anions and cations

DRAFT DOCUMENT LAB 15

IDENTIFICATION OF ANIONS AND CATIONS IN SOLUTION

In this activity you will become a chemical detective. You will conduct analyses to determine the ionic composition of two unknown solutions. The process of determining the composition of a sample of matter by conducting chemical tests is called qualitative analysis. Solutions of unknown ions can be subjected to chemical and flame tests and the results can be compared to the results given by known ions, in the same tests. By conducting the appropriate tests and applying simple logic, the identities of the ions present in the unknown solutions can be determined.

To help you identify the composition of two unknown metallic solutions, you will use flame tests to identify the cation and chemical tests (e.g. silver nitrate, lead nitrate) to identify the anion. 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 (blue light is more energetic than red light). As each element emits a characteristic pattern of light energies, excited electrons fall back to lower energy levels. The color of the light given off is characteristic of a specific element.

Qualitative tests for anions are based upon the idea that each ion reacts in its own characteristic way in a chemical reaction (no two ions react the same). Reactions involving color changes, the evolution of gas, and/or the formation of a precipitate (a solid product) are indicative of the presence of specific anions.

As you complete this activity, remember that careful observation and logical reasoning are crucial if you are to be a successful chemical detective.

OBJECTIVES

At the end of the activity, you will be able to:

1. Perform flame tests to determine the characteristic flame colors of known cations.

2. Perform qualitative tests to characterize anions in solution.

3. Devise a schematic plan to identify anions and cations in solution.

4. Identify the composition of two unknown ionic solutions.

5. Write the formulas and names of specific ionic compounds.

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PART 1: Flame Tests for Cations

MATERIALS

Goggles Solutions of: potassium nitrate strontium nitrate

Splints calcium nitrate barium nitrateCobalt blue glass copper(II) nitrate lithium nitrateBunsen burner sodium nitrate

PROCEDURE

1. Dip a wooden splint into a test tube of metallic salt solution. Remove splint after 10 seconds.

2. Wave the splint SLOWLY back and forth through the flame just above the tip of the inner blue cone. (DO NOT LET THE SPLINT CATCH ON FIRE.) Record your observations in the chart below.

3. 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.

4. Dispose of used splints.

DATA

Table of Flame TestsIon Flame Color

sodium, Na+ __________________________sodium, Na+ (cobalt glass) __________________________potassium, K+ __________________________potassium, K+ (cobalt glass) __________________________calcium, Ca2+ __________________________barium, Ba2+ __________________________strontium, Sr2+ __________________________lithium, Li+ __________________________copper(II), Cu2+ __________________________

QUESTIONS

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

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2. Explain why some elements are difficult to identify by flame color.

3. Which element produces the most intense color?

4. Would flame tests be useful for detecting specific cations present in a mixture of metal ions? Why?

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

6. 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.

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PART 2: Qualitative Tests for Anions

MATERIALS:Known anions: Indicators: goggles

5% NaCl 0.2 M AgNO3 depression plates5% NaNO3 0.2M Pb(NO3)2 China marker5% Na2S 3.0 M HCl5% NaI5% Na2SO4

0.6 M NaHCO3

PROCEDURE

1. Label 8 troughs on the depression plate with the anion being tested.

2. Add the following chemical reagents to each trough in order and record your observations.

Qualitative Tests for AnionsKnown Anion Indicator Observations

a) 4 drops 5% NaCl 2 drops 0.2 M AgNO3 ___________________________

b) 4 drops 5% NaNO3 2 drops 0.2 M AgNO3 ___________________________

c) 4 drops 5% Na2S 2 drops 0.2 M AgNO3 ___________________________

d) 4 drops 5% Na2SO4 2 drops 0.2 M AgNO3 ___________________________

e) 4 drops 5% Na2SO4 2 drops 0.2 M Pb(NO3)2 ___________________________

f) 4 drops 5% NaI 2 drops 0.2 M Pb(NO3)2 ___________________________

g) 4 drops 0.6 M NaHCO3 2 drops 0.2 M Pb(NO3)2 ___________________________

h) 4 drops 0.6 M NaHCO3 2 drops 3.0 M HCl ___________________________

QUESTIONS1. Underline the anion being tested in 2a-h.

2. Explain why it is necessary to test sodium sulfate with lead nitrate.

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CAUTIONREAD LABELS CAREFULLY!!!!!!

Avoid getting AgNO3 on your hands and clothing.


3. Justify your observation in 2b. Explain why you did not observe a similar result when AgNO3 was added to NaCl or Na2S.

4. Write a balanced equation for each chemical reaction that gave a positive qualitative test.

PART 3: Determining the Composition of Unknown Ionic Solutions

PROCEDURE

1. Devise a plan to identify the composition of each unknown. List the steps on the worksheet.

2. Draw a flow chart summarizing the expected results that would lead to a clear identification of an unknown anion.

3. Perform the qualitative tests (chemical and flame tests) for each unknown. Record your observations in an organized fashion.

WORKSHEET / DATA

1. Procedural Plan

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2. Flow Chart

3. DATA

ANALYSIS

1. List the anion and cation present in each unknown. Give the chemical name and the formula of each compound.

Unknown Cation Anion Name Formula

2. Complete the chart (Compounds Formed From Ions) by legibly writing the formula and name of each compound.

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EXTENSION

THINK and then WRITE: Use your knowledge about valence electrons, atomic radii, ionization energy, orbital energies, the process of emitting light, and additional reference material as needed to explain why the emitted light in the flame test for sodium is yellow while the light emitted for strontium is bright red-orange. For full credit you must give a thorough explanation in a well-written paragraph.

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

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Directions: 1) Write the charge of each ion.2) Write the formula of the compound formed by the two ions.3) Give the classical and/or Stock name of each compound.

Cl CO3 NO3 S PO4 H2PO4 N

HHCl

hydrochloric acid

NaNaCl

sodium chloride

Zn

NH4

Al

Fe2+

Pb4+

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Compounds Formed From Ions


PRELAB: IDENTIFICATION OF ANIONS AND CATIONS IN SOLUTION

Read the introduction to the lab activity, highlighting key information as you read. Use the information in the introduction and your class notes to answer the following questions.

1) What is an ionic solution composed of?

2) List three examples of ionic solutions.

3) Answer a or ba. Why do elements emit visible light?

b. What is the relationship between emitted visible light and energy?

4) Differentiate between a qualitative test and quantitative test.

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anions and cations