Chemistry Open Book Paper

Chemistry Open Book Paper Atomic spectroscopy is a chemical analysis technique and it is used to identify what elements are in a compound. It uses the idea of a photon being absorbed or emitted whenever an electron changes from one energy level to another. The diagram below shows sodium salt being sprinkled onto a flame and yellow light being emitted. Emission spectra are produced by thin gases in which the atoms do not experience many collisions (because of the low density). The emission of yellow light occurs because the electrons of the sodium salt have been promoted to a higher electronic energy state but have then fallen back down and emitted the energy as an electromagnetic wave, in the wavelength corresponding to yellow which is 600 nm.

The more intense that the yellow band is the more abundant the sodium salt present. The diagram (left) shows an atomic excitation caused by the absorption of a photon and an atomic de-excitation caused by emission of a photon. In each case the wavelength of the emitted or absorbed light is exactly such that the photon carries the energy difference between the two orbits. This energy may be calculated by multiplying the Plank constant by the wavelength of the light. Thus, an atom can absorb or emit only certain discrete wavelengths (or equivalently, frequencies or energies).

This diagram shows white light being shone through sodium vapour and the resulting spectra on a board. An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the spectrum. Absorption spectroscopy can only be carried on a substance in solution or gaseous form. The presence of the dark band shows that the sodium vapour had absorbed the light in the yellow region. Sodium salt has absorbed energy but it is not re-emitted or just not re-emitted efficiently and so the wavelength of the light increases, leaving the observed dark band where yellow was expected.

The Sun appears yellow because that is the main wavelength the sun emits radiation at. This is shown in a graphically below. For each hot object there is a corresponding colours. Those stars with colours of lower wavelengths are lower in temperatures. For example something that appears red has a temperature of 3000 K. But something blue has a temperature of 10000 K.

So finding the predominant colour of the sun then its temperature could be determined. The Suns spectrum resembles that of something around 5000 K. By studying the emission spectra captured on the photographic film for dark bands , the composition can be found. Because interstellar clouds have a temperature of between 10-50 K, radiation emitted has much shorter wavelengths, so different techniques to the ones above have to be used. The wavelengths are in fact in the order of 0.001 m so the can be picked up by radio telescopes on earth. Here is a picture of one below.

A neutral hydrogen atom (H I) consists of 1 proton and 1 electron. The proton and electron spin like tops with their spin axes either parallel or anti-parallel. When hydrogen atoms switch from the parallel to the anti-parallel configuration they emit radio waves with a wavelength of 21 centimetres and a corresponding frequency of exactly 1420 MHz. This is called the 21-centimetre line. Thus, radio telescopes tuned to this frequency can be used to map the great clouds of neutral hydrogen found in interstellar space. Radio telescopes identify which elements are present and how abundant they are and then the conditions are replicated here on earth.

But this requires keeping the elements in gaseous form at low temperatures (as low as 7 K) without condensing. This requires using CRESU apparatus .It takes advantage of the flow properties of gaseous expansions from convergent-divergent Laval nozzles into low-pressure environments, producing a flow of gas, which is uniform in temperature, density and velocity, and carries on for hundreds of millimetres and hundreds of microseconds after leaving the nozzle exit. Frequent collisions occur during the controlled expansion within the nozzle .The expansion is slow enough to maintain thermal equilibrium, but rapid enough that condensation is avoided. A uniform, ‘collimated’ flow results at the exit of the nozzle. This uniform supersonic flow provides a good environment in which to perform experiments on “collisional” processes at extremely low temperatures.

The said gas is made up of three components: the source of the radicals (which are to be broken up by a generating laser), a molecule to react with the radicals and a chemically inert gas to carry the other two gases. Removal of the radicals is followed by another laser which exites the radical and fluorescence can be observed .The rate constant can be determined by increasing the time delay between the generating laser and the detecting laser because the fluorescence will fall as the radical is being removed due to reaction. Abstract A photon is absorbed when an electron is raised to a higher energy level and emitted when falling to a lower level. Each element has discreet energy levels, which can be identified by looking at absorption and emission spectra so the composition of anything which emits radiation can be found. Acknowledgements The sources of additional information used were: Salters Advanced Chemistry “Chemical Ideas”. Internet resource pages / Encarta 2000.

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