Black who named it “fixed air.” Information about “fixed air” was recieved through the equation: limestone + acid –> a salt + fixed air (modern equation: CaCO3 + 2HCL –> CaCl2 +H2O + CO2) While Black did not find any commercial use for it, Joseph Priestly prepared carbonated beverages as early as 1772 in London
- Nitrogen (N2) : Dead Air
The discovery of Nitrogen is clearly distinguished to be around 1772 though the actual discoverer is unclear. Scheele noted that when air was trapped, part of the air could be removed by a reaction with moist iron fillings and that the remaining air could not support combustion. This remaining air, was not “fixed air” because it would not precipitate with limewater as in the above equation. Since it did not support life or combustion, it was called “dead air.”
- Hydrogen (H2) : Inflammable Air
Hydrogen was discovered by Henry Cavendish who described it in his work “On Factitious Airs” published in 1766. This gas, which he created by mixing a metal with an acid (equation: HCl or H2SO4 + Zn or Sn –> inflammable air + a salt), also had a very low density and was originally called “inflammable air.”
- Oxygen (O2) :Fire Air
Oxygen, which makes up about one fifth of our atmosphere, was originally given the names “fire air” and “dephlogisticated air.” It was discovered by Joseph Priestly who prepared it by heating the red oxide of mercury in 1774. However, while in modern terms the reaction was HgO + heat –> Hg +.5O2 Priestly considered the air to have lost phlogiston.
That is :
air + X = phlogisticated air (Nitrogen)
Antoine Laurent Lavoisier’s
Theory of Combustion
Antoine Laurent Lavoisier, a French scientist during the eighteenth century, is commonly referred to as the Father of Modern Chemistry. He achieved such a distinguished title by his ingenious discoveries in the areas of respiration and combustion. He revealed the true nature of science by transforming the accepted theoretical structure at the time. While his contributions to chemistry are multitudinous, his study of combustion is particularly interesting when analyzing the process of discovery. His “discovery” of combustion followed the five main generalizations characterized by Dr. Gorman in his book Invention and Discovery: A Cognitive Quest. These generalizations are as follows:
- Establishing a problem significant enough to be labeled an important achievement
- Transforming the problem into a form that suggests a promising path to solution
- Finding or inventing good data
- Using the right combination of stubbornness and flexibility
- Using writing as both a means to record and explore
In particular, this paper will show how Lavoisier changed the scientific view of combustion by disproving the phlogiston theory and replacing it with his own ideas. Lavoisier began working on the problem of combustion in 1773 with his friends Trudaine and Montigny. They became interested in combustion because of the similarities they saw between it and respiration. The prevailing view at the time was that the combustion of candles somehow decreased the volume of air. Mr. Priestley, a very prominent English chemist and a strong proponent of the phlogiston theory, considered this diminution to be a characteristic of the “goodness” of the air. Lavoisier and his friends decided to test the theory on their own. The first experiment they performed tested whether a candle would burn in the air left after a bird had been suffocated in a closed bell jar. The second was to test the properties of the air left over after a candle had extinguished in pure or dephlogisticated air. For the second experiment the residual air extinguished a second flame and, when agitated in water, lost 1/3 of its volume. The other 2/3 reacted to flame and his nitrous air test just as common air does. Lavoisier concluded that after the candle went out, the air remaining was 1/3 common air and 2/3 fixed air. Rather than assume that combustion decreased the volume of air, Lavoisier hypothesized that it converted common air to fixed air and that the diminution was fixed air combining with water. Therefore, Lavoisier did not believe that the diminution of the air was a measure of the “goodness” of the air but rather was influenced by the amount of pure air in the sample
He decided to test his new hypothesis by observing the effects of combustion on airs containing different proportions of pure air. He hypothesized that if pure air is converted to fixed air, the more pure air in the sample, the more the volume of air will decrease. While both of the first experiments were done in bell jars inverted over water, for this experiment he decided to invert the bell jars over mercury so that the fixed air created would not react. Sure enough, when he burned a candle in this new setting, the volume of the air remained basically constant suggesting that combustion does not cause a diminution of gas. Reacting the residual gas with caustic alkali he then determined the amount of pure air which was converted to fixed air. The result was that more fixed air was formed when more pure air was in the original sample which supported his initial hypothesis that pure air was somehow converted into fixed air.
Lavoisier noticed that the series of experiments he’d been working on strongly resembled the experiments he had done with birds. Respiration was believed to be similar to calcination for various qualitative reasons. However, Lavoisier suggested the correct correlation between combustion and respiration as early as November of 1773 when he wrote a memoir on respiration for the Academy of Science. At this point he had already started on the path that would lead him away from the phlogiston theory.
