The History of Chemistry
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Diposting oleh
Awal
Chemistry has evolved over thousands of years to become the sophisticated science it is today. As long ago as 3500 B.C., the early Egyptians were skilled in the production of wine and had discovered that certain metals such as copper and tin could be obtained by roasting metal ores in a fire with charcoal.
By about 1500 B.C., the Hittites discovered that when iron and charcoal are heated a much harder from of iron is produced. This material, which is today called steel, was used to produce a range of tools and weapons.
The Greek philosophers, including Democritus and Aristotle, were the first to attempt to understand the nature of matter. In fact, Democritus proposed the existence of atoms. He believed that the universe consisted of one kind of atom of varying sizes and shapes. About 350 B.C., Aristotle rejected the idea of atoms. He considered that the matter consisted of different proportions of four elements – earth, water, air and fire. Thisview of ma tter dominated Western scientific thought for about 2000 years until the development of modern chemistry.
In the period A.D. 500 – 1600 attention shifted from an interest in the nature of matter to more practical concerns. The ‘chemists’ of that time, better known as the alchemists, had two main aims. The first was to find a method of converting metals such as iron, zinc and copper into gold. The second aim was to discover an ‘elixir of life’ which would prolong life indefinitely. The alchemists relied heavily on experimentation but their activities were not guided by the development of scientific theories. As well, much of their work was done in secret and their findings were not subject to the opendebate which is characteristic of science today.
Despite the limitations of the alchemists’ approach, many important chemical substances were produced in this period. These included alcohol, arsenic, zinc, and hydrochloric, sulfuric and nitric acid.
By about 1500 B.C., the Hittites discovered that when iron and charcoal are heated a much harder from of iron is produced. This material, which is today called steel, was used to produce a range of tools and weapons.
The Greek philosophers, including Democritus and Aristotle, were the first to attempt to understand the nature of matter. In fact, Democritus proposed the existence of atoms. He believed that the universe consisted of one kind of atom of varying sizes and shapes. About 350 B.C., Aristotle rejected the idea of atoms. He considered that the matter consisted of different proportions of four elements – earth, water, air and fire. Thisview of ma tter dominated Western scientific thought for about 2000 years until the development of modern chemistry.
In the period A.D. 500 – 1600 attention shifted from an interest in the nature of matter to more practical concerns. The ‘chemists’ of that time, better known as the alchemists, had two main aims. The first was to find a method of converting metals such as iron, zinc and copper into gold. The second aim was to discover an ‘elixir of life’ which would prolong life indefinitely. The alchemists relied heavily on experimentation but their activities were not guided by the development of scientific theories. As well, much of their work was done in secret and their findings were not subject to the opendebate which is characteristic of science today.
Despite the limitations of the alchemists’ approach, many important chemical substances were produced in this period. These included alcohol, arsenic, zinc, and hydrochloric, sulfuric and nitric acid.
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Rutherford Atomic Model
By the early 1900s, it was clear that each atom contains regions of both positive and negative charge. The question was, how are these charges distributed? The dominant view of that time was summarized in J. J. Thomson’s model of the atom; the positive charge was assumed to be distributed evenly throughout the atom. The negative charges were pictured as being imbedded in the atom like plums in a pudding (the “plum pudding model”). Soon after Thomson developed his model, tremendous insight into atomic structure was provided by one of Thomson’s former students, Ernest Rutherford (1871–1937), who was the outstanding experimental physicist of his time.By 1909, Ernest Rutherford had established that alpha particles are positively charged particles. They are emitted at high kinetic energies by some radioactive atoms, that is, atoms that disintegrate spontaneously. In 1910, Rutherford’s research group carried out a series of experiments that had an enormous impact on the scientific world. They bombarded a very thin piece of gold foil with alpha-particles from a radioactive source. A fluorescent zinc sulfide screen was placed behind the foil to indicate the scattering of the alpha-particles by the gold foil (Figure 5-4). Scintillations (flashes) on the screen, caused by the individual alpha-particles, were counted to determine the relative numbers of alpha-particles deflected at various angles. Alpha particles were known to be extremely dense, much denser than gold.
If the Thomson model of the atom were correct, any alpha-particles passing through the foil would have been deflected by very small angles. Quite unexpectedly, nearly all of the alpha-particles passed through the foil with little or no deflection. A few, however, were deflected through large angles, and a very few -particles even returned from the gold foil in the direction from which they had come! Rutherford was astounded. In his own words, It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you.
