Line Spectra Lab

Objective

The objective of this lab is broken into two parts. The first is to recognize the principles of flame ionization and atomic spectra. The second is to observe and thus further understand the line spectra for multiple elements or molecules as well as determine the correlation between emission spectra and atomic structure.

Experimental Procedure

Procedure 1: Observation of Line Spectra by Discharge Tubes Six discharge lamps were selected – argon, carbon dioxide, helium, hydrogen, iodine, mercury, and xenon. Using the spectroscopes, the wavelengths, or the average of them, were recorded. It was important to turn on the discharge tubes for only 10 seconds or less at a time with the spectroscope only approximately one inch from the
tube for the greatest results.

Procedure 2: Observation of Line Spectra by Flame Ionization A Bunsen burner was used to produce a source of energy by means of flame. Each of the six ions were obtained: Li+, Na+, K+, Ca2+, Sr2+, and Ba2+. A labeled Nichrome wire was first dipped into water and then immediately dipped into the salt matching the label on the Nichrome wire, resulting in the compilation of several salt crystals on the loop. The loop was then placed in the flame, resulting in the ionization of the salt. While one person held the wire in place, the line spectrum was recorded using the spectroscope. The color of the flame was also recorded. When complete, the Bunsen burner was shut off and the wire was once again dipped in water to rinse and cool.

Results

Table 1, below, shows the resulting energies of each discharge tube. In observation of the discharge tubes in part one of the experiment, the average wavelength was recorded, table 1, in order to calculate the energy. This energy was calculated using the equation shown in equation 1 below.

Table 1: Line Spectra by Discharge Tubes

Element Name
Color
Wavelength (nm)
Energy (J)
Ar
Purple
420
4.73 x 10-19
CO2
continuous
400-680
4.97 x 10-19 - 2.92 x 10-19
He
Blue – Red
440-680
4.52 x 10-19 - 2.92 x 10-19
H2
continuous
400-680
4.97 x 10-19 - 2.92 x 10-19
I
Red
640
3.11 x 10-19
Hg
Purple - Yellow
400-580
4.97 x 10-19 - 3.43 x 10-19
Xe
Blue
460
4.32 x 10-19

During the second part of the experiment, the flame color was recorded as well as the wavelength of that color shown on the spectroscope, used to calculate its energy using equation 1. The color, wavelength, and energy was used to determine the two unknowns.

Table 2: Line Spectra by Flame Ionization
Element Name
Color
Wavelength (nm)
Energy (J)
K+
Pink
420
4.73 x 10-19
Na+
Orange
590
3.37 x 10-19
Sr2+
Pink
615
3.23 x 10-19
Ca2+
Orange
620
3.21 x 10-19
Li+
Red
680
2.92 x 10-19
Ba2+
Orange
580
3.34 x 10-19 unk A
Orange
590
3.37 x 10-19 unk B
Red
620
3.21 x 10-19

Equation 1: Sample Calculation of Energy for K+

Discussion/ Conclusion

The determination of the wavelength of both the discharge tubes and flame ionization can be accomplished by looking through the spectroscope. The spectroscope depicts colored lines at various wavelengths (line spectrum), representing the separation in energy levels. Each atom has a different line spectrum. Atoms with a continuous spectrum contains all wavelengths of visible light. The energy of each atom represents the differences in energy between the energy levels.

When observing the energy calculations in Table 1, it is clear that tubes with lower wavelengths result in higher energies and ones with higher wavelengths result in lower energies. For example, the molecule CO2 has a continuous spectrum. An example calculation of the lowest to greatest wavelengths is below:

As shown in these equations, the purple visible light, beginning at 400nm, has the greatest energy while the red visible light, ending at 680nm, has the lowest energy.
Thus the order of color from least energetic to most energetics is purple, blue, green, yellow, orange and red. It seems that all atoms and ions emit a line spectra if there is a separation in energy levels. This speaks to the reason that different atoms have different line spectra, as the separation in energy levels from Na to Mg or Ba to Li is different due to their atomic structures.

In the second part of the lab, it was possible to identify unknown A and unknown B by comparing color of the flame, observed wavelength and thus the energy. Unknown A appears to be NaCl due to the fact that it emitted an orange color at 590nm with the same energy. They shared the same line spectra and qualities, leading to the conclusion that Unknown A was certainly NaCl. Unknown B was also determined through these three qualities, leading to the understanding that Unknown B was CaCl2 as they also shared the same line spectra and very similar flame color. Regardless of the excitation source (flame or electricity), the line spectrum would be the same, resulting in these same answers, as it is still the emittance of visible
light.

Conclusion Through determining the wavelengths and energies of the elements, ions, and molecules, it was possible to further understand atomic line spectra. Unfortunately, there were errors associated with this lab due to human error, as it is very possible to misread the line spectra. In order to assure more accuracy in the future, it would be best to record all wavelengths observed on the spectroscope rather than just one or the average.