Laboratory Experiment pubs.acs.org/jchemeduc
Following Precipitation Reactions with Conductivity Measurements Zeynep Eslek and Aysen Tulpar* Division of Chemistry, Dogus University, Istanbul 34722, Turkey S Supporting Information *
ABSTRACT: This general chemistry laboratory exercise is based on the analysis of precipitation reactions via conductivity. In the first part of the experiment, the aim is to teach students how to prepare solutions from a solid (Na2CO3) and by dilution from a stock solution [Cu(NO3)2]. In the second part of the experiment, the students use the solutions to perform precipitation reactions: (1) Cu(NO3)2 (aq) + Na2CO3 (aq) and (2) Cu(NO3)2 (aq) + 2 NaOH (aq). Observations are made on the appearance of the solutions and the precipitates. Conductivities of the solutions are measured at each step of the experiment. For the laboratory report, students perform stoichiometric calculations to determine the theoretical conductivities of the reaction mixture. The students also calculate hypothetical conductivities of the mixture in the case of no precipitation reaction and compare them with the measured data. Most of the students conclude that the number and concentration of free ions decrease during these precipitation reactions. Overall, the students learn how to prepare solutions and gain experience in doing conductivity measurements and stoichiometric calculations. KEYWORDS: First-Year Undergraduate/General, Physical Chemistry, Laboratory Instruction, Collaborative/Cooperative Learning, Hands-On Learning/Manipulatives, Aqueous Solution Chemistry, Conductivity, Precipitation/Solubility, Solutions/Solvents, Stochiometry
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solutions and the precipitates) before, during, and after precipitation. After the lab, students perform stoichiometric calculations to determine the concentration of species remaining in solution after the precipitation reaction, and using the appropriate molar conductivity values for those species, they determine the theoretical conductivity of solutions following the precipitation reaction. The students also predict the conductivity of the reaction mixture if there were no precipitation reaction, assuming that the conductivities are additive. This analysis leads to the conclusion that the solution conductivity decreases in these precipitation reactions. Two precipitation reactions are studied
recipitation reactions have been used in general chemistry laboratories1 to teach solubility rules,2 the stoichiometry of reactions, and the concepts of the limiting reactant, percent yield, and equilibrium.3 The commonly employed method for quantitative investigation of precipitation reactions is to determine the mass of dried precipitates. For cases where drying time is a constraint, we offer a new approach for investigating precipitation reactions via conductivity measurements. Conductivity measurements are ideal for studying precipitation reactions as only the free ions in solution will elicit a response. Conductimetric titrations using precipitation reactions have been used to determine the concentrations of species in solution in general chemistry laboratories.4 The measured conductivity minimum is used to determine the stoichiometric point. However, there is no formal calculation of conductivity of reaction mixtures in those experiments. We propose an experiment that analyzes a single point on the titration curve and provides a formal calculation of conductivity. Students compare the conductivities of solutions before and after a precipitation reaction using their conductivity data and their calculated conductivity values. A drawback for conductivity measurements may be the cost of providing an instrument for each student pair.5 In our lab, a single portable conductivity meter was used to perform all the measurements for 8 student pairs. There are two major aims of this experiment during the lab period: (1) to teach students how to prepare solutions (from a solid and by dilution from stock solution)6 and (2) to encourage students to make observations (the color of © 2013 American Chemical Society and Division of Chemical Education, Inc.
Cu(NO3)2 (aq) + Na 2CO3(aq) → 2NaNO3(aq) + CuCO3(s) (1)
Cu(NO3)2 (aq) + 2NaOH(aq) → 2NaNO3(aq) + Cu(OH)2 (s) (2)
For the first reaction, the amounts of the reactants are equal, so the reaction is stoichiometric. For the second reaction, Cu(NO3)2 is the limiting reagent.
