Acid-Base Reactions﻿: Dissolving Alchemy Into Chemistry

Acid-Base Reactions: Dissolving Alchemy Into Chemistry

3/29/2016

Acids are perhaps the first chemicals that humans recognize. They readily react to most materials, and the result of those reactions would appear magical. Although at first they became the essential tools of alchemists, they provide much of the answers in explaining the world in less mystical terms.

This article will deliver the story of how this transition occurred.

"Perhaps the most important of all concepts related to electronegativity and polarity is that of acidity and basicity. We’ll soon see, in fact, that the acid–base behavior of organic molecules explains much of their chemistry." - John McMurry, "Organic Chemistry", 2012

A funny thing about the history of chemistry; new things tend to be discovered inadvertently. Sometimes a chemist (or in earlier cases, alchemist) would seek to solve a specific problem but end up solving another. Thus is the case with acids and base. Probably the first chemical reactions to be recognized as they are: a process that involves rearrangement of the molecular or ionic structure of a substance, as opposed to a change in physical form.

Views of Antiquity

Probably the earliest discovery of acid-base reactions were brought on by the launderers of ancient Babylonia. The (likely apocryphal) story was that when animals were sacrificed on top of a certain hill, the fats and ash mixed together as the carcasses were burned in the sacrificial pyre, and as it rained, the mixture leeched into a nearby river of which the city’s launderers find to be particularly good for their job.

A little nearer to the present. During the time of the Ancient Greeks, the properties of acids and bases were only vaguely understood. During their attempts to categorize substances and try to bring balance, harmony, and perfection to the universe, they used a variety of tests to distinguish compounds. One of these was taste, and they divided substances according to whether they were sour, bitter, salty or sweet.

Savon de Marseille, a direct descendant of Roman soaps

As the Greek influence waned and their knowledge passed on to the Romans, they began to refer to sour substances, such as vinegar or lemon juice, as acids. The words 'acid' and 'acetic,' are both derived from the Latin word for 'sour-tasting,' acere. Bases, by contrast, were not as well studied, although they were recognized as substances that would neutralize acids, something that fitted well with the desire of the ancients to find balance and harmony in everything.

Antoine Lavoisier

Antoine Lavoisier

Lavoisier’s basic idea was that acidity was caused by the presence of oxygen in the compound. This idea turned out to be wrong, but it is historically important since it is the first systematic attempt to chemically characterize acids and bases. In early 1776, Lavoisier was able to write this: "It appeared proven . . . that the air we respire contains only a quarter of true air; that this true air is mixed, in our atmosphere, with three or four parts of an injurious air, a species of mophette, which causes most animals to perish, if the quantity of it is a little greater."

The "true air" is, of course, oxygen and the "species of mophette" is nitrogen. The more complex nature of the atmosphere, with carbon dioxide and the noble gases became apparent later. However, it was Lavoisier that was the first to show the atmosphere was composed of more than one substance.

At this time in scientific history, it was a common belief that the properties of acids could be traced back to a single substance. Lavoisier had studied the combustion of phosphorus and sulfur in 1772-73 and had proven that they combine with something in the atmosphere. He also knew that, when dissolved in water, phosphorus and sulfur oxides made acidic solutions.

Also, the test Lavoisier used to demonstrate the presence of "true air" was the nitrous air test, devised by Joseph Priestley. Lavoisier knew that nitrous air combined with oxygen and the resulting compound made nitric acid in water.

Hence, his conclusion (published in 1776) was that oxygen was the component in a compound that was responsible for the generic property of acid. The other portion of the compound combined with the oxygen was called an 'acidifiable base" and was responsible for the specific properties of the compound.

Humphry Davy & Justus Liebig

Events remained unchanged until about 1809-1810, when Davy entered the scene. He reacted many metals and non-metals with oxymuriatic acid and never obtained oxygen nor did he obtain any oxygen compounds. He heated charcol to white-hot temperatures in the presence of oxymuriatic gas and did not get a reaction, much less any oxygen evolved.

Here is some of what he wrote in 1810: "One of the singular facts I have observed on this subject, and which I have before referred to, is, that charcoal, even when ignited to whiteness in oxymuriatic or muriatic acid gases, by the Voltaic battery, effects no change in them; if it has been previously freed from hydrogen and moisture by intense ignition in vacuo. This experiment, which I have several times repeated, led me to doubt the existence of oxygen in that substance."

