Atomic Spectra/Flame Tests Report
Atomic Spectra/Flame Tests Practical Task (DCP)(CE)
Background Information:
What do fireworks, lasers, and neon signs have in common? In each case, we see the brilliant
colours because the atoms and molecules are emitting energy in the form of visible light. The
chemistry of an element strongly depends on the arrangement of the electrons. Electrons in an
atom are normally found in the lowest energy level called the ground state. However, they can
be "excited" to a higher energy level if given the right amount of energy, usually in the form of
heat or electricity. Once the electron is excited to a higher energy level, it quickly loses the
energy and "relaxes" back to a more stable, lower energy level. If the energy released is the
same amount as the energy that makes up visible light, the element produces a colour.
Objectives:
•
Observe/record the colours produced by metal salts in a flame
•
Use flame colours to identify unknown salts
•
Observe/record line spectra of hydrogen gas
•
Calculate frequency and energy values of specific wavelengths
Apparatus and materials:
- A nichrome wire,
- The heating flame of a Bunsen,
- Two watch glasses,
- Water in the beaker,
- Several bottles with different salts and acid,
- Tissues, matches, spoon,
- A hydrogen gas discharge tube,
- The spectrometer.
Safety:
There are several specific safety concerns about this experiment: Firstly, take care with the
heating flame and matches which are burning. Secondly, the burning flame should also be
considered. Hence students should were the safety goggles.
Method:
Part A – Flame Tests:
1. Clean a nichrome wire by dipping into HCl solution. Heat the wire in the heating flame
of a Bunsen until no more colour is produced.
2. Mix a little of the salt to be tested with the acid solution on a watch glass and dip the
end of the clean wire in the solution.
3. Hold this end of the wire in the outer blue part of the flame and note any flashes of
colour that may appear and the intensity. Note that these may fade quickly.
4. Test subsequent salts in the same manner, cleaning the wire between each test as
described in #1.
5. Tabulate your results making sure to include qualitative and quantitative data.
6. Use the information you have collected to identify the cations in each of the unknown
samples provided to you.
Part B – Line Spectra of Gases:
1. Go to the station where a hydrogen gas discharge tube is set up. Here, an induction coil
is being used to provide energy to “exite” the gas atoms.
2. Using the spectrometer, look at one of the fluorescent light in the room. You will see
coloured lines corresponding to specific wavelengths emitted by the white light.
3. Record the values for the violet and green lines. These should be 4360 Å and 5460 Å
respectively. Note any error in these readings so that adjustments can be made in
subsequent readings.
4. Turn on the electricity to the hydrogen gas tube (for no more than 30 sec at one time to
prevent burn out).
5. Look through the spectrometer and observe/record the coloured lines that are
produced.
6. Tabulate your results, including qualitative and quantitative data.
7. Process your data thoroughly, providing as much information as possible about each line
found including error and uncertainty.
Results:
(a) Flame Tests
Salt
Colour
Sodium
Chloride
NaCl
Yellow
Salt
Wavelength
(nm)
NaCl
around
600nm
Strontium Magnesium
Iron
Chloride
Chloride
Chloride
SrCl2
MgCl2
FeCl3
Red
Orange
Green
(or red)
(or blue)
SrCl2
around
700nm
MgCl2
around
620nm to
720nm
FeCl3
around
450nm
to
550nm
Copper
Chloride
CuCl2
Blue (or
green)
Potassium
Chloride
KCl
Yellow (or
orange)
Calcium
Chloride
CaCl2
Red
CuCl2
around
450nm
to
550nm
KCl
around
600nm
CaCl2
Around
700nm
From the diagram showed above, I had a guess of the wavelengths of those salts according
their light colours. The wavelength of sodium chloride around 600nm; the wavelength of
strontium chloride around 700nm; the wavelength of magnesium chloride around 620nm to
720nm; the wavelength of iron chloride around 450nm to 550nm; the wavelength of copper
chloride around 450nm to 550nm; the wavelength of potassium chloride around 600nm and
the wavelength of calcium chloride around 700nm. (As shown the table above)
(b) Flame Tests
Unknown A: Orange (red) ------ CaCl2
Unknown B: Green (blue) ------ CuCl2 or FeCl3
In this experiment, we were given two unknown salts and we needed to identify these by using
same process as the front experiments (Flame Tests). For unknown A, the colour I saw is orange
and a little bit resemble to red, thus I guess it’s CaCl2. As for unknown B, the colour I saw is
green and a little bit resemble to blue, thus I guess it’s CuCl2 or FeCl3.
(c) Line Spectra of Gases
Hydrogen Gas
Colour
Wavelengths/ Å
Violet
4400
Green
5500
Orange
5900
Red
6200
Table 1
The diagram I saw in the spectrometer is similar to the diagram shown above and there are
scales for us to measure the vertical line which represents the wavelengths emitted by the
hydrogen gas. The results are shown in the table above. (Table 1)
Colour
Energy/ kJ.mol-1
Red
182
Blue
246
Violet 1
276
Violet 2
292
Table 2
Above are the standard energy values (accept value) that are produced by “excited” hydrogen
gas. (Table 2)
I will present the process that how I got the values of the energy start form the wavelengths
values which I measured in the experiments.
Calculations:
There are mathematical relationships between some key terms.
