Identification of Anions and Cations in Solutions

Name(s): ______

Signature(s): ______

FLAME TESTS FOR CATIONS

Background:When solids, liquids, and gases under high pressure are heated, they release a continuous spectrum of light. The visible region of light ranges from wavelengths of 400 nm (violet) to 700 nm (red). In contrast, fluorescent, or electrically excited gases under low pressure produce bright line spectra indicating that energy is emitted only at specific wavelengths. Each line in the spectrum corresponds to a particular frequency of light emitted by the atoms. It is now known that each element has a unique emission spectrum. Compare the positions of the bright lines for each element, and you can see that every element produces a unique spectrum that can be used to identify the element, much as a fingerprint may be used to identify a person.

On the basis of this data, the German physicist Max Plank proposed that atoms and molecules cannot absorb or emit any arbitrary amount of radiant energy, but instead can absorb or emit radiant energy only in discrete quantities. The name quantum was given to the smallest quantity of energy that can be absorbed or emitted as electromagnetic radiation. Albert Einstein used Plank’s ideas and suggested that light was a stream of particles which he called photons. He conjectured that each photon possesses energy E as indicated by E = hv, where E is energy, v is the frequency of light, and h is Plank’s constant.

In 1913, Niels Bohr was able to explain the hydrogen-atom line spectrum by hypothesizing that an electron of the hydrogen atom can circle the nucleus is fixed orbits. An electron in any of these orbits has a definite (fixed) amount of energy and is said to be quantized. Thus a line spectrum is produced when an electron drops from a higher-energy orbit to a lower-energy orbit because as the electron drops, a photon is emitted that has an energy equal to the difference in energy between the initial higher energy orbit and the final lower energy orbit. Likewise, to move an electron from a lower energy orbit to one of higher energy requires that the electron absorb a fixed amount of energy.

It is rather easy to excite the electrons of the group 1 and 2 elements by heating solutions containing these elements. The flame test is based on the concept that when elements are heated to high temperatures, some of their electrons are excited to higher energy levels. When these excited electrons fall back to lower energy levels, they release excess energy as light (photons). The color of the emitted light depends on its energy and can be used to identify an element. For two cations, cobalt blue glass will be used. Cobalt blue glass acts as a filter 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.

Objective:

At the end of the activity, you will be able to perform flame tests to determine the characteristic flame colors of known and unknown cations.

Materials:

goggles Solids:potassium chloridestrontium chloride

wire loopscalcium chloridebarium chloride

cobalt blue glasscopper(II) chloridelithium chloride

Bunsen burnersodium chloridecobalt chloride

deionized water wash bottlenickel chloride

Beaker

3M Hydrochloric Acid, HCl

Safety: Safety goggles and aprons should be worn at all times.

Procedure:

Wire Loop Method:Heat the metal of the flame loop only, not the glass.

1. _____ Clean the wire loop by dipping the loop in hydrochloric or nitric acid, followed by rinsing with deionized water. Test the cleanliness of the loop by inserting it into the flame. If a burst of color is produced, the loop is not sufficiently clean.
2. _____ Dip the clean loop in the powder of the ionic (metal) salt.
3. _____ Place the loop in the clear or blue part of the flame. Observe the color and record.
4. _____ After you have viewed the flame color produced by sodium nitrate and potassium nitrate, use the cobalt blue glass to view the flame color again. Record new observations in the appropriate space below.
5. _____ The loop must be cleaned between tests as you did in step 1.

Unknown:

1. _____ Perform a flame test on the unknown cation and record your observations in the data table.

Data:

Analysis: Write your answers in complete sentences unless otherwise indicated.

1. What is the identity of the unknown? How do you know?

1. Each of the solutions used contained chloride, yet each compound produced a flame of a different color. Explain.

1. List the elements that could be identified based on their characteristic flame color.

1. Why do different chemicals emit different colors of light?

1. Problem Solve: You are enjoying a July 4th fireworks display when one of the rockets explodes prematurely. The color of the explosive mixture is yellow, green, and red-orange. Which three metals were most likely present in the rocket? Support your answer with evidence from the flame tests you performed.

Name: ______Pd: ______Date: ______

Flame Tests Pre-Lab

Read the entire lab on the wiki. Write your answers in complete sentences unless otherwise stated.

1. Line spectra may be used as “fingerprints” to identify the elements. Explain.

1. Identify the following element based on its line spectrum.______

1. How will the electrons be excited in this lab?

4. Why is it important that a new wooden split, or a clean wire loop is used for each flame test?

5. What is the purpose of using cobalt glass in the identification of sodium and potassium?

6. What safety precautions should you take in this lab?

Adapted from: Addison-Wesley: Identification of Anions and Cations in Solution and Flame Tests for Metals.; Chemistry: Non-Consumable Laboratory Handbook. 4th Ed. 1995