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Lab #4: Observations of Chemical Reactions of Ionic CompoundsPurpose

In this experiment you will conduct a series of tests on five different ionic compounds. You will carefully observe all reactions, and record everything that you see. You will conduct the same tests on an unknown compound, and use the results to determine the identity of the unknown. You do not need any knowledge of the chemistry in the reactions to determine the unknown, just complete observations. In the second part of the experiment you will practice naming ionic compounds by either writing the name of a compound from the chemical formula or the formula from the name.

Introduction

Chemistry is a science that is built upon the scientific method. In the scientific method, hypotheses are tested with experiments, and experiments may lead to theories. These theories then lead to further experimentation, which involve making observations. Making good observations is a skill that is needed when working in the laboratory.

Although we cannot observe individual molecules reacting, there are many signals that indicate that a chemical reaction has taken place. Sometimes, observations that may not seem important at the time, can give a scientist additional insight into a process later on. Therefore, it is important to make complete and thorough observations of every chemical reaction to maximize the amount of useful information that can be obtained from an experiment. Additionally, it is important for the study of chemistry to understand how to name compounds and write the correct chemical formula.

Ionic Compounds and Formulas

In an ionic compound, metals, which have lost electrons to form positively charged cations, combine with nonmetals, which have gained electrons to form negatively charged anions. The charge is equal to the number of electrons lost or gained and can often be determined by the group number on the periodic table. Metals and nonmetals combine in combinations that cancel the charges and make the compound neutral. For example the calcium ion has a +2 charge and the chloride ion has a -1 charge. Therefore, you will need two chloride ions, for a total charge of -2, to cancel out the +2 charge on calcium: CaCl2.

Some transition metals such as iron (Fe+2 and Fe+3) can form more than one kind of positive ion. To distinguish between the two ions, a Roman numeral that gives

1 Lab 4

  

the ionic charge of that ion follows the element name. Other transition metals, such as Zn+2 and Ag+ only have one charge and do not need a Roman numeral in their names.

A compound that consists of three or more kinds of atoms will contain a polyatomic ion, which is a group of atoms with an overall charge. Polyatomic ions are often named with the ending -ate or -ite. When writing formulas, parentheses are put around the polyatomic ion if more than one is needed to balance the charges. Refer to your ion handout from class to assist in naming ionic compounds.

In this lab is test five different ionic compounds: NaCl, NaI, Na2CO3, Na2HPO4, and Na2SO4. You will use both solid samples and solutions of these compounds dissolved in water. You will test these compounds with solutions of nitric acid (HNO3), barium nitrate (Ba(NO3)2), and silver nitrate (AgNO3). At the same time, you will test an unknown sample that is identical to one of the five known samples. You can determine its identity by matching your results with the results for the known compounds.

Reaction Signals

Evidence that a reaction has occurred include the following:

1. The color of a solution changes.

2. A gas is formed in the reaction.

3. A precipitate appears or disappears.

When a gas is formed during a reaction, you will see bubbles form in the solution and rise upwards. If there are only a few bubbles, it may just be from mixing the two solutions, not from gas formation. Make sure you look for bubbles immediately after adding the test solution.

A precipitate is a solid that forms, but often the solution just appears milky because the solid particles are so fine. Anytime the solution is no longer transparent, a precipitate has formed! Eventually, the solid particles will settle to the bottom.

Procedure

1. Obtain an unknown solid from the supply hood. Record the number of the unknown.

2. Place a pea-sized portion of your unknown in a 250-mL beaker. Add 100 mL of distilled water using a graduated cylinder. Mix with a glass stir rod until the entire solid is dissolved. Save this solution for use later in the experiment.

2 Lab 4


Testing with HNO3

3. Obtain 6 large test tubes from your cabinet and check that they are clean. Use a marking pencil to number the test tubes 1 through 6.

4. Place pea-sized portions of SOLID samples of each compound in six different test tubes using a clean spatula: 1. NaCl; 2. NaI; 3. Na2CO3; 4. Na2HPO4; 5. Na2SO4; 6. Unknown.

