Download Emission Spectroscopy Lab

Chemistry
Name _________________________________
Emission Spectroscopy Lab
Date _____/_____/_____
Period _____
Pre-Lab Discussion
The normal electron configuration of an atom or ion(s) of an element are known as the “ground
state”. In the most stable energy state, all electrons are in the lowest energy state possible.
(According to the AUFBAU Principle, electrons will occupy the lowest energy state possible.)
When energy is added to an atom or ion, the electrons of the atom or ion may absorb enough
energy to allow them to “jump” to a higher energy state, called the “excited state”. The atom or
ion is said to be in an excited state. This excited state configuration is not stable and the
electrons return to their normal ground state, releasing energy in the form of electromagnetic
radiation (EMR). Some of this EMR may have wavelength that allows us to see the energy as
visible light. The color of this light can be used as a means of identifying the elements involved.
Purpose
Metals, with their loosely held electrons, are excited in the flame of a laboratory burner. Gases
can be excited by running high voltage through the gaseous atoms under low pressure. In
either case, the excited atoms will, in each case, produce a characteristic spectrum. The goal of
this lab is to excite metal ions or gaseous atoms, record the gross color of emitted light, use
spectroscopes to get a more complete emission spectrum from light sources and the bright-line
spectra of some gaseous elements, and calculate the energy of released photons.
Safety
 Do NOT touch the high-voltage power supply.
 Follow appropriate procedures when operating the burners. Tie back long hair and
secure loose clothing.
 Many of the salts used in this experiment are toxic. Be sure to wash your hands
thoroughly after coming into contact with any of the substances.
Procedure
Part I – Emission Spectra of Gaseous Elements
1. Using a spectroscope, examine the spectrum emitted by an incandescent light source.
Record your observation by sketching the continuous spectrum. Record all the colors
and the relative amounts of each present.
2. Aim your spectroscope at a fluorescent light source. Record your observation by
sketching the continuous spectrum. Record all the colors and the relative amounts of
each present.
Part II – Emission Spectra of Gaseous elements
View each gas tube and record its gross color (what you see with just your eyes) beneath
its name in the Part II Data Chart. View each of the following gas tubes using a spectroscope.
Record the line spectrum in the top half of each box in the chart (Part II Data Chart). If you see
one red line, then draw one line in the box below “Red” for the gas tube that you are viewing. If
you see two red lines, then draw two lines in the box below “Red” for the gas tube that you are
viewing. After viewing all of the gas tubes and recording their observed line spectrums in the
top half of each box in the chart, refer to a wall chart to obtain and record the accepted line
spectrum of each gas in the bottom of each box.
Part III – Flame Tests of Metal Ions
1. Using the striker, ignite a Bunsen burner. Adjust the burner so that the flame is almost
invisible.
2. Note: Be sure that the wire loop is correctly matched to the correct chemical before using it!
a. If the chemical (salt) is solid, moisten the wire loop in some distilled water. Then
dip the loop in the solid chemical so that some of the chemical sticks to the loop of
the wire.
b. If the chemical (salt) is liquid, dip the wire loop in the liquid chemical so that it
moistens the loop of the wire.
3. View the flame test of each salt and gross color (what you see with your eyes) just
beside its name in the Part III Data Chart A.
4. Repeat Steps 2 and 3 using the other salts of known identity. To prevent contamination of
the samples, be sure to use each loop for only one salt.
5. Record the number of the unknown chemical for your lab group in Part III Data Chart
(example: U5). The unknown is one of the chemicals used in Part III. Repeat
steps 2 and 3 using the unknown salt.
6. Compare the line spectrum of the unknown salts to the line spectrum of the known salts and
determine the identity of the unknown salt. Record the identity of the unknown salt in Part
III Data Chart A.
7. After viewing all of the flame tests of the salts, refer to a wall chart to obtain and record the
accepted line spectrum of each salt in Part III Data Chart B.
Questions
1. What form of energy emission accompanies the return of excited electrons to the ground
state?
2. Explain why metal atoms can be excited using a Bunsen burner whereas gaseous atoms
need a high voltage power source?
3. What happens to the distance between successive energy levels as the number of
energy levels increases?
What happens to the energy difference
between successive energy levels with each additional energy level?
4. What are some practical applications of spectroscopy? Your answer should be thorough
and reflect research done on your part.
Observations
Record your observations as directed in the procedure.
Part I
Incandescent light
Fluorescent light
Part II Data Chart: Gas Tube Line Spectrum
Gas Tube
Red
Orange
Yellow
Observed
Helium
Green
Blue
Indigo
Violet
Accepted
Hydrogen
Observed
Accepted
Krypton
Observed
Mercury
Observed
Accepted
Neon
Observed
Air
Observed
Part III Data Charts: Flame Test
A. Gross Color (All work for the calculations must be included in your write-up.)
Substance
Gross Color
Wavelength
Frequency
Energy
(in meters)
(in Hertz)
(in Joules)
Ba(NO3)2
Ca(NO3)2
Cu(NO3)2
KNO3
LiNO3
NaNO3
Sr(NO3)2
Unknown Chemical:
B. Emission Spectra of the Seven Metal Ions
Metal Ion
Red
Orange
Yellow
Accepted
Barium
Accepted
Calcium
Accepted
Lithium
Accepted
Sodium
Accepted
Strontium
Green
Blue
Violet