Precipitation Reactions and Percent Yield – Background

The ability to recognize, and predict the products for a precipitation reaction are critical skills for all General Chemistry students to master. Precipitation reactions are a class of chemical reaction in which a solid product is formed by combining at least two fully soluble, aqueous solutions. They are also known as double displacement, exchange, or metathesis reactions. These secondary names are a nod to the "chemical square dance" which occurs during the course of the reaction, and that must be understood to predict the products of the reaction. Ionic compounds are composed of cations (+) and anions (-), in proportion to each other so that the total charge of the cations equals the total charge of the anions. When fully dissolved in water, the ions separate, with the appropriate stoichiometric coefficient for each ion. + 2- Na2SO4(aq) → 2Na (aq) + SO4 (aq) (1) or 2+ - Ba(NO3)2(aq) → Ba (aq) + 2NO3 (aq) (2) The stoichiometry of each dissociated ion comes from the formula of the ionic compound. The charge of the dissociated ion can be determined from the periodic table trends for the monatomic ions (Na+ and Ba2+ in equations 1 and 2), or memorization for the polyatomic ions. It is worth noting that you must recognize the presence of a polyatomic ion and understand that polyatomic ions DO NOT dissociate further into their constituent elements/ions.

When two aqueous, ionic solutions such as Na2SO4(aq) and Ba(NO3)2(aq) are combined, the dissociated cations and anions switch partners. The Na+ ion, once paired with the negatively charged sulfate, ditches the sulfate ion and pairs up with the negatively charged nitrate ion. 2- Likewise, the SO4 ion, which just lost his/her partner, now pairs up with the positively charged barium ion. Remember, cations always pair up with anions and vice versa.

(3) Once you have the ions paired with their new partners, you must write the correct chemical formula for each new ionic compound. At this point, do not concern yourself with the existing stoichiometry of any particular ion; just write the correct chemical formula that balances the charges of the cation and anion. Also, remember that the cation is always written first and the anion follows. In our example, sodium is a (1+) ion and nitrate is a (1-) ion, therefore the

Precipitation Reactions – Background B-1 correct formula for sodium nitrate is NaNO3; we need only one of each ion to balance the charges. Meanwhile, barium is a (2+) ion and sulfate is a (2-) ion. Once again, we need only one of each ion to balance the charges and the correct formula for barium sulfate is BaSO4.

Na2SO4(aq) + Ba(NO3)2(aq) → NaNO3 + BaSO4 (unbalanced) (4) Lastly, the equation must be balanced, and states of matter must be assigned to the newly predicted products. Trends of solubility predict that NaNO3 is soluble and we will assign (aq) as the state of matter. Barium sulfate is insoluble and will receive (s) as the state of matter. Balancing the equation and including the states of matter gives the final equation.

Na2SO4(aq) + Ba(NO3)2(aq) → 2NaNO3(aq) + BaSO4(s) (5) Every General Chemistry student is expected to be able to write a complete precipitation reaction when given a set of reagents. In today's experiment, you will mix aqueous solutions of cobalt(II) nitrate and sodium phosphate.

Co(NO3)2(aq) + Na3PO4(aq) → products (6) As part of your pre-lab preparation, you should complete equation 6 by predicting the products with states of matter, and balance the equation. While cobalt can take on various charges, as most transition metals do, equation 6 is not a redox reaction and the charge of the cobalt ion is consistent from reactant to product. In today's experiment, you'll learn new techniques such as vacuum and the proper use of a graduated pipet. You'll also be asked to calculate the percent yield for each of your precipitation reaction trials. Based upon the molar amounts of Co(NO3)2 and Na3PO4 used, you'll need to determine the limiting reagent, calculate the theoretical yield of the isolated precipitate, and finally calculate the percent yield for the formation of the precipitate. actual yield % yield " !100 (7) theoretical yield

Calculations: Prior to doing any calculations, you must write the complete balanced chemical equation for the precipitation reaction performed. In fact, this should be done as part of your pre-lab preparation. For each precipitation reaction trial, perform the following steps. A. Calculate the mass of dried precipitate.

