Emission Spectroscopy

Emission Spectroscopy

Experiment #1

Abstract:

The purposes of this experiment were to look at the colors of elements emitted in flame tests and to be able to identify an unknown metal from the colors and to examine the atomic emission spectra of mercury and hydrogen and use the spectra to find the wavelength of each color emitted. The following metal salts were tested in the flame of a Bunsen burner:  calcium chloride, sodium chloride, barium chloride, potassium chloride and copper chloride. As each metal produced a unique and vibrant color it was determined that unknown # 3 was light purple.  The Rydberg equation was used to calculate the wavelengths for the hydrogen lines produced from a hydrogen gas tube. The calculated values were 434 nm for purple, 486 nm for blue and 656 nm for red. A spectroscope was used to view the lines produced from a mercury vapor tube and a hydrogen vapor tube. The colors and positions were recorded. Using the known wavelength values for the four mercury lines a calibration curve was made and used to find the wavelength values for the three hydrogen lines.  They were found to be 406.202 nm for purple, 476.6827 nm for blue and 641.137 nm for red.

Background:

There are energy levels or “shells” in atoms and molecules and when electrons jump between these shells they emit light and certain colors. When an atom is in ground state, the electron in the atom is in the lowest energy level. When the electron experiences specific amounts of energy, it jumps up to the next energy level to become in its excited state. This energy can be added to the atom in the form of light, heat, or an electric discharge. This extra energy is then released when the excited electron falls back to its ground state and emits light. Because each element has a unique energy level system, each element emits different colored light.

        In this experiment emission colors are observed in two ways: performing a flame tests on various metals and using a spectroscope. In the case of the flame test, a Bunsen burner will be used to excite the ions of the ionic salt compounds. Because of the added heat, the electrons emit energy and a color will be produced. This color will characterize the element and will help identify the unknown element.

In the case of using the spectroscope, the color of light emitted by excited gases of elements in sealed glass will be observed. Electricity is used to excite the atoms in the tubes and the excited atoms will then release the energy they gain through heat and light. Each atom that goes back to its ground state releases a single pulse and since there are so many pulses it seems continuous.  The color seen by the human eye is a combination of many colors or light. The spectroscope separates these different lights into a color spectrum so one can see the specific colors emitted by the light source. When pointing the spectroscope at the light source, the emission lines can be seen. The spectrum for mercury and hydrogen will be observed  and then the spectral lines can be observed.

The mercury lamp will be used to calibrate the spectroscope. Mercury emits four distinct lines: violet (404.7 nm), blue (435.8nm), green(546.1 nm), yellow (579.0 nm). Using the known wavelengths values as x values and the observed positions seen through the spectroscope as y values, a calibration curve can be prepared. Then the observed positions can be compared to the calibration curve to get the wavelength of the lines observed for hydrogen. The relationship between wavelength and energy levels for hydrogen can be demonstrated by the Rydberg equation 1/λ=R[(1/n2²−1/n1²)]. λ is the wavelength of light, R is a constant (1.097x 107 m-), and n1 and n2 are the energy levels which the electrons travels.

Experimental:

Frist in part 1, the normal lights were looked at through the spectroscope and the colors and range on the spectrum where the light was visible was recorded. In part 2, looking at the mercury lamp, 4 lines, violet, blue, green, and yellow, were observed and the positions were recorded. For the hydrogen lamp, 3 lines, violet, blue, and red, positions were recorded.  Then, a calibration curve from the mercury lamp data was made using the graphical analysis program. The wavelength was the x-axis and the observed position was the y-axis. Then the slope equation was found for this graph. In part 3, a 400ml beaker was filled with distilled water. The Bunsen burner was lit with the striker. For the control, a cotton swab was dipped in water and the flame was observed. The unknown metal salt was recorded for future reference.  A small amount of each metal salt was added to a small labeled weighing dish. A new cotton swab was dipped in the distilled water and placed into the first known metal salt. This was then placed in the Bunsen burner and the color was observed. This action was then repeated with each metal salt using a new cotton swab each time. When finished, the weighing dishes were rinsed in a beaker and poured into the proper waste container.