On September 23, 1809, Davy had written in a letter to a friend: "...the substance we took for Sulphuretted Hydrogene is telluretted Hydrogene... , a substance affording another proof that Hydrogene is an oxide."

Davy was reasoning that, since H2S and H2Te are acids, their common component (hydrogen) must contain oxygen, the principle of acidity according to Lavoisier. His ideas we’re not complete though until Justus Liebig came along.

Humphry Davy

Liebig proposed that an acid was a hydrogen-containing substance in which the hydrogen could be replaced by a metal. This definition proved sufficient for over 50 years until the advent of Svante Arrhenius. In fact, it is still compatible with modern definitions. He had no corresponding theoretical definition for bases. There were still identified, in an empirical fashion, as substances which neutralized acids. However, no one could explain why they did so. Arrhenius will be the first person to offer a more-or-less correct explanation for acid-base neutralization.

Svante Arrhenius

Svante Arrhenius

Svante Arrhenius was one of the towering giants of chemistry in the years surrounding the turn of the century. His most important contribution to chemistry was also his first - the idea of electrolytic dissociation. This idea, first published in 1883 and in refined form in 1887, was the mainstay of his doctoral dissertation. It was the source of much hurt in his life.

The basic idea is that certain substances remain ionized in solution all the time. Today, everyone accepts this without question, but it was the subject of much dissention and disagreement in 1884, when a twenty-five year old Arrhenius presented and defended his dissertation.

He was bitterly disappointed when the dissertation was awarded a fourth class (non since laude approbatur - approved not without praise) and his defense a third class (cum laude approbatur - approved with praise). Essentially, he got a grade of D for the dissertation and a C for his defense.

He could not obtain a job within his native Sweden, but he did get a travel grant and worked outside the country for several years. He did return in 1891, but even in 1895, his elevation to Professor of Physics was bitterly opposed as was his overdue election to the Swedish Academy of Sciences in 1901. However, he received the 1903 Nobel Prize in Chemistry for his electrolytic dissociation theory and that effectively ended public criticism.

Modern Acid-Base Theory

The Arrhenius definitions of acids and bases describe the characteristics of aqueous solutions of acids and bases, however this is still somewhat incomplete. In 1923, T. M. Lowry in England and J. M. Brønsted in Denmark proposed a system that defines acids and bases in terms of the mechanism by which they react. According to the Brønsted-Lowry definitions: An acid is a proton (H+) donor, and a base is a proton (H+) acceptor.

Because a hydrogen ion consists of a nucleus containing a single proton, the terms hydrogen ion and proton are synonymous. These definitions somewhat broaden the category of substances that are acids or bases. The category of acids now includes ions such as ammonium ion, NH4+, and bicarbonate ion, HCO3-. Among Brønsted-Lowry bases are the hydroxide ion, OH-; the anion of any acid; and ammonia, NH3. Many substances such as water, bicarbonate ion, and ammonia can act as either an acid or a base.

Johannes Bronsted (left), and Thomas Lowry

In the Brønsted-Lowry system, an acid reacts by donating a proton to a base. In doing so, the acid becomes its conjugate base. The formula of the conjugate base is the formula of the acid less one hydrogen. The reacting base becomes its conjugate acid. The formula of the conjugate acid is the formula of the base plus one hydrogen ion.

Conclusion

From the article, you can surely see the train of thought of chemists throughout the ages. From the simplistic views of the ancient Greeks to Brønsted and Lowry’s subatomic rationalization, acid-base reactions had provided much of the clues to how chemistry actually worked. As a matter of fact, acid-base reactions became the key to transitioning of chemistry from alchemy.

Ponder this

Lavoisier's most notable discovery is oxygen, of which is also the basis of his theory on acids. How likely it is that he blinkered himself into explaining everything through this worldview? The saying goes that "a man with only a hammer will see all problems as nails"; are there any assurances in the scientific method that is designed to avoid this pitfall?

Discuss

Discuss the train of though throughout the history of the acid-base reaction. What led to the thinkers in each successive era to their respective conclusion? What did they get wrong, and why? We're there any gaps of knowledge between the eras, if yes, what are they? How did other branches of chemistry and physics assist in the final conclusion?