Name Period Date

21. Double Replacement Reactions

Driving QuestionsLimestone (CaCO3) and washing soda (Na2CO3) are similar to each other; both are solid white powders containing a carbonate anion. However, washing soda dissolves in water and limestone does not. Is there a predictable way of determining if an ionic compound is likely to dissolve? What happens if the product of a reaction in solution is an insoluble compound?

BackgroundA solute is a substance being dissolved. A solvent is a substance that does the dissolving. Dissolving occurs when solute particles are completely surrounded by solvent particles. This forms a jacket or “solvent cage” around each solute particle, allowing the particle to be incorporated into the solvent. Some substances do not dissolve because the attractions between the individual components of the substance are stronger than the attractions to the components of the solvent. They are insoluble in that solvent. For water, a set of rules assists in predicting whether or not an ionic compound is soluble or not. See Table 1 for a list of solubility rules.

In a double replacement reaction, the reactants switch anions to form new products. The reaction occurs only if one of the resulting products is insoluble. The insoluble product formed is called a precipitate. Knowing the solubility rules assists in predicting the products in a double replacement reaction.

A double replacement reaction can be written as a molecular equation showing all of the reactants and products. However, when in water, the soluble components exist in their ionized forms and the insoluble components exist as solids. This can be shown using an ionic equation. To focus only on the ions that are involved in making a new product, a net ionic equation can be written by removing those ions that appear on both sides of the chemical equation. This leaves only those ions producing the precipitate.

Molecular equation: AgNO3(aq) + NaCl (aq) → AgCl(s) + NaNO3(aq)

Ionic equation: Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) → AgCl(s) + Na+(aq) +

NO3–(aq)

Net ionic equation: Ag+(aq) + Cl–(aq) → AgCl(s)

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Double Replacement ReactionsTable 1: Solubility rules

Rule Number

Rule

1 Any compound with Na+, K+, Li+, or ammonium (NH4+) cations or with acetate

(C2H3O2–) or nitrate (NO3

–) anions is soluble.

2 Any compound with carbonate (CO32–) or chromate (CrO4

2–) or phosphate (PO4

3–) anions is insoluble, except if with those cations listed in rule number 1.

3 All halides (F–, Cl–, Br–, I–, and At–) are soluble, except those of Ag+, Hg22+, and

Pb2+.

4 All sulfates (SO42–) are soluble, except those of Sr2+, Ba2+, and Pb2+.

5 All sulfides (S2–) and hydroxides (OH–) are insoluble, except those of Ca2+, Sr2+, and Ba2+ and those cations listed in rule number 1.

Materials and Equipment

For each student or group:

Ring stand Erlenmeyer flask (4),125-mL

Buret clamp Graduated cylinder, 50-mL

Buret, 50-mL 0.2% Disodium salt fluorescein indicator, 2 mL

Funnel 1% Dexrin solution, 100 mL

Magnetic stirrer 0.020 M Silver nitrate (AgNO3), 200 mL

Micro stir bar 0.010 M Sodium chloride (NaCl), 100 mL

Transfer pipet Swimming pool water, 100 mL

Waste container

SafetyAdd these important safety precautions to your normal laboratory procedures:

Silver nitrate (AgNO3) and the resulting silver chloride (AgCl) precipitate in this lab are hazardous to the environment and should not be disposed down the drain. Make sure you follow your teacher’s instructions on how to properly dispose of the chemicals.

The fluorescein indicator is a dye and may stain your skin or clothes. The silver nitrate (AgNO3) solution may also stain your skin and clothes, especially after being exposed to bright sunlight. If either solution contacts your skin, wash it with soap and water immediately.

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Student Inquiry Worksheet

Sequencing ChallengeThe steps below are part of the Procedure for this lab activity. They are not in the right order. Determine the proper order and write numbers in the circles that put the steps in the correct sequence.

ProcedureAfter you complete a step (or answer a question), place a check mark in the box () next to that step.

Set Up

1. Set up the titration apparatus, using the illustration as a guide. a. Assemble the ring stand. b. Position the magnetic stirrer next to

the base of the ring stand. c. Place a waste container on the

magnetic stirrer. d. Use the buret clamp to attach the

buret to the ring stand.