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PRELABORATORY EXERCISES This is the first experiment of the semester; therefore, one week before this experiment, students go through a safety training and watch a demonstration performed by the instructors in the laboratory for a total of two hours. The details of the prelaboratory exercises are explained elsewhere.6 Published: November 20, 2013 1668
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Laboratory Experiment
EXPERIMENTAL PROCEDURE
also show the students a mixture for the reaction of eq 1 and a mixture for the reaction of eq 2 that have been standing for 15 days so that they can observe the greenish CuCO37 and the black CuO, respectively. The color of the solutions and the precipitates is the most well liked part of the experiment.
Overview
This laboratory exercise takes two hours of laboratory time. The students work in pairs. Each student pair prepares two solutions at the same concentrationone Na2CO3 solution from solid, one Cu(NO3)2 solution by dilution from 2.0 M Cu(NO3)2 solutionand measures the conductivities of these solutions before and after mixing the two solutions (eq 1). Each student pair also mixes the Cu(NO3)2 solution with a 0.25 M NaOH solution already prepared by the instructors (eq 2) and measures the conductivity. All conductivity measurements are normalized to a standard temperature of 25 °C. Each student uses his or her group’s data and observations to turn in a formal laboratory report in one week.
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EXPERIMENTAL DATA AND ANALYSIS The experimental conductivity data for the first reaction (eq 1) are listed in Table 1. For analysis, the students are asked to Table 1. Experimental Conductivity Data for Na2CO3 and Cu(NO3)2 Solutions and Their Mixture Concentration/ M 0.020 0.040 0.060 0.080 0.100 0.120
Apparatus and Materials
Portable conductivity meter, balance (±0.001g), volumetric flask (100 mL), pipet (5 mL), graduated cylinder (50 mL), pipet bulb, funnel, stirring rod, 5 beakers (100 mL), 0.25 M sodium hydroxide (NaOH) solution, 2.0 M copper(II) nitrate [Cu(NO3)2] solution, sodium carbonate (Na2CO3), and deionized water.
κNa2CO3/ (mS/cm) 4.1 7.3 10.0 13.1 14.9 17.8
± ± ± ± ± ±
0.5 0.4 0.3 0.7 0.5 0.3
κCu(NO3)2/ (mS/cm) 3.3 6.3 9.4 13.2 14.1 17.0
± ± ± ± ± ±
0.1 0.4 0.2 1.0 0.5 0.2
κafter rxn/ (mS/cm) 2.2 4.1 5.8 7.6 9.3 11.4
± ± ± ± ± ±
0.1 0.2 0.1 0.2 0.4 0.3
compare their experimental conductivity data of the reaction mixture with the theoretically determined values. Only the ions from NaNO3 contribute to conductivity. The theoretical conductivity of the reaction mixture is determined by the product of the molar conductivity of NaNO3 [98 mS/(cm M)] and the molarity of NaNO3 at the end of the reaction. This conductivity, κ, is calculated for the whole range of concentrations by eq 3
Preparation of Solutions
Each student group is asked to calculate the mass (in grams) of solid Na2CO3 to be weighed out to prepare 100 mL of one of the following solutions: 0.0200, 0.0400, 0.0600, 0.0800, 0.1000, and 0.1200 M Na2CO3. Each student group is also asked to calculate the volume (in milliliters) of the stock 2.0 M Cu(NO3)2 solution needed to prepare 100 mL of one of the following solutions: 0.020, 0.040, 0.060, 0.080, 0.10, and 0.12 M Cu(NO3)2. Students follow the procedures in the laboratory handout to prepare the solutions in 100 mL volumetric flasks and measure the conductivity of the resulting solution using the conductivity meter.
κ = 98
mS × [NaNO3] cm M
(3)
Equation 3 assumes that conductivity of NaNO3 is directly proportional to concentration and, therefore, the molar conductivity is constant within the concentration range of the experiment. The molarity of NaNO3 in eq 3 is determined using stoichiometric calculations as shown in eq 4
Precipitation Reactions
Students mix 40 mL of Cu(NO3)2 solution with 40 mL of Na2CO3 solution in a 100 mL beaker and measure the conductivity of the resulting mixture. Students also mix 40 mL of Cu(NO3)2 solution with 40 mL of 0.25 M NaOH solution and measure the conductivity of the resulting mixture. The NaOH solution is readily available for students in order to save time.