Rutherford’s mathematical analysis of his results showed that the scattering of positively charged alpha-particles was caused by repulsion from very dense regions of positive charge in the gold foil. He concluded that the mass of one of these regions is nearly equal to that of a gold atom, but that the diameter is no more than 1/10,000 that of an atom. Many experiments with foils of different metals yielded similar results. Realizing that these observations were inconsistent with previous theories about atomic structure, Rutherford discarded the old theory and proposed a better one. He suggested that each atom contains a tiny, positively charged, massive center that he called an atomic nucleus. Most alpha-particles pass through metal foils undeflected because atoms are primarily empty space populated only by the very light electrons. The few particles that are deflected are the ones thatcome close to the heavy, highly charged metal nuclei.
Rutherford was able to determine the magnitudes of the positive charges on the atomic nuclei. The picture of atomic structure that he developed is called the Rutherford model of the atom.
If the Thomson model of the atom were correct, any alpha-particles passing through the foil would have been deflected by very small angles. Quite unexpectedly, nearly all of the alpha-particles passed through the foil with little or no deflection. A few, however, were deflected through large angles, and a very few -particles even returned from the gold foil in the direction from which they had come! Rutherford was astounded. In his own words, It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you.
Rutherford’s mathematical analysis of his results showed that the scattering of positively charged alpha-particles was caused by repulsion from very dense regions of positive charge in the gold foil. He concluded that the mass of one of these regions is nearly equal to that of a gold atom, but that the diameter is no more than 1/10,000 that of an atom. Many experiments with foils of different metals yielded similar results. Realizing that these observations were inconsistent with previous theories about atomic structure, Rutherford discarded the old theory and proposed a better one. He suggested that each atom contains a tiny, positively charged, massive center that he called an atomic nucleus. Most alpha-particles pass through metal foils undeflected because atoms are primarily empty space populated only by the very light electrons. The few particles that are deflected are the ones thatcome close to the heavy, highly charged metal nuclei.
Rutherford was able to determine the magnitudes of the positive charges on the atomic nuclei. The picture of atomic structure that he developed is called the Rutherford model of the atom.
"Atoms consist of very small, very dense positively charged nuclei surrounded by clouds of electrons at relatively great distances from the nuclei".
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Milikan Experiment Animation
Once the charge-to-mass ratio for the electron had been determined, additional experiments were necessary to determine the value of either its mass or its charge, so that the other could be calculated. In 1909, Robert Millikan (1868–1953) solved this dilemma with the famous “oil-drop experiment,” in which he determined the charge of the electron.The Millikan oil-drop experiment. Tiny spherical oil droplets are produced by an atomizer. The mass of the spherical drop can be calculated from its volume (obtained from a measurement of the radius of the drop with a microscope) and the known density of the oil. A few droplets fall through the hole in the upper plate. Irradiation with X-rays gives some of these oil droplets a negative charge. When the voltage between the plates is increased, a negatively charged drop falls more slowly because it is attracted by the positively charged upper plate and repelled by the negatively charged lower plate. At one particular voltage, the electrical force (up) and the gravitational force (down) on the drop are exactly balanced, and the drop remains stationary. Knowing this voltage and the mass of the drop, we can calculate the charge on the drop.
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Limiting Reagent
From the balanced equation for a chemical reaction it is possible to calculate the exact quantities of reactants which are consumed and products which are formed. For example, consider the reaction between nitrogen momooxide gas and oxygen gas to form nitrogen dioxide. The equation for the reaction is :
3 NO (g) + O2 (g) → 3 NO2 (g)
From the equation it is evident that every mole of O2 which reacts, three moles of NO are needed and three moles of NO2 will be produced. However, consider the situation where the ratio of coefficients in the balanced chemical equation. In this situation, one of the reactants will be the limiting reagent and the other will be present in excess.
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3 NO (g) + O2 (g) → 3 NO2 (g)
From the equation it is evident that every mole of O2 which reacts, three moles of NO are needed and three moles of NO2 will be produced. However, consider the situation where the ratio of coefficients in the balanced chemical equation. In this situation, one of the reactants will be the limiting reagent and the other will be present in excess.
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