2. Make sure the stopcock on the buret is off.

3. With a funnel, fill the buret with approximately 50 mL of AgNO3 solution (titrant).

4. Drain a small amount of the titrant into the waste beaker to remove any air in the tip of the buret.

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Clamp a buret to the ring stand and fill the buret with 50 mL of silver nitrate solution. Record the initial buet volume.

Add 50.0 mL of sample solution, dextrin, and the indicator to the flask. Place the flask under the buret filled with silver nitrate.

Stop adding titrant when the solution turns pink.

Slowly add titrant to the flask.

Record the final buret volume in the data table. Use the results to calculate the concentration of chloride ion in the sample solution.

Buret

Waste container

Magnetic stirrer

Double Replacement Reactions5. Why is it important to remove air from the tip of the buret?

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6. Place a micro stir bar in a clean 125-mL Erlenmeyer flask.

7. Using the 50-mL graduated cylinder, measure 50.0 mL of sodium chloride known solution (0.010 M) and add it to the Erlenmeyer flask.

8. Why is a solution with a known concentration being tested first?

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9. Using the same 50-mL graduated cylinder, measure 25 mL of 1% dextrin solution and add it to the Erlenmeyer flask containing the sodium chloride solution.

10. Using a transfer pipet, add 1 or 2 drops of the fluorescein indicator to the solution in the flask.

Collect Data

11. Read the initial volume of the AgNO3 solution that is in the buret. Record this value in Table 2 below, in the NaCl Known, Trial 1 column (record within 0.01 mL).

Table 2: Initial and final buret readings

NaCl Known (0.010 M) Swimming Pool Water

Trial 1 Trial 2 Trial 1 Trial 2

Initial buretvolume

(mL)

Final buretvolume

(mL)

12. Position the flask containing the sodium chloride solution on the magnetic stirrer.

13. Turn on the magnetic stirrer and begin stirring at a slow to medium speed.

14. Why is it necessary to stir the solution during a titration?

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Student Inquiry Worksheet___________________________________________________________________________________________

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15. Open the stopcock on the buret carefully, allowing the titrant to drip about 2 or 3 per second into the sodium chloride solution.

16. Reduce the flow of the titrant when the solution becomes cloudy.

Note: The solution becoming cloudy occurs when the solution begins to precipitate.

17. Continue to add titrant slowly until the solution turns pink. When the solution turns pink, close the stopcock.

18. ¨ What substance is being formed in the beaker? What type of reaction is occurring?

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19. ¨ What equation represents the chemical reaction that is occurring in the beaker?

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20. Record the final volume of AgNO3 that is in the buret. Record this value in Table 2 above, in the NaCl Known, Trial 1 column (record within 0.01 mL).

21. Turn off the magnetic stirrer.

22. Remove the flask and dispose of its contents according to your teacher’s instructions.

23. Clean the flask and the magnetic stir bar with distilled water.

24. For "NaCl Known Trial 2," repeat the titration so you can record the initial and final volumes in the buret of the titrant AgNO3. Refer to the process above for details.

25. For "Swimming Pool Water Trial 1," repeat the titration so you can record the initial and final volumes in the buret of the titrant AgNO3. Refer to the process above for details.

26. For "Swimming Pool Water Trial 2," repeat the titration so you can record the initial and final volumes in the buret of the titrant AgNO3. Refer to the process above for details.

27. Clean your lab station according to your teacher’s instructions, especially those regarding unused and waste chemicals.

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Double Replacement Reactions

Data Analysis

1. Determine the volume of titrant added to the flask by subtracting the initial volume from the final volume. Record this value in the Table 3 below.

Table 3: Concentration of chloride ions

NaCl Known (0.010 M)

Swimming Pool Water

Trial 1 Trial 2 Trial 1 Trial 2

Total silver nitrate added, final minus initial (mL)

Chloride ion in flask (mg)

Chloride ion in 1000 mL of sample (mg/L)

Average chloride ion concentration (mg/L)

2. Determine the amount of chloride ion in each flask using the following hint. Record the values in Table 3 above. Show your work for the NaCl Known, Trial 1, below.