B. Calculate the moles of Na3PO4 used:

* volume of Na3PO4, mL ' moles Na3PO4 " #molarity of Na3PO4, mole/L$!( % ) 1000 &

C. Calculate the moles of Co(NO3)2 used:

* volume of Co(NO3)2, mL ' moles Co(NO3)2 " #molarity of Co(NO3)2, mole/L$!( % ) 1000 &

D. Using the balanced chemical equation, convert the moles of Na3PO4 to moles of

Precipitation Reactions – Background B-2 precipitate:

* stoich. coeff. precipitate ' moles Na3PO4 !( % " moles of precipitate ) stoich. coeff. Na3PO4 &

E. Using the balanced chemical equation, convert the moles of Co(NO3)2 to moles of precipitate:

* stoich. coeff. precipitate ' moles Co(NO3)2 !( % " moles of precipitate ) stoich. coeff. Co(NO3)2 &

F. What is the limiting reagent and what is the theoretical yield (moles) of precipitate?

G. Calculate the molar mass of the precipitate: Note: The precipitate formed in this experiment is actually a hydrate! precipitate • X H2O Do some literature research to determine the most common hydrate of the precipitate, and then, based upon your answer, calculate the correct molar mass of your precipitate.

H. Convert the theoretical yield (moles) of precipitate to grams:

thoretical yield (g) " theoretical yield (moles) ! #molar mass of precipitate + X H2O, g/mole $

I. Calculate the percent yield of the reaction:

step A % yield " !100 step H

Precipitation Reactions – Background B-3 Precipitation Reactions and Percent Yield – Procedure

Purpose: In this experiment, the precipitation reaction between cobalt(II) nitrate and sodium phosphate is performed, and the percent yield of the reaction is calculated.

Procedure: Specific reaction mixtures formulated in Table 1 will be assigned to each group. In each case, a precipitate is formed upon mixing Co(NO3)2(aq) and Na3PO4(aq). The precipitate will be collected by filtration, washed, dried, and weighed.

• In those mixtures where Co(NO3)2 is the limiting reagent, the moles of precipitate will be related to the moles of Co(NO3)2 originally present, by the balanced reaction stoichiometry, and the filtrate will be colorless.

• In those mixtures where Na3PO4 is the limiting reagent, the moles of precipitate will be related to the moles of Na3PO4 originally present, by the balanced reaction stoichiometry. These mixtures will contain an excess of Co(NO3)2, and the filtrate will be pink/red due to the presence of the red cobalt(II) ion (Co2+) remaining in solution. Each group will be responsible for preparing three solutions from Table 1 as assigned by your instructor. Each group will determine the mass of precipitate formed in each case as well as the color of the filtrate (remaining solution). For each of the three solutions assigned to your group, the limiting reagent must be determined mathematically, and the percent yield of precipitate calculated. The mathematically determined limiting reagent should agree with the color of the filtrate, as described above. Table 1.

solution number mL of Na3PO4(aq) mL of Co(NO3)2(aq)

1 10.0 2.0 2 10.0 3.0 3 10.0 4.0 4 10.0 6.0 5 10.0 7.0 6 10.0 8.0 7 10.0 10.0 8 10.0 11.0 9 10.0 12.0

The Precipitation Reaction

Precipitation Reactions – Procedure P-1 Your instructor will assign a single group letter, A-L, to you and your lab partner. Each group letter has three correlating solution numbers according to the table below. For instance, group A will perform three trials of the precipitation reaction, using the quantities given in solution numbers 1, 5, and 8. group solution # A B C D E F G H I J K L (Table 1) 1 • • • • 2 • • • • 3 • • • • 4 • • • 5 • • • • • 6 • • • 7 • • • 8 • • • • • • 9 • • • • Clean three beakers (100 mL, 150 mL, or 250 mL). Label each beaker with the number of each one of your three solutions. To each of the three labeled beakers, 10.0 mL of the Na3PO4 solution, and then pipette the appropriate volume of Co(NO3)2 solution. The best way to do this is to rinse the pipette with a little of the Na3PO4 solution, then pipette the 10.0 mL into each of the beakers. Next rinse the pipette with deionized water and then with a little of the Co(NO3)2 solution. Finally, pipette the required volumes of the Co(NO3)2 solution into each beaker. Rinse the pipette with deionized water before you return it. Swirl each beaker to thoroughly mix the reagents and form the precipitate. Make sure to record in your notebook the concentrations of both the Na3PO4 and Co(NO3)2 solutions. In a , set up a vacuum filtering apparatus using your Buchner funnel, filter adapter, filtering flask, and rubber hose, and securely clamp it to a ring stand (Figure 1).

