Lab Report 10 PDF

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Lab Report 10...

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Fall 2017 CH 227- Afternoon Class

Experiment #10: Emission Spectroscopy

Prepared for: Emma Tran Prepared by: David McNeely

January 30, 2018

Introduction Intro and Theory Emission spectroscopy is an invaluable technique used in many different facets of scientific research and data collection. Its uses range from material identification of distant stars to precise analysis of many different pharmaceuticals. Emission spectroscopy allows one to view the emission spectra of a light source through a specialized tool (spectroscope) in order to analyze the source’s constituent wavelengths. Beginning Questions The questions that were recorded before this experiment was conducted were “is there a predictable pattern to the emission spectra?” and “is there a relationship between wavelength and frequency?”. Safety Care should be taken when using the box knife during the construction of one’s DIY spectroscope. Procedure The steps to complete this experiment are very straightforward. First, one must construct a spectroscope using a cardboard box and assorted materials received in the assembly kit. Then, the spectroscope must be calibrated using a sample CRT (mercury, in this case). Finally, other CRT samples can be observed and their constituent wavelengths recorded. Experimental Data and Observations

Table 1: Mercury Calibration Data Color orange green blue violet

Position (mm) 155 148 127 118

Table 2: Hydrogen Spectroscope Data

Color violet blue cyan red

Position (mm) not observed 118 121 179

Neon CRTMany light wavelengths (12+), lines are very crowded, colors include: dark green, violet, yellow, orange, many shades of red Helium CRTOnly ~6 wavelengths, lines are spread out, colors include: violet, dark blue, cyan, light green, yellow, light red

Calculations and Graphs

Wavelength: λ(nm)=

distance (mm )−(y intercept ) slope

Ex. Hydrogen Wavelengths Using Calibration Data from Figure 1 λ=

118 mm−38.964 =398 nm 0.1986

Energy: Δ E ( J )=

hc λ

Ex. Hydrogen Energy from shortest wavelength Δ E=

6.63 ¿ 10−34 J∗s ∙ 2.998∗108 m /s =4.99∗10−19 J −7 3.98∗10 m

Hydrogen Final Energy Level:

√

Rh(i . e . slope) y intercept

√

Rh(i . e . slope) −3∗10−17 = =3.16 y intercept 3∗10−18

nf =

Ex. Hydrogen Final Energy Level using data from Figure 2 nf =

√

Mercury Calibration 160.0

Position (mm)

150.0

f(x) = 0.2 x + 38.96 R² = 0.99

140.0 130.0 120.0 110.0 100.0 350.0

400.0

450.0

500.0

550.0

Wavelength (nm)

Figure 1: Mercury Calibration Graph

600.0

Rydberg Constant Derivation 0 0

Delta E (J)

0 0

f(x) = − 0 x + 0 R² = 0.63

0 0 0 0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

N sub i

Figure 2: Rydberg Constant Derivation Graph from Hydrogen Energy Data

Claims and Evidence Based on the results from this experiment, it seems that there is not a clear pattern on which one can predict the emission spectrum of any given element. On the other hand, there is a clear relationship between wavelength and frequency; they are inversely related to one another. Due to a lack of easily-identifiable patterns between emission spectra, it seems safe to say that one cannot predict what the emission spectrum would look like for any random atom. Conversely, one can logically explain the relationship between wavelength and frequency. As the wavelength of a wave decreases, more oscillations can occur in the same amount of space compared to when the wavelength was longer, therefore increasing the frequency.

Analysis and Discussion

The correct value for nf is 2. Since my calculations show that nf is 3.16, it is clear that an error occurred somewhere during the experimentation process (likely in the wavelength position measurement). The accepted value (provided in the lab notebook) for the Rydberg constant is 2.18∗10−18 J . My calculations predicted the Rydberg constant to be −3∗10−17 J (Figure 2). Again, this error likely arises from a mismeasurement of the wavelength position in the DIY spectroscope. *****CORRECRED SECTION- THANK YOU EMMA *********** The class value for the Rydberg constant was RH = ( 2.2 +/- 0.5 ) x 10-18 J . When compared to my prediction of −3∗10−17 J , it is safe to say that there was a fair amount of error involved in my calculations. This was determinate error due to the fact that it likely occurred during the reading of the distance marks in the spectrometer (Table 1). The class value for the final state quantum number was nf = 2.0 +/- 0.1. Again, my prediction of 3.16 is far from the actual value. Since my calculated Rydberg constant value was incorrect, this error then skewed my value for nf. Due to the fact that nf was derived from the calculated Rydberg constant, both of these errors can likely be attributed to the same source.

