Atomic Emission Spectra and Flame Test Lab

Objectives SWBAT complete flame tests of various metals to demonstrate that spectral lines are the result of transition of electrons between energy levels.

Agenda I Do: Flame Test lecture (10 min) We Do: Pre-observation questions and Flame test demo (25 min) You Do: Data Analysis and Conclusion questions (35 in)

Recap: Electromagnetic Spectrum Light can be understood as either waves or particles. All waves and particles have a certain amount of energy. c = λν and E = hν

How do we see this in REAL LIFE? Light consists of electromagnetic waves The different wavelengths of light can be separated into a spectrum of colors RAINBOWS!

Atomic Emission Spectrum: Atoms Absorb Energy When electrons have the lowest energy possible, they are at their ground state. When atoms absorb (gain) energy their electrons move to higher energy levels. This is called the excited state.

Atomic Emission Spectrum: Atoms Emit Energy Electrons return to lower energy levels when they lose energy and emit (give off) photons of light When the emitted light is passed through a diffraction grating, the atomic emission spectrum appears

Atomic Emission Spectrum When the emitted light is passed through a prism, the atomic emission spectrum appears The atomic emission spectrum is a set of frequencies of the electromagnetic waves emitted by the atoms. Each element has its own atomic emission spectrum.

Now onto the videos….

What is the unknown element?

Quantization of Energy Einstein calculated that a photon’s energy depends on its frequency. When electrons move between energy levels, they give off photons of energy in the form of light. We can use the following equation to describe the energy of the photon: Ephoton = h ν, h = Planck’s constant = 6.626 x 10-34 Js (J = Joule, a unit of energy 

Example 2 Tiny water drops in the air disperse the white light of the sun into a rainbow. What is the energy of a photon from the violet portion of the rainbow if it has a frequency of 7.23 x 1014 s-1? Show step by steps using SmartBoard.

Now let’s really take a look at the Atomic Emission!! But how……

Flame Test The heat of the flame excites the electrons in the metal ions The electrons ‘jump’ from the ground level to a higher energy level in the atom As electrons fall back to their ground state, energy is released and the excited electrons can be seen as a colored flame If you want to do a demo!

Flame Test The excited electron produces one or more specific lines in the visible light spectrum that corresponds to the element’s distinct electron configuration.

Pre-Observation Questions Explain the difference between the ground state and the excited state of an atom. What does it mean to excite an electron? What happens when an electron moves from a higher energy level to a lower energy level? Draw the orbital diagram for Lithium when its electrons are in an excited state and jump to the next energy sublevel.

Flame tests demo Watch and record observations as the teacher: Clean the loop by dipping into acid and burn the acid off in the flame. Dip the loop into metal salt soln. Put the loop in the flame and note the color. Eye protection Nichrome wires bonded to glass handles Watch glasses with the samples on Bunsen burner + safety equipment Samples of chlorides of the liquids you wish to test. Conc. HCl to clean wires (this may be left out for safety reasons, but the wires should be heated to burn off contaminants). Safety – chloride are irritants, HCl is corrosive. This activity should be completed in a well ventilated area.

Flame Test Purpose: Write what the purpose for the lab is… Materials: Bunsen burner flint striker or matches Water soaked wood splint 6 unknown metals (salts) from Group I & II

Flame Test Procedure: Light Bunsen Burner so it has a low hot flame Soak 6 wood splints in distilled water (Done for you) Dip one wood splint into the container of a substance collecting a small amount of substance on the end of the wood splint Carefully pass the wood splint through the flame Observe the color the flame changes; record the color in the table Repeat steps 3 through 5 for each substance three times each Clean lab station when finished Return to your desk and get a sheet from the instructor to identify each unknown substance Answer the conclusion questions

Unknown substance Name Flame Test Data Table: Unknown substance Trial 1 Trail 2 Trial 3 Unknown substance Name 1 2 3 4 5 6

Flame Test Conclusion Questions: Answer in complete sentences or be deducted 10%. What inaccuracies (errors) may be involved in using flame test for identification purposes? Explain how difficult or easy it was to decide what elements were in the unknown substances using your results. Explain why identification of a substance with only one property is valid or invalid. Explain how the colors observed in the flame tests are produced? Be specific. The energy of colored light increases in the order of red, yellow, orange, green, blue, and violet. List the metallic elements used in the activity in order of increasing energy emitted (lowest to highest). Why do you think the chemicals have to be heated in the flame before the colored light is emitted?

Guided Practice: Guess the metal. If there is time! Na K Li Ba/Cu

Data Analysis and Conclusion Answer the data analysis and conclusion questions in your lab notebook. WRITE THE QUESTION

Closing Essential Question: How can we understand color and light in terms of energy? How is this energy related to electron configurations?