Class Note for CHEM 111 at UMass(27)
Class Note for CHEM 111 at UMass(27)
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This 2 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Massachusetts taught by a professor in Fall. Since its upload, it has received 12 views.
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Date Created: 02/06/15
Light behaves as both waves and particles E hc t ht Vibrations of atoms in solids are quantized For visible light photon energies have similar values to enthalpies of reaction few hundreds of kJ per mol Light can cause chemical reactions photochemistry Lower wavelengths can be suf ciently energetic to remove electrons from molecules and atoms hence the term ionizing radiation How to explain the limited number of wavelengths in the emission spectrum of hydrogen Bohr suggested that the wavelengths correspond to the transition of electrons between quantized energy levels whose energies were given by En Rhcn2 where R is the Rydberg constant 10974 X 107 m391 and n is an integer 1 2 3 known as the principal quantum number Rhc 2179 x 1018 J per atom Bohr imagined that the electrons moved in orbits around the nucleus like planets around the sun The higher the value of n the further away from the nucleus and the less negative the energy the higher the energy The lowest energy level n 1 is known as the ground state All other energy levels are excited states The visible lines in the H spectrum are due to transitions to the n 2 level If light energy is emitted then AE must be negative Example Calculate the wavelength ofthe red line in the spectrum if the initial energy level is n 3 AE E2 E3 nal state initial state Rhc4 Rhc9 536 x Rhc hC7 so 365R m 36x1095x10974x107 nm 6561 nm Also calculate the ionization energy of hydrogen as the energy needed to remove an electron from n 1 to n infinity IE o Rhc1 2179x103918 Jatom 2179x103918x6022x1023Jmol391 13121938 J mor1 1312 kJ mor1 Atoms have characteristic emission spectra Summary energy level diagram Ladder rungs not equally spaced in a well see Fig 710 The intensity of emission is directly proportional to the number of atoms and atomic emission spectroscopy is widely used in chemical analysis for quantitative measurements It is also quite useful for qualitative analysis The intensity of emission is directly proportional to the number of atoms and atomic emission spectroscopy is widely used in chemical analysis for quantitative measurements It is also quite useful for qualitative analysis Atoms also absorb radiation though this is more difficult to demonstrate The wavelengths are the same as those at which they emit The amount of light absorbed is directly proportional to the number of atoms Atomic absorption spectroscopy is widely used for quantitative analysis The light source is a special lamp which emits the spectrum of the element to be determined Most materials for analysis are solids or liquids aqueous solutions In atomic spectrometers atoms are formed by one of several different means the most common is spraying solutions into hot gases The hot gases are produced by either a combustion ame air and acetylene or by the radio frequency induction heating of ionized argon the socalled inductively coupled plasma With these techniques metals and metalloids can be quantified at concentrations down to a few ng per liter parts per billion ppb Further information http instruments perkinelmer com ai atomic index asp
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