Unknown B
Under test conditions the flame colour was identified as orange / yellow which when confirmed against the spectra is identifiable as sodium (Na+).
Unknown C
Under test conditions the flame colour was identified as violet; which when confirmed against the spectra is identifiable as potassium (K+).
Unknown D
In order to test this unknown compound for the presence of the anion carbonate, dilute nitric acid was added to the solid and the resulting reaction was noted as effervescence. The fumes of this were passed through to a sample of lime water, producing a chalky precipitate within the lime water.
When the cation test was performed the flame colour recorded was a yellow / orange colour and this confirmed that the cation involved was sodium (Na+)
Therefore we have identified the unknown substance as NaCO3.
Conclusion
To conclude, we understand that bright light emission occurs when energy in the form of an electron, travels from a higher energy level to a lower energy level. The colour of the light emitted is characteristic to the electronic structure of the atom or element involved. During the flame test the activation energy supplied through the bunsen burner excites the atomic structure; the electrons under the influence of this activation energy move up to the next highest energy level. This movement is only brief however as the electron must return to the original energy level to retain atomic integrity; when the electron returns back down to the atoms ground state a photon of light is given off.
Photons are recognised visually as a colour and using set calculations can be converted to a wavelength. Using this information we successfully compared the known cation to its documented characteristic light emission using current literature in order to determine that whether we had correctly identified the unknown compound.
We identified the unknown compounds A and C as potassium, with a bright light emission of approximately 420nm to 430nm according to the light emission spectra. The actual λ of K+ = 420nm; confirming that we have correctly identified the unknown cation as K+. The frequency of the cation can be calculated by using the formula ν=C/λ; therefore:
ν = 3x108 m/s = 7.05x1014 s-1 or 7.05 MHz (705GHz)
425x10-9 m
We identified the unknown compounds C and D as Na+, with a bright light emission of approximately 580nm to 590nm according to the light emission spectra. The actual λ of Na+ = 589.2nm; confirming that we have correctly identified the unknown cation as Na+. The frequency of the cation can be calculated by using the formula ν=C/λ; therefore:
ν = 3x108 m/s = 5.08x1014 s-1 or 5.08 MHz (508GHz)
590x10-9 m
Analysis
Although the results eventually reflected the correct compounds we initially determined that no anion was present in the compound. Upon analysis of our procedure it became apparent that in the initial test we did not introduce dilute HNO3 at all during the procedure and this also affected the result for the cation test, where we recorded an initial result of a yellow flame.
We re-ran both tests using a different strategy and the results were recorded and are displayed on the results page.
Upon reflection there are some improvements that could take place in order to produce a more accurate set of results; they would include:
- Smaller teams controlling the practical, potentially even consider using just 2 people maximum per group in order to reduce any potential random errors.
- Ensure that one person has one role in the practical and that the method is fully understood prior to the commencement of the practical.
- Allow more time for preparation for the practical; even though we had materials to test, not all of the materials on offer reflected the materials detailed on the method script.
- All visual interpretation of the bright light emissions could be confirmed using a mass spectrometer, to take away any potential systematic errors in interpretation of information.
Calculations and Questions
Q1
In order to make a bright white sparkler we need to use a metal which during energy transfer will emit a bright white light. Bright white light sits in midrange ultraviolet wavelength part of the light spectrum. “The work function is essentially the work that is required to separate an electron from the metal surface. In order that there by ionization by light - photo-ionization- the incident light quantum hv must be at least equal to the work of ionization. Calling the latter eVi, we have hv >= eVi” (Carnicorn, 2001). Carnicorn also confirms, “If we characterize the light by its frequency instead of by its wavelength, insertion of the numerical values yields the easily-remembered relationship between wave-length and equivalent electron energy:
ν (in nanometers) x Vi (electron energy in volts) = 1238”.
Applying this formula to various metal ions using their documented work function (eV) we can calculate the position of each ion within the spectrum. For a bright white light the best metal ion to use would be Mg+; in practical tests where Mg+ is added to H20, the resulting light is so intensely white that is should not be viewed through direct eye contact. Using this information against the formula written above the wavelength of light emitted from the material can be calculated as: ν = 1238 / 3.7eV = 335nm. There are other required aspects involved in the process of building a sparkler, beyond the cation required for specific colour; they would have include a fuel source and binding agent combined in the form of a dextrin solution; and also require the use of an oxidizing agent, such as potassium chlorate. The oxidizing agent readily give up their oxygen molecules when activation energy is applied:
2KClO3 + ΔE 2KCl + 3O2.
Q2
Energy of a photon of light from K+ = hC/λ = (6.634x10-34 J.s X 3x108 m/s)
425nm.
Therefore: E = 6.634x10-34 X 3x108 = 4.68x10-19J
4.25x10-7m
Energy of a photon of light from Na+ = hC/λ = (6.634x10-34 J.s X 3x108 m/s)
590nm.
Therefore: E = 6.634x10-34 X 3x108 = 3.37x10-19J
5.9x10-7m
Energy to vaporize 0.1ml water using a bunsen flame = m X ΔH vap
Therefore E = 0.1g X 2256 J/g = 225.5 J or 2.26x10-2J to heat 0.1ml water.
The amount of ΔE required to cause an e- to jump energy levels in order to emit light is infinitely small compared to the amount of ΔE required for 0.1ml of water to vaporize. However the bonds which hold the water molecules together are hydrogen bonds and are generally quite weak. The bond energy of water molecules and therefore between the H-O atoms is 463 Kj/mol; therefore 463 Kj of ΔE is required to break 1 mole of H-O bonds; whereas the energy required to make the e- of Mg or Na jump energy levels is considerably lower as the ΔE needs only to overcome the electromagnetic force within the one atom and not a combined EN of two or more atoms with added dipole attraction.
References
Carnicorn, C E; 2001. Ionization Apparent [online]. Available at URL:
[Accessed 10th December 2009].
Helmenstine, A M; Ph.D, 2009. “How Do Sparklers Work” [online]. URL Accessed at : -
[Accessed 10th December 2009].
