Chemistry Flame Test

Document Sample
Chemistry Flame Test Powered By Docstoc
					Chemistry: Flame Test

Introduction: Electrons can absorb energy and jump to an excited energy state, a quantum jump if given an
appropriate amount of energy, then return to their ground state by emitting a photon of light. The amount of
energy in the photon is determined by the change in energy states as electrons “fall”, which is evident by
different colors of light that are emitted. Wavelengths outside of the visible spectrum, ultraviolet and infrared,
are also emitted. The arrangement of electrons in an atom determines the sizes of the quantum jumps, and thus
the energy and colors of the collection of photons emitted, known as emission spectrum. In this way the
emission spectrum serves as a „fingerprint‟ of the element to which the atoms belong. We can view the
emission spectrum using a spectroscope, which bends light of different energies differently.

Objectives:

   1.   In this lab we will record the flame test color of several metals, including two unknowns. The first
        unknown will have just one solute and the second will have a mixture of two solutes.
   2.   We will examine the emission spectra of two elements through a spectroscope and you will record the
        energies of each spectral band.

Cobalt blue glass filters are often used when viewing mixtures of metals to screen out light that is yellow in
color. The human eye sees yellow very well, since it is in the middle of the spectrum visible to the eye. Colors
at the edges of the visible spectrum, especially violet, are more difficult to see. Cobalt glass absorbs light in the
yellow wavelengths, but is transparent to light of higher energy (this is why it looks blue!). Viewing a yellow
flame through cobalt glass will allow us to see if there is any higher energy light present.

Additional Resource: Read through this resource for additional information: Flame test

Materials:

       1 M HCl solution
       Metal salt solutions (CaCl2, K2SO4, Li2SO4, SrCl2, Na2SO4, NaCl, Unknown)
       Bunsen burner and related equipment
       Cobalt glass plates
       Flame test wire
       Well plate- 3 small test tubes
       Spectroscope

Procedures:

  1. Clean loop. Place about 2 mL of 1.0 M HCl into each of two small test tubes and 10 mL of distilled water
     in a third test tube. Dip the flame test wire into the first test tube of HCl, then hold it in the flame for 10
     seconds. Repeat the process for the 2nd and 3rd test tubes.
  2. Rinse and dry the well plate.
  3. Pour a few drops of test solution into a well.
  4. Dip the wire into the well to capture a droplet of solution.
  5. Hold the wire loop in the flame and record your results in the data table.
  6. Using a spectroscope, view the emissions generated from the spectra tubes. Record the spectral lines and
     their wavelengths.
Chemistry: Flame Test Results                              Names:

Data: Flame test

        Sample            Flame Color- no cobalt glass   Flame Color- with cobalt glass
         CaCl2

          KCl

         CuCl2

         NaCl

         SrCl2

         BaCl2

         LiCl2

     Unknown #1

     Unknown #2


Data: Atomic Spectra

     Element                    Spectra line color                       Wavlength




    Hydrogen




     Mercury
Questions:

1. What color of light is lowest in energy? What is highest?



2. What is the purpose of a spectroscope?



3. Before we begin each flame test we will clean the wire loops with 1.0 M HCl several times. Why is it
    essential that the wire loop be clean?



4. How can cobalt blue glass help us to see violet and blue light?



5. If you test two samples and find that both produce a red flame, how can you determine for sure whether they
    contain the same metal?

6.
  a) If the frequency of a red spectrum line is at 1.60 x 1014 Hz, how much energy does each photon of this light
      have?


  b) If the frequency of a violet spectrum line is at 2.50 x 1014 Hz, how much energy does each photon of this
      light have?

  c) On the far ends of the visible spectrum of light, there exists ultraviolet (UV) radiation and infrared (IR)
      radiation. Based on what you calculated in parts a & b, explain -why- UV is more dangerous than IR
         - UV radiation is dangerous. UV radiation is located just past violet on the spectrum.
         - IR radiation is harmless. It is located just past red on the spectrum.

7. Each spectral line represents a single photon of energy. Where does this energy come from?



8. Which element had more spectral lines? Which emitted the highest energy visible light photon?



9. Identify the metals solutions in the unknown test tubes

     Unknown #1_______________________                       Unknowns #2 _____________________________