In 1776, Lavoisier ran into a problem in the development of his new views on combustion. From his own experiments, he had hypothesized that phlogisticated air was air deprived of matter of fire and from which fixed air is precipitated. Therefore to recompose air one would have to restore its fixed air and its matter of fire. Lavoisier was writing a memoir on this subject when he realized the problem with this logic. He had observed in his experiments the recomposition of air simply in the presence of water. He knew that water was not composed of fixed air and matter of fire and even further that water did not give any substance to the air during the reaction. Rather than adding something, it seemed more probable that in recomposing air, water must remove a substance from the fixed air. Since the Priestley’s explanation, which delineates the phlogiston theory, was that residual air was phlogisticated and the water removes the phlogiston, Lavoisier was being forced back into a view of combustion which he had tried to depart from three years earlier.
Given all this uncertainty as to the composition of fixed air and its role in combustion, Lavoisier designed some new experiments. He set up some candle experiments which reinforced his results from 1773 that the volume of fixed air produced in combustion was equal to the volume of pure air consumed. Since the weights were so close, he thought that the two gases differed by some weightless substance such as matter of fire or phlogiston. In a few memoirs he even entertained the idea that phlogiston was matter of fire. However, another experiment in which he reacted charcoal and mercury calx and obtained fixed air, contradicted the idea that fixed air was merely a combination of common air and phlogiston or matter of fire. The results of this experiment implied (correctly) that fixed air was the combination of common air with charcoal. Clearly Lavoisier was far from solving the puzzle. However, even at this stage in his pathway toward discovery he sensed his inevitable deviation from the accepted norm when he wrote, “according to Mr. Priestley and many other physicians, the air in which one has burned candles is partly phlogisticated. I think, on the contrary, and I believe I have proven, that this air is only atmospheric air deprived of its pure respirable portion.
He decided that in order to fully comprehend the process of combustion he had to first understand the nature of matter of fire. He was confused because matter of fire had never been sufficiently defined by the proponents of the phlogiston theory. Therefore, early in 1777 he began a series of experiments with Pierre Laplace which greatly effected his mental model. Their experiments with matter of fire showed that the temperature of the surroundings fell whenever something evaporated. They explained this phenomenon by saying that the fluid they started with absorbed matter of fire and then turned into a gas. This drastically changed his mental model of even the existence of gases. In grade school, he was taught that liquids dissolved in the air in a similar manner as salts dissolve in water. It was also thought that only specific materials could become gases. Lavoisier showed, however, that all materials can exist in each of the three forms of matter; gas, solid, and liquid. He was able to accept these results when he changed his mental model from the one he had been taught as a child to one in which a “base” of a material combined physically with matter of fire to create the phase changes. The large amount of volume that gases take up in comparison to liquids and solids could then be explained by a large amount of massless matter of fire combined with a small amount of “base” matter. However, if matter of fire only changed materials from one state to another, he could no longer hypothesize that it combined with pure air in combustion to create fixed air.
This dilemma was still looming in his mind when he began a series of experiments involving pyrophor. Pyrophor was a substance obtained by distilling fecal matter and had the interesting property of catching fire spontaneously in air. During this process of combustion it gained weight. Intrigued by its bizarre physical properties, Lavoisier performed another experiment where he burned pyrophor under a jar and then analyzed the results. He concluded that pyrophor contained some amount of sulfur, which absorbed the pure air and accounted for the weight gain. He also hypothesized that it contained some charbon which changed the pure air consumed into fixed air. This latter portion, which he wrote in a memoir to the academy early in 1777, was the most important because he had finally formed a connection between all of his previous experiments. By suggesting that pyrophor contained charbon, he was implying that all of the substances being burned in combustion must contain some charbon-like substance to combine with pure air to form fixed air.
To test this new idea, he revived the calx of mercury in the presence of “matieres charbonneuse” and without it. The result was that without charbon, the reaction produced pure air and that with charbon the reaction produced fixed air. This gave qualified support for his idea that charbon combined with pure air to form fixed air. The most important aspect of his new theory was that the concept of phlogiston was completely unnecessary.
He finally had enough evidence backing his new theory to make some very broad and substantial attacks of the phlogiston theory. In a paper on combustion written in the middle of 1777, he wrote “I am at the point of attacking the entire doctrine of Stahl concerning phlogiston, and of undertaking to prove that it is erroneous in every respect. If my opinions are well founded, Mr. Priestley’s phlogisticated air will find itself entangled in the ruins of edifice.” However, even at this time he was having trouble overcoming the mental barriers and throwing out the old theory. Throughout this paper, he repeatedly wrote “dephlogisticated air” before catching himself, crossing it out and replacing it with “pure air.” In a personal sense, attacking the phlogiston theory was difficult because of his extreme respect for Mr. Priestley and his other colleagues. He constantly wrote about how much he had benefited from Priestley’s wisdom and it was extremely difficult to separate himself from those personal contacts for whom phlogiston was one of the set pieces of chemistry. When the academy met on September 5th, these feelings of hesitation kept him from sharing his results to the scientific community. Rather, he read a less controversial paper he had prepared on pyrophor.
