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A summary of a chemistry experiment about acids

Theoretical definitions of acids and bases Hydrogen and hydroxide ions The first attempt at a theoretical interpretation of acid behaviour was made by Antoine-Laurent Lavoisier at the end of the 18th century.

8: Acid, Bases and pH (Experiment)

Lavoisier supposed that all acids must contain oxygenand this idea was incorporated in the names used for this element in the various languages; the English oxygen, from the Greek oxys sour and genna production ; the German Sauerstoff, literally acid material; and the Russian kislorod, from kislota acid.

Following the discovery that hydrochloric acid contained no oxygen, Sir Humphry Davy about 1815 first recognized that the key element in acids was hydrogen.

Not all substances that contain hydrogen, however, are acids, and the first really satisfactory definition of an acid was given by Justus von Liebig of Germany in 1838.

According to Liebig, an acid is a compound containing hydrogen in a form in which it can be replaced by a metal. This definition held the field for about 50 years and is still considered essentially correct, though somewhat outmoded. The whole subject of acid—base chemistry acquired a new look and a quantitative aspect with the advent of the electrolytic dissociation theory propounded by Wilhelm Ostwald and Svante August Arrhenius both Nobel laureates in the 1880s.

The principal feature of this theory is that certain compounds, called electrolytes, dissociate in solution to give ions. It was also realized at that time that there is a correspondence between the degree of acidity of a solution as shown by effects on vegetable dyes and other properties and the concentration of hydrogen ions in the solution. This led naturally to the simple definition that acids and bases are substances that give rise, respectively, to hydrogen and hydroxide ions in aqueous solution.

This definition was generally accepted for the next 30 or 40 years.

Cabbage Chemistry--Finding Acids and Bases

Nevertheless, there is a great advantage in the definition of acids and bases in terms of hydrogen and hydroxide ions, and this advantage lies in its quantitative aspects.

Because the concentrations of hydrogen and hydroxide ions in solution can be measured, notably by determining the electrical conductivity of the solution its ability to carry an electrical currenta quantitative measure of the acidity or alkalinity of the solution is provided. Moreover, the equations developed to express the relationships between the various components of reversible reactions can be applied to acid and base dissociations to give definite values, called dissociation constants.

These constants can be used to characterize the relative strengths degrees of dissociation of acids and bases and, for this reason, supersede earlier semiquantitative estimates of acid or base strength.

As a result of this approach, a satisfactory quantitative description was given at an early date of a large mass of experimental observations, a description that remains essentially unaffected by later developments in definitions of acid—base reactions.

Acid–base reaction

The success of these quantitative developments, however, unfortunately helped to conceal some ambiguities and logical inconsistencies in the qualitative definitions of acids and bases in terms of the production of hydrogen and hydroxide ions, respectively. For example, it was not clear whether a substance like anhydrous hydrogen chloridewhich would not conduct electricity, should be regarded as an acid or whether it should be considered an acid only after it had come in contact with water.

These considerations led to the development of definitions of acids and bases that depended on the solvent see below Alternative definitions. In spite of this change, however, the difficulty still remained that typical acid—base properties, such as neutralization, indicator vegetable dye effects, and catalysis, often took place in solvents such as benzene or chloroform in which free ions could barely be detected at all by conductivity measurements.

Even for aqueous solutions a particular ambiguity arises in the definition of bases, some of which for example, metallic hydroxides contain a hydroxyl group, whereas others such as amines do not. The latter produce hydroxide ions in solution by reacting with water molecules.

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