Discussion Questions Emission spectroscopy involves the study of the spectra of electromagnetic radiation (i.e. light, in this case) that is emitted from a particular sample. The source of the spectral lines is the radiation emitted by the electrons of an atom as they go from an excited state to their natural energy states. The color of any given wave of light is directly related to the wavelength of the wave. The frequency of a wave of light is inversely proportional to its wavelength. The energy of a wave of light is directly related to its frequency. The energy calculated from the energy of the lines represents the amount of energy released by an excited electron as it leaves its excited states and enters its natural state. The class value for the Rydberg constant was quite close to the actual Rydberg constant (falling within the class value’s standard deviation). This indicates that any calculations made using the class value for the Rydberg constant should be relatively accurate compared to using the actual value of the Rydberg constant. Having an increasingly large number of measurements increases the confidence one should have for the accuracy of any calculated values. Since the sample size grows larger, there is less chance that only a few of the measurements are skewing the average value. Given the accurate class values for both the Rydberg constant and the final energy state quantum number, it is safe to assume that the laboratory techniques used for this experiment were efficient and accurate. The accuracy and precision are limitations of my spectroscope. With the lack of clear grid lines on the graph paper used in my spectroscope came an increased difficulty to obtain accurate and repeatable measurements of the spectral lines. For the most part, I feel I was able to assemble the spectroscope quite well; however, my calculated Rydberg constant seems to imply otherwise. I am not entirely sure where I went wrong in the construction and use of my spectrometer, but something definitely went wrong during the process. A practical application of spectroscopy is the identification of the composite elements of distant stars by astrophysicists. The principles

observed in this experiment can be used to lead toward greater energy sustainability by potentially allowing one to analyze and compare the amount of type of light emitted by different elements I order to determine which can be used most efficiently. The downside I terms of environmental impact for this idea could be the energy and amount of resources required for the initial testing and comparison. However, it seems as though once the most efficient element has been determined, the environmental benefits of using it would easily outweigh the initial negative aspects.

***********END OF CORRECT SECTION*********

Unique Discussion Topics 1. Overall, I think the instructions were quite clear and concise. I was able to stay on track just by reading the instructions, but having the TA provide a visual aid was very helpful. 2. The actual location of the spectrum is on the surface of the diffraction grating. This is where the light wave is separated and able to be viewed. The graph paper simply allows a scale to compare the wavelengths to. 3. If the spectroscope were to have been made using a pizza box, I would predict that the spectrum would appear the same as the shirt box, but the scaling would be different due to the larger distance between the graph paper and the diffraction grating. 4. The light spectrum varies in brightness with the change in slit size. The larger the slit size, the brighter the picture- the smaller the slit size, the dimmer the picture. 5. The slit does not have to be a long, thin rectangle, but this shape allows for the clearest spectral image to be observed. 6. The light spectrum becomes brighter and more visible the closer one gets to the light source. 7. Craftsmanship- 9 Aesthetics- 5 Spectroscopic Efficiency- 9

Reflection Topics Extend If I were to extend this experiment, it would be interesting to create multiple spectroscopes using different shapes and sizes of boxes in order to find the most effective design using the least amount of material. CH222 Lecture Topic One lecture topic relevant to this experiment is that of energy levels. It was discussed in class that the reason hydrogen has four strips of light in its emission spectrum is due to

the number of energy levels that its electron can occupy when in an excited state, and when the electron decreases an energy level, it emits EMR in the visible spectrum. Applications The concept of emission spectroscopy can be used in astrophysics. If a scientist were to want to know the atomic composition of a distant star, he might use a spectroscope to analyze the light emitting from that star to determine the chemical makeup of it. Related Reading According to Lewen, emission spectroscopy is oft used to determine trace elements in pharmaceuticals. This allows pharmacologists to accurately determine the chemical composition of a drug in order to make its use more efficient (Lewen). This example shows a common us of the exact procedure conducted in this experiment. Green Chemistry Due to the simplicity of this experiment, there was practically no chance that any tenet of Green Chemistry could have been violated. However, sustainability seems to be a core idea that was upheld throughout this experiment. Environmentally-safe materials were used, and there was little waste left over.

Works Cited Lewen, N. “The Use of Atomic Spectroscopy in the Pharmaceutical Industry for the Determination of Trace Elements in Pharmaceuticals.” Journal of Pharmaceutical and Biomedical Analysis., U.S. National Library of Medicine, 25 June 2011, www.ncbi.nlm.nih.gov/pubmed/21159460....