CAUTION HNO3 is a strong acid and can cause chemical burns. If any is spilled in your area, wash with tap water and report the incident to your instructor.

5. Add 5 drops of HNO3 to each test tube and record your observations in Column 1 of the results table. If nothing happens, write NO REACTION in the table.

6. Discard the contents of the test tubes in the waste hood, clean the test tubes and rinse with distilled water.

Testing with barium nitrate

7. For this test you will use SOLUTIONS of all compounds, which means the solid is dissolved in water. For your unknown, use the solution that you prepared in Step 2.

8. Place 20 drops of NaCl in test tube 1; 20 drops of NaI in test tube 2; 20 drops of Na2CO3 in test tube 3; 20 drops of Na2HPO4 in test tube 4; 20 drops of Na2SO4 in test tube 5; and 20 drops of the unknown solution in test tube 6.

9. Add 3 drops of ammonia (NH3) to each test tube. For each test tube, stir the solution with a stirring rod, remove the stirring rod and touch the end of it to a piece of red litmus paper. If the paper does not turn blue, add drops of ammonia until it does. (You do not need to record this in the table)

CAUTION NH3 is a base and can cause chemical burns. If any is spilled in your area, wash with tap water and report the incident to your instructor.

10. Add 5 drops of the solution of Ba(NO3)2 to each test tube. Shake gently to make sure the solutions are mixed. Examine each test tube and look for precipitates. Remember, any cloudiness indicates the presence of a precipitate. Also note the color of any precipitate. Record your observations in Column 2 of the results table.

CAUTION Barium is poisonous, so wash your hands thoroughly after using the solution.

3 Lab 4

  REMINDER

  You are looking for gas bubbles in this test, so watch closely as soon as you add the HNO3 and complete the observation before adding HNO3 to the next test tube.

   REMINDER

  NH3 is a base, and red litmus paper will turn blue when touched with a basic solution. You do not need to record any observations yet, you are just preparing the right conditions for the reaction with barium nitrate.

    

11. FOR THE TEST TUBES THAT HAVE A PRECIPITATE, add 10 drops of the solution of HNO3. Test each solution with blue litmus paper (turns pink with an acidic solution) using a clean stir rod. If the paper does not turn pink, continue to add drops of HNO3 until it does turn pink.

12. For the test tubes in Step 11, record your observations in Column 3 of the results table. In some cases, the precipitate may have dissolved and the solution should be clear again. State clearly whether a precipitate is still present or if it has dissolved.

13. Discard the contents of the test tubes in the waste hood, clean the test tubes and rinse with distilled water.

Testing with silver nitrate

14. For this test you will use SOLUTIONS of all compounds, which is, dissolved in water. For your unknown, use the solution that you prepared in Step 2.

15. Place 20 drops of NaCl in test tube 1; 20 drops of NaI in test tube 2; 20 drops of Na2CO3 in test tube 3; 20 drops of Na2HPO4 in test tube 4; 20 drops of Na2SO4 in test tube 5; and 20 drops of the unknown solution in test tube 6.

16. Add 5 drops of the solution of AgNO3 to each test tube. Shake gently to make sure the solutions are mixed. Examine each test tube and look for precipitates. Remember, any cloudiness indicates the presence of a precipitate. Also note the color of any precipitate. Record your observations in Column 4 of the results table.

17. FOR THE TEST TUBES THAT HAVE A PRECIPITATE, add 10 drops of the solution of HNO3. Test each solution with blue litmus paper (turns pink with an acidic solution) using a clean stir rod. If the paper does not turn pink, continue to add drops of HNO3 until it does turn pink.

18. For the test tubes in Step 17, record your observations in Column 5 of the results table. In some cases, the precipitate may have dissolved and the solution should be clear again. State clearly whether a precipitate is still present or if it has dissolved.

19. Discard the contents of the test tubes in the waste hood, clean the test tubes and rinse with distilled water.

Ionic compounds and formulas

20. Use the periodic table or an ion table to write the positive and negative ion in each compound in part "B" of the results section.