Check your filter flask for visible cracks or star cracks. If your flask is cracked, discard it and obtain a new flask from the stockroom. Attach one end of the thick-walled rubber hose to the

Precipitation Reactions – Procedure P-2 side arm of the filter flask and attach the other end to the yellow house-vacuum inlet in the fume hood. The use of vacuum, versus gravity alone, will speed the filtration and isolation of the precipitate. Obtain three pieces of Whatman #4 , sized specifically for your Buchner funnel (4.25 cm diameter), and with a pencil, label them with the numbers of your solutions. Also label three watch glasses with the numbers of your solutions. Place each piece of filter paper onto its corresponding , and record the combined mass of each watch glass/paper pair. Be certain that the balance is set to zero grams before weighing. Place one of the pieces of filter paper in the Buchner funnel and fully turn on the vacuum valve. Make sure the filter paper is sitting flat, secure, and covers all of the holes in the funnel. Pour the gelatinous precipitate from the corresponding numbered solution into the Buchner funnel. Allow the vacuum to pull through the liquid filtrate until you are left with only the solid precipitate in the Buchner funnel. Rinse the reaction beaker with no more than 5 mL of deionized water, dislodging any precipitate which remained in the beaker with your rubber policeman. Transfer this first rinse to the Buchner funnel and again allow the vacuum to pull through the liquid phase. Repeat this rinsing process two more times, for a total of three rinses. The purpose of these rinses is two-fold: 1. completely transfers the precipitate to the Buchner funnel. The yield is an important part of your grade so work carefully. 2. washes the precipitate of excess, unreacted aqueous reagent. If not removed, this will falsely add to the mass of your dried precipitate. Finish rinsing the precipitate by adding two 5 mL portions of acetone. Add the first 5 mL portion of acetone to the Buchner funnel and allow it to be pulled through before you add the second portion. Acetone is a volatile (low boiling point, evaporates easily, do not breathe the vapors) organic solvent which is soluble in water. The two acetone rinses will wash away the excess water, allowing the precipitate to dry faster and dry to the correct mass. *** Acetone is extremely flammable! *** Absolutely no use should occur during this entire experiment! Once the second portion of acetone has been pulled through, let the vacuum pull air through the sample for a minute or two. During this time you may notice the precipitate lighten in color and it may begin to crack and pull away from the walls of the funnel. Pulling air through will help to dry the precipitate and also aid in its removal from the funnel. Remove the funnel from the filter flask and turn off the vacuum. Using a spatula, pry the filter paper from the bottom of the Buchner funnel. Lift the paper and precipitate out of the funnel and place it on the correspondingly labeled watch glass. If you are careful, you can remove the paper and precipitate as an entire "cake". Scrape any remaining precipitate from the walls of the Buchner funnel and add this to your watch glass. Record the color of the filtrate (filtered solution) in your notebook. Waste Disposal: Dispose of the liquid filtrate in the appropriate liquid waste container. Repeat the entire vacuum filtration/isolation procedure for the two remaining precipitates. The filtration and isolation of all three precipitates will require approximately 60 minutes to complete. Come to lab prepared and make sure to give yourself enough time to properly isolate all three precipitates without rushing through or skipping steps. Carefully store all three watch glasses in your drawer and leaved them there for one week to air dry.

Precipitation Reactions – Procedure P-3 The Dried Precipitates Now that the precipitates have been dried, weigh the precipitate-watch glass-filter paper combinations and record the masses in your notebook. Waste Disposal: Dispose of the dried precipitate and the filter paper in the appropriate solid waste container.

Results: For each trial, report the limiting reagent, the reagent in excess, actual yield of the precipitate, theoretical yield of the precipitate, and percent yield.

Conclusion: Your conclusion must be a well written synopsis that addresses the experimental purpose and restates the results. Did the color of the filtrate agree with the calculated limiting reagent? What is the most desirable percent yield and how do your values from each trial compare? Suggest sources for possible errors that could have affected your percent yields. Include the balanced overall chemical reaction as well as the net ionic equation as part of your conclusion.

Precipitation Reactions – Procedure P-4