Chemistry: Animation, Video, and Laboratory Activity

The effect of changing the temperature of equilibrium system can be predicted from a knowledge of the heats of reaction. If the temperature of a system is lowered, the exothermic reaction is favoured. If the temperature isincreased, the endhotermic reaction is favoured.

The forward reaction therefore absorbs heat from the surroundings and the reverse reaction releases heat to the surroundings. According to Le Chatelier’s principle, if the temperature of an equilibrium system is increased the system will adjust to re-establish equilibrium in such a way as to decrease the temperature. Therefore in re-establishing equilibrium the endothermic reaction is favoured as this absorbs heat from the surroundings and would tend to decrease the temperature. In the N2O4/NO2 system this would result in an increase in the concentration of NO2 and a decrease in the cpncentration of N2O4.
If the temperature of the equilibrium system was reduced the system would re-establish equilibrium by favouring the exothermic reaction. This would tend to increase the temperature of the surroundings, partially counteracting the change. In the N2O4/NO2 system, more N2O4 and less NO2 would be present when equilibriuym was re-established.

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.

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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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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If electricity is passed through a discharge tube containing hydrogen gas at very low pressure, the hydrogen molecules are split into individual atoms.these excited hydrogen emit a violet light which, when passed through a prism, produces a specrtum consisting of a series of bright lines separated by dark spaces.

The spectrum produces is called a line emission spectrum and indicates that excited atoms emit light of certain frequencies only. The occurrence of line emission spectra for hydrogen and other elements could not be satisfactorily explained in terms of classical physics at the beginning of the twentieth century. To account for these observations a radical revision of the laws and theories of physics was required.

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NaCl (s) → Na+(aq) + Cl-(aq)

This process is known as dissociation. The ions in the ionic solid have been separated in the solution process.

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In gases the particles are widely spaced and move virtually independently of one another. Because of this a gas will become dispersed throughout any container into which it is placed. Also diffusion can occur rapidly. Gases are very compressible because the particles are relatively widely spaced compared with the size of the particles.

The Geiger-Marsden experiment (also called the Gold foil experiment or the Rutherford experiment) was an experiment done by Hans Geiger and Ernest Marsden in 1909, under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester which led to the downfall of the plum pudding model of the atom. They measured the deflection of alpha particles (helium ions with a positive charge) directed normally onto a sheet of very thin gold foil. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees; some were even scattered back toward the source. From this observation Rutherford concluded that the atom contained a very physically-small (as compared with the size of the atom) positive charge, which could repel the alpha particles if they came close enough, subsequently developed into the Bohr model.

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‘If a chemical system at equilibrium is subjected to a change in conditions, the system will adjust to re-establish equilibrium in such a way as to partially counteract the imposed change.’

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