21. Use charge balance to write the correct formula using subscripts when two or more ions are needed.

22. From the formula of each ionic compound, write the compound name be placing the metal name first, then the nonmetal name ending in -ide.

4 Lab 4


Ionic compounds with transition metals

23. Use the periodic table or an ion table to write the positive and negative ion in each compound in part "C" of the results section.

24. Use charge balance to write the correct formula using subscripts when two or more ions are needed.

25. From the formula of each ionic compound, write the compound name be placing the metal name first, then the nonmetal name ending in -ide. Be sure to indicate the ionic charge if the transition metal has a variable charge by using a Roman numeral.

Ionic compounds with polyatomic ions

26. Use the periodic table or an ion table to write the positive and negative ion in each compound in part "D" of the results section.

27. Use charge balance to write the correct formula using subscripts when two or more ions are needed. Use parentheses when two or more polyatomic ions are needed for charge balance.

28. Name the compounds listed, using the correct names of the polyatomic ions.

5 Lab 4


 Name __________________ Pre-Lab done on time Partner _________________

Date ________ Drawer Checkout

Pre-laboratory Assignment

1. Write the name for each compound tested in this experiment:

 a. Na2HPO4 b. NaCl

c. NaI

d. Na2SO4 e. Na2CO3

________________

________________

________________

________________

________________

2. What are the signals for chemical reactions that you will look for in this experiment?

3. What does red and blue litmus paper test for?

4. What are the rules for naming ionic compounds when the metal can have more than one charge (like Fe2+ or Fe3+)?

5. What safety precautions must be observed during this experiment?

6 Lab 4


Results

Unknown #__________

TABLE A: Results

   Column 1

HNO3

  Column 2

Ba(NO3)2

 Column 3

Precip+HNO3

  Column 4

AgNO3

  Column 5

Precip+HNO3

 1. NaCl

    2. NaI

         3. Na2CO3

    4. Na2HPO4

       5. Na2SO4

         6. Unknown

     7 Lab 4


Ionic compounds and formulas

TABLE B1: Formulas of ionic compounds

  Name

Magnesium chloride Calcium oxide Lithium phosphide Aluminum sulfide Calcium nitride

Positive Ion

Mg2+

Negative Ion

Cl-

Formula

MgCl2

            TABLE B2: Names of ionic compounds

K2S Potassium sulfide BaF2

MgO

AlCl3

Mg3P2

Ionic compounds with transition metals

TABLE C1: Formulas of ionic compounds

        Name

Positive Ion

Negative Ion

Cl-

Formula

FeCl3

  Iron(III) chloride Fe+3 Iron(II) oxide

Copper(I) sulfide

Copper(II) nitride

Zinc oxide

          8 Lab 4


TABLE C2: Names of ionic compounds

Cu2S Copper(I) sulfide Fe2O3

CuCl2

FeS

FeBr2

Ionic compounds with polyatomic ions

TABLE D1: Formulas of ionic compounds

        Name

Potassium carbonate Calcium phosphate Aluminum hydroxide Ammonium oxide Potassium sulfate

Positive Ion

K+

Negative Ion

CO3-2

Formula

K2CO3

            TABLE D2: Names of ionic compounds

 CaSO4 Al(NO3)3 MgSO3 Cu(OH)2 Mg3(PO4)2

Calcium sulfate

     9 Lab 4


Questions

1. Using the results from table A, what is your unknown compound?

2. Discuss how you determined your unknown and indicate if there were any differences in the results of your unknown from the known compound.

3. Another student tested one of the unknown compounds (not yours) with Ba(NO3)2 and a white precipitate formed. The precipitate DOES NOT dissolve after the addition of HNO3. From your table, can you determine which of the compounds it could be (you may have more than one)?

4. Another student tested one of the unknown compounds (not yours) with AgNO3 and a white precipitate formed. From your table, can you determine which of the compounds it could be (you may have more than one)?

5. If you have more than one possible compound for your answer in #4, what would you do next to determine which compound it is (Hint: suggest another test from this lab)

Step-by-step answer

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Subject: Chemistry, Science

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