Energy is related to the frequency in a direct proportion and wavelength is related to frequency
in an inverse proportion. Specifically,
E = hv
c = λv
Where:
E = energy (J or kJ)
h = Planck’s constant = 4.00 x 10-13 kJ.s.mol-1
v = frequency (Hz or s-1)
c = speed of light = 3.00 x 108 m.s-1
λ = wavelength (nm, cm, Å) (1Å (angstrom) = 1 x 10-10m)
From the results of wavelengths that I measured (Table 1), we can get the frequency and
energy ultimately. The process is shown as below.
1. Violet: ------compare with colour Violet1
4400 Å = 4400 × 1 × 10-10m = 4.4 × 10-7m
c = λv
3.00 × 108 m.s-1 = 4.4 × 10-7m × v
V = 6.82 × 1014 s-1 (3s.f’s)
E = hv
E = 4.00 × 10-13 kJ.s.mol-1 × 6.82 × 1014 s-1 = 273 kJ.mol-1 (3s.f’s)
Percentage error= (273 - 276) ÷ 276 × 100% = 1.09% (3s.f’s)
2. Green: ------compare with colour Blue
5500 Å = 5500 × 1 × 10-10m = 5.5 × 10-7m
c = λv
3.00 × 108 m.s-1 = 5.5 × 10-7m × v
V = 5.45 × 1014 s-1 (3s.f’s)
E = hv
E = 4.00 × 10-13 kJ.s.mol-1 × 5.45 × 1014 s-1 = 218 kJ.mol-1 (3s.f’s)
Percentage error= (218 - 246) ÷ 246 × 100% = 11.4% (3s.f’s)
3. Orange: ------compare with colour Red
5900 Å = 5900 × 1 × 10-10m = 5.9 × 10-7m
c = λv
3.00 × 108 m.s-1 = 5.9 × 10-7m × v
V = 5.08 × 1014 s-1 (3s.f’s)
E = hv
E = 4.00 × 10-13 kJ.s.mol-1 × 5.08 × 1014 s-1 = 203 kJ.mol-1 (3s.f’s)
Percentage error= (203 - 182) ÷ 182 × 100% = 11.5% (3s.f’s)
4. Red: ------compare with colour Red
6200 Å = 6200 × 1 × 10-10m = 6.2 × 10-7m
c = λv
3.00 × 108 m.s-1 = 6.2 × 10-7m × v
V = 4.84 × 1014 s-1 (3s.f’s)
E = hv
E = 4.00 × 10-13 kJ.s.mol-1 × 4.84 × 1014 s-1 = 194 kJ.mol-1 (3s.f’s)
Percentage error= (194 - 182) ÷ 182 × 100% = 6.59% (3s.f’s)
Violet
Green
Orange
Red
Colour
Wavelengths
Frequency
λ/Å
4400
5500
5900
6200
v/ s-1
6.82 × 1014
5.45 × 1014
5.08 × 1014
4.84 × 1014
Red
Blue
Energy
E/ kJ.mol-1
Compare With
Percentage
Error (%)
273
218
203
194
Violet1
Blue
Red
Red
1.09%
11.4%
11.5%
6.59%
Violet 1
Violet 2
Energy (kJ.mol-1)
182
246
276
292
Discussion:
An atom will be excited when heating or electricity. When an atom is exited its electrons gain
energy and move to a higher energy level. In order to return to lower energy levels, the
electron must lose energy. It does this by giving out light. In other words, the electron loses
energy in the form of light.
In general, different salts will produce different colours of light when it is heating. Because the
atoms and molecules are different in various substances. Hence, the levels of emitted energy
are distinct. Then, it causes the different colours of the visible light which formed by the
emitting energy.
Evaluation:
In general, the reliability of my data is high. I collected enough data and repeated the
experiments enough times in order to minimise the impact of human error and differences; the
repeated results were also fairly similar to each other. However, there were still random error
and systematical error exist. The problems are followed: we used our own visible sight to
measure, thus the accuracy and precision could not be ensured. Furthermore, results might be
different if various people do the experiments together as every one’s identification to the
different colours might be different. Moreover, the substances left on the nichrome wire and
the spoon may influence the final results of the experiments. In order to improve the
experiments, we can adopt following measurements: Firstly, we should repeat the experiments,
at least to repeat 3 or 4 times, even more times. Besides, if we clean the nichrome wire by
dipping into HCl solution and clean it through as well as the spoon, the accuracy of the
experiments might be improved. The best situation is that to change a new nichrome wire and
the new spoon once we have done one of the experiments.
The method was valid as it allowed me to collect relevant data and get the relatively accurate
results. While, it could be improved by adding the cautions of each step in the experiments.
Hence, students will pay attention to them.
Having done this experiment, it inspired me a further idea to investigate what colour will be
seen if we use different elements rather than salts. Is the light the only way for atoms to give
away energy? Despite the light which we can see, is there any other tangible or intangible form
of energy giving up exist? What are the other features when atoms lost or gain energy?
Conclusion:
In general, the experiments are successfully completed. Through the experiments, I have
proved the theory I learned in the textbook and accomplish the assignments which given by the
teacher. It also encourages me to do further and deeper investigation about the energy in an
atom.
Bibliography:
-
Text book
(John Green & Sadru Damji, 2007)
Marks