[NaNO3] =
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=
HAZARDS Skin protection is necessary while using NaOH and Cu(NO3)2 solutions. Sodium hydroxide is caustic and causes burns to any area of contact. Leftover Cu(NO3)2 solutions and all mixtures are collected in waste containers at the end of the experiment. The Cu(NO3)2 solution is precipitated using Na2CO3 and reused for other purposes. The mixtures are filtered and the solid is collected for reuse for other purposes. Leftover solutions of NaOH and Na2CO3 go down the drain.
[Cu(NO3)2 ] × VCu(NO3)2 × 2 VCu(NO3)2 + VNa 2CO3 [Na 2CO3] × VNa 2CO3 × 2 VCu(NO3)2 + VNa 2CO3
(4)
where the values of [Cu(NO3)2] and [Na2CO3] are different for each student group and are listed in Table 1 in the concentration column, the factor 2 is the stoichiometric coefficient of NaNO3 in eq 1, and VCu(NO3)2 and VNa2CO3 are the volumes of Cu(NO3)2 and Na2CO3 solutions, respectively, each being equal to 40 mL. The students also calculate a hypothetical conductivity value for the mixture if there were no precipitation reaction. They use their measured conductivity values for Cu(NO3)2 and Na2CO3 solutions and assume that the conductivity is directly proportional to concentration and is additive (eq 5) κCu(NO3)2 κ Na 2CO3 κ= + (5) 2 2 The same amount of each solution (40 mL) is used, so the molarity of each compound decreases by half in the final mixture. Because conductivity is directly proportional to
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OBSERVATIONS Students are asked to note down the color of the solutions and the precipitates. For the preparation of Cu(NO3)2 solutions by dilution of 2.0 M Cu(NO3)2, the students are expected to notice the effect of dilution on the color of Cu(NO3)2 solution. For precipitation reactions of eq 1 and eq 2, the students can clearly see the turquoise color of solid CuCO3 and the blue Cu(OH)2, respectively, and the remaining clear solution. We 1669
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Laboratory Experiment
column, the factor 2 is the stoichiometric coefficient of NaOH in eq 2, and VCu(NO3)2 and VNaOH are the volumes Cu(NO3)2 and NaOH solutions, respectively, each being equal to 40 mL. The students use eq 5 to determine the hypothetical conductivity of the mixture in the case of no precipitation reaction, but this time, they replace Na2CO3 with NaOH.
concentration, the conductivity of each compound also decreases by half, as seen in eq 5. Table 2 reports the calculated conductivity in case of no precipitation reaction as eq 5. Table 2. Comparison of the Experimental Conductivity Data with the Calculated Values for Mixtures of Cu(NO3)2 and Na2CO3 Solutions Concentration/ M 0.020 0.040 0.060 0.080 0.100 0.120
κafter rxn/ (mS/cm)
Eq 3a/ (mS/cm)
Percent Error (%)
± ± ± ± ± ±
2.0 3.9 5.9 7.8 9.8 11.8
10 5 2 3 5 3
2.2 4.1 5.8 7.6 9.3 11.4
0.1 0.2 0.1 0.2 0.4 0.3
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DISCUSSION Students discuss the possible sources of error in their lab reports. Most of them mention the uncertainty of volume, mass, and conductivity measurements. A few of them mention the assumptions made during the theoretical determination of conductivity values. The students also discuss the reason why their calculated values from eq 5 are larger than the measured conductivity data for the precipitation reactions. The answer of 80% of the students is that the number of free ions decrease during these precipitation reactions.
Eq 5b/ (mS/cm) 3.7 6.8 9.7 13.2 14.5 17.4
± ± ± ± ± ±
0.3 0.3 0.2 0.6 0.4 0.2
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a
The calculated conductivity after the reaction. bThe calculated conductivity in the case of no precipitation reaction.
SUMMARY During the lab period, students learn how to prepare solutions, and then by mixing these solutions, they observe precipitate formation. The choice of the chemicals is such that colorful precipitates form and the remaining solution is clear. To follow the precipitation reactions, the conductivity of solutions is measured before and after the reaction. While writing the lab report, students learn to write and practice writing balanced equations, to perform stoichiometric calculations, and to calculate conductivity values for the reaction mixtures.