Hint:

3. Determine the concentration of chloride in each flask by dividing the amount of Cl– by the volume of the water sample (50.0 mL). You will also have to convert from milliliters to liters. Record the values in Table 3 above. Show your work for the NaCl Known, Trial 1, below.

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Student Inquiry Worksheet4. Calculate the average concentration of Cl– for the different water samples by

adding the values of the two trials and dividing by the number of trials (2). Record this value in Table 3 above. Show your work for the NaCl Known, Trials 1 and 2, below.

5. The chloride concentration for the NaCl Known sample is 355 mg/L. Using your calculated average for the NaCl Known, report your percent error for this experiment.

Analysis Questions

1. For the titration of the swimming pool water chloride, what are the equations?

a. Molecular equation:

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2. What is it likelihood that the amount of silver ion and chloride ion is exactly equal at the point at which the solution turns pink and the titration is stopped (equivalence point)? Explain your reasoning.

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Double Replacement Reactions3. If more silver nitrate is added after the indicator changes color, what effect would this have on the calculated results for the chloride ion concentration in the sample?

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Synthesis QuestionsUse available resources to help you answer the following questions.

1. Based on the results for both the sodium chloride known and the swimming pool water chloride, what are their concentrations converted into parts per million (ppm)?

Known:

Pool water:

2. Based on the results for both the sodium chloride known and the swimming pool water chloride, what are their concentrations converted into units of molality?

Known:

Pool water:

3. What is the source of chloride ions in the swimming pool water? Is it the same as the chlorine used to kill bacteria?

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Student Inquiry Worksheet4. Why are multiple trials of the same sample analyzed and then averaged together? What effect do multiple trials have on the precision of the results?

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5. What is the maximum chloride ion concentration that can be determined by one buret (50.00 mL) of the silver nitrate solution? How would you analyze a sample that had a greater chloride concentration?

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Multiple Choice QuestionsSelect the best answer or completion to each of the questions or incomplete statements below.

1. Which of the following reactions represents a double replacement?

A. 4Fe + 3O2 → 2Fe2O3

B. 2NaHCO3 → Na2CO3 + H2O + CO2

C. Pb(C2H3O2)2 + Na2CrO4 → 2NaC2H3O2 + PbCrO4

D. CuSO4 + Zn → Cu + ZnSO4

2. Which of the following compounds is soluble in water?

A. CaCO3

B. CaI2

C. AgID. PbI2

3. Which of the following compounds is not soluble in water?

A. LiOHB. FePO4

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Double Replacement ReactionsC. (NH4)2SD. NiSO4

4. Which two reactants form a precipitate when mixed in solution?

A. 2NH4OH + BaI2 → 2NH4I + Ba(OH)2

B. Li2S + Ca(C2H3O2)2 → 2LiC2H3O2 + CaSC. 3KCl + FeI3 → 3KI + FeCl3D. Mg(OH)2 + Na2SO4 → 2NaOH + MgSO4

5. Which of the following reactants forms a precipitate when mixed with NaCl in solution?

A. (NH4)2SO4 B. Pb(NO3)2

C. Fe2(OH)3

D. None of the above

Key Term ChallengeFill in the blanks from the list of words in the Key Term Challenge Word Bank.

1. Substances that dissolve have ____________ attractions with solvent particles than with themselves. Substances that do not dissolve are ____________. The solubility rules assist chemists in predicting if a substance dissolves in water. For example, any compound having Na+, K+, Li+, NH4+, C2H3O2

–, or NO3– ions is ____________. There are no

exceptions for these components. For other components, some exceptions apply. For instance, all ____________ are soluble except those of Ag+, Hg22+, and Pb2+.

2. In a ________________________ reaction, the reactants switch anions to form new products. The reaction takes place only if one of the resulting products is insoluble. The insoluble product formed is called a ________________________. Knowing the solubility rules assist in predicting the products in a double replacement reaction. By using a ________________________ equation, only those ions and products undergoing a change in the reaction are shown in the written chemical equation.

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Student Inquiry Worksheet

Key Term Challenge Word BankParagraph 1 Paragraph 2carbonates

dissolve

halides

insoluble

soluble

stronger

sulfates

sulfides

weaker

combination

decomposition

double replacement

ionic

molecular

net ionic

precipitate

single replacement

solute

solvent

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