When he finally did present his theory on November 12th, he did so very carefully. Rather than focus his attention on destroying the old chemical edifice of combustion, he made it clear that he was merely offering a substitute perspective. “In attacking the doctrine of Stahl, I claim to substitute for it not a rigorously demonstrated theory, but only a more probable hypothesis which presents fewer contradictions.” However, towards the end of the paper he could not help getting excited over the thoroughness of his theory writing that it “explains with marvelous ease almost all of the phenomena of physics and chemistry.” This difference from the beginning of the paper to the end suggests that in the process of writing he found that the theoretical structure he had outlined was even more promising than he had expected.
Thomas Kuhn, a man knowledgeable regarding scientific revolution stated in one of his numerous works that a scientists shift from one conceptual model to another occurs momentarily. However, Lavoisier’s process for “discovering” combustion was so prolonged and complex that it does not seem adequate to describe it with words like “shift” or “switch.” His change was more of an evolution in which he wavered back and forth before settling on one definite idea.
Lavoisier’s process fits quite well into the five generalizations of discovery outlined in Dr. Gorman’s book. Combustion was certainly a significant problem because it is the foundation on which modern chemistry builds. He transformed the problem into one which suggested a promising path to solution by using quantitative experiments in which the amounts of both reactants and products were heavily scrutinized. By structuring his experiments so that all the materials could be accounted for during and after the reaction, the combustion phenomenon became readily explained through quantitative analysis. If anywhere Lavoisier’s path towards discovery strayed slightly from the five generalizations it would be with the concept of good data. He was known for being willing to settle for imperfect results. However, this realistic rather than idealistic approach towards science is what led him to success. He was not bogged down with seeking a perfect analytical solution to a given problem but rather moved on and tackled the next problem. “His readiness to publish results based on experiments containing obvious flaws, in order to get on with his general program, made the most efficient use of his limited research time.” However, just because he didn’t test all of his thoughts in Karl Popper’s philosophical style of falsification does not mean that he ignored the effects of the results on his ideas. Rather, he simply made the judgment that it would be better to devote his time to creating a comprehensive theoretical structure than to put additional effort into some small areas to clear up minor discrepancies. In one of his classic essays, Jacques Hadamard stated that “in research, it may be detrimental to scatter our attention too much, while overstraining it in one particular direction may also be harmful to discovery.” Lavoisier managed to avoid both situations.
Lavoisier was extremely proficient with regards to the fourth generalization of finding the right combination of flexibility and stubbornness. In particular, he combined short range flexibility in regard to experimental impediments, such as weather conditions and obtaining necessary equipment, with a long-term persistence. He organized his life to direct himself for long periods of time toward coherent objectives. This can be seen by the fact that he split up his experimenting to answer specific questions. When he became blocked for some reason on a particular project, he always had another to be working on which usually illuminated problems in his other areas of study. This sustained effort over the course of four years was at least as important as any creativity or brilliance he was endowed with. In the end, he was able to combine all of the results from his experiments into a consistent theory.
The process of writing was a powerful tool Lavoisier used in the formation of new insights. While scientific papers usually represent the best combination of argument and evidence that the author can bring together to justify conclusions he has already reached, Lavoisier used them to transform clusters of ideas into organized units. In terms of the specific case of combustion, it was not until he began writing out the results of his data that he realized the contradictions with the accepted phlogiston theory. Sometimes he would use writing as a means for scientific investigation. He titled numerous pages in his logbooks “Ideas” and would then proceed to bring together multiple experiments and rationalize how their results coexisted in logical harmony. The purpose of this is relatively easy to comprehend. The complexity of the ideas we can hold in our head at any given time is very limited and we are more likely to elaborate on them when we write them out. Furthermore, one must remember that Lavoisier gathered all of his information on combustion over the course of four years. Often times people assume that scientists are able to hold in their minds continuously the entire conceptual field they have spent years developing. However, just as we can only focus on one object visually at a given time, we also are limited to considering only a small portion of a mental field at one point. Therefore, it makes sense that it was not until he was finishing his final paper regarding his new theory of combustion that Lavoisier filled in all the loopholes and blurred edges of ideas to bring his mental model into sharp focus.
Lavoisier’s road toward discovery fit well into the perspective of these five generalizations. His consistent production of incredible scientific studies up until his inopportune death in 1794 shows that a combination of personal creativity/intelligence and a strong adherence to Dr. Gorman’s five generalizations is an impressive recipe for discovery. Lavoisier recognized combustion as an important process from a chemical process and proceeded to change the problem into smaller questions which he could easily answer. While at times he would use data known to be slightly flawed so that he could work on other projects, he did not settle for such data in experiments which were dire to the fundamentals of his new principle. His combination of short term flexibility with long term stubbornness allowed him to “discover” over the course of four years after many scientists would have given up. Finally, he used writing as a means of both recording the results of his numerous experiments and exploring the nature of his investigation. The most befitting quote made referring to Lavoisier could have been the one made by Joseph Lagrange a few days after his beheading during the French Revolution. “It took only an instant to cut off that head, and a hundred years may not produce another like it.”