Table 3 shows both the theoretical and experimental conductivity data for the mixture of Cu(NO3)2 and 0.25 M Table 3. Comparison of the Experimental Conductivity Data with the Calculated Values for Mixtures of Cu(NO3)2 and 0.25 M NaOH Solutions Concentration/ M 0.020 0.040 0.060 0.080 0.100 0.120
κafter rxn/ (mS/cm)
Eq 6a/ (mS/cm)
Percent Error (%)
± ± ± ± ± ±
24.2 21.9 19.7 17.4 15.1 12.8
4.1 3 12 5 9.3 2
25.2 22.5 22.1 18.3 16.5 13.0
0.4 1.0 0.9 1.3 1.8 0.8
Eq 5b/ (mS/cm) 28.2 29.7 31.2 33.1 33.6 35.0
± ± ± ± ± ±
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1.0 1.0 1.0 1.1 1.0 1.0
* Supporting Information
Instructor notes and student handout. This material is available via the Internet at http://pubs.acs.org.
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a
The calculated conductivity after the reaction. bThe calculated conductivity in the case of no precipitation reaction.
*Corresponding Author E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Ricci, R. W.; Ditzler, M. A. Discovery Chemistry: A LaboratoryCentered Approach to Teaching General Chemistry. J. Chem. Educ. 1991, 68, 228−231. (2) Blake, B. Solubility Rules: Three Suggestions for Improved Understanding. J. Chem. Educ. 2003, 80, 1348−1350. (3) DeMeo, S. Using Limiting-Excess Stoichiometry to Introduce Equilibrium Calculations: A Discrepant Event Laboratory Activity Involving Precipitation Reactions. J. Chem. Educ. 2002, 79, 474−475. (4) Randall, J. Advanced Chemistry with Vernier; Vernier Software & Technology: Beaverton, OR, 2004; p 16−1. (5) Baksa, K. Advanced Chemistry with Vernier (Jack Randall). J. Chem. Educ. 2007, 84, 1611. (6) Eslek, Z.; Tulpar, A. Solution Preparation and Conductivity Measurements: An Experiment for Introductory Chemistry. J. Chem. Educ. 2013, No. 10.1021/ed300593t. (7) The students are asked where they have seen this compound in daily life and some of them mentioned old copper Turkish coffee pots and some mentioned the Statue of Liberty.
⎛ 53 ⎞ mS mS ⎟ × [NaNO3] + ⎜ × [NaOH]excess ⎝ 0.25 ⎠ cm M cm M (6)
The molarity of NaNO3 is determined using [Cu(NO3)2], as shown in the first half of eq 4, because Cu(NO3)2 is the limiting reagent. The molarity of excess NaOH is calculated as [NaOH]excess =
AUTHOR INFORMATION
Corresponding Author
NaOH solutions. A solution of 0.25 M NaOH with a conductivity of 53 ± 2 mS/cm is used for the entire concentration range of Cu(NO3)2 solutions. The students determine the theoretical conductivity for the mixture by accounting for the conductivities of NaNO3 and the excess reagent NaOH, as seen in eq 6. The conductivity of NaNO3 is determined by the product of the molar conductivity of NaNO3 [98 mS/(cm M)] and the molarity of NaNO 3 . The conductivity of excess NaOH is determined by the product of the molar conductivity of NaOH [(53/0.25) (mS cm−1 M−1)] and the molarity of excess NaOH. κ = 98
ASSOCIATED CONTENT
S
[NaOH]initial × VNaOH − ([Cu(NO3)2 ] × VCu(NO3)2) × 2 VCu(NO3)2 + VNaOH (7)
where [NaOH]initial is 0.25 M, [Cu(NO3)2] has a different value for each group and is listed in Table 3 in the concentration 1670
dx.doi.org/10.1021/ed300594f | J. Chem. Educ. 2013, 90, 1668−1670
