Limiting Reagent. Introduction

Transcription

1 Limiting Reagent Introduction The concept of the limiting reagent can be demonstrated by an analogy: Jerry works in the Purchasing Department at a specialty car manufacturing plant in Detroit. The plant gets an order for 100 limited edition cars for Macy s parade. Jerry purchases 100 car bodies, 100 steering wheels, and 100 tires. Sadly, Jerry gets fired. Do you know why? You probably realized that Jerry did not order enough tires to make the 100 cars required. With his order Jerry could only make 25 cars, and he would have 75 leftover car bodies and steering wheels. In this story the tires are the limiting reagent and the bodies and steering wheels are in excess. Now let s apply this concept to chemistry. Consider the balanced combustion reaction of methane gas: CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) According to the reaction one mole of methane gas will react with two moles of oxygen gas to produce one mole of carbon dioxide gas and two moles of steam. Worked example: When a reaction involves a solution, the concentration of the solution is generally reported as molarity (shown in units of molar, M, which is equal to moles of solute per liter of solution) e.g. a 1.50M sodium chloride solution has 1.50 moles of NaCl per liter of 1.50 mol NaCl solution or. Given the volume and molarity of any solution, we can calculate the L number of moles of solute present. For example, given 50.0 ml of a 1.50M sodium chloride solution, we can calculate the number of moles of NaCl present as follows: æ 1L öæ1.50 mol NaCl ö 50.0 ml ç ç = mol NaCl è1000 ml øè L ø What mass of precipitate could be produced if 10.0 ml of 1.500M aluminum chloride reacts with 10.0 ml of 1.500M sodium hydroxide? First, we write the balanced chemical equation: AlCl(aq) + NaOH(aq) Al(OH)(s) + NaCl(aq) The precipitate formed is aluminum hydroxide. In this example we must first calculate moles for each reactant, then continue with the molar ratio step, then convert moles of the precipitate back to grams. GCC CHM 151LL: Limiting Reagent GCC, 2016 page 1 of 6

2 æ 1L öæ1.500 mol AlCl ö æ1mol Al(OH) ö æ 10.0 ml ç ç è1000 ml øè L ç 78.0 g Al(OH) ö ø è 1mol AlCl ç = 1.17 g Al(OH) ø è 1mol Al(OH) ø æ 1L öæ1.500 mol NaOH ö æ 10.0 ml ç ç ç è1000 ml øè L ø è 1mol Al(OH) mol NaOH ö æ 78.0 g Al(OH) ö ø ç = 0.90 g Al(OH) è 1mol Al(OH) ø Notice that the two product amounts are very different. Even though there is enough aluminum chloride to make 1.17 grams of Al(OH) precipitate, there is only enough sodium hydroxide to make 0.90 grams of Al(OH). Once that amount of precipitate is made, we run out of sodium hydroxide. Thus, in this example, sodium hydroxide is the limiting reagent, aluminum chloride is in excess, and 0.90 grams of aluminum hydroxide precipitate can be produced. While these calculations may not be as exciting as manufacturing specialty cars, if a chemist miscalculates limiting reagent problems he or she could get fired, cause a serious accident, and (in the extreme case) kill someone due to an explosion or other serious accident! In this lab you will conduct several trials of a reaction, determine the limiting reagent and the reactant in excess for each trial, and record your observations. Procedure: In your Lab Notebook, before beginning the experiment, prepare a data table to record the following data for all six trials (A1, A2, A, B1, B2 and B) for each solution, CaCl 2 (aq) and Na 2CO (aq) : Data Table 1 (All Six Trials): Data Table 2 (Trials A1, A2, A): For both solutions include: mass of weighing paper initial volume mass of weighing paper with precipitate final volume mass of precipitate (actual yield) total volume Directly below the data table, label a space to record the exact concentrations of CaCl 2 (aq) and Na 2CO (aq) Part I. Precipitation reactions. 1. Clean and label six test tubes: A1, A2, A, B1, B2 and B. In each of the six test tubes, add approximately 4 ml of 0.5 M calcium chloride solution to the six test tubes. Record initial and final volumes for calcium chloride in the Lab Notebook. Also record the exact molarity from the reagent bottle in the Lab Notebook. Calculate total volume delivered for each test tube and record this result in the lab notebook. 2. Now, in test tubes A1 and B1, add approximately 1mL of 0.5 M sodium carbonate solution to each. In each of test tubes A2 and B2, add approximately 4 ml sodium carbonate solution. In each of test tubes A and B, add approximately 6 ml of sodium carbonate solution. Record initial and final volumes for sodium carbonate in the Lab Notebook. Also record the exact molarity from the reagent bottle in the Lab Notebook. Calculate total volume delivered for each test tube and record this result in the lab notebook.. Place each test tube in the Vortex Mixer (set to Auto/Touch; adjust the dial to a low-medium setting (a 4-5 setting on the vortex) for about 25-0 seconds. Once all six solutions have been thoroughly mixed, place them in two separate test tube racks, separated by letter (A1, A2 and A in one rack and B1, B2 GCC CHM 151LL: Limiting Reagent GCC, 2016 page 2 of 6

3 and B in a different rack). These two sets will be treated differently. Place the B rack in an area on your bench-top where it will not be disturbed. Record the time in your Lab Notebook and do not touch these test tubes ( set B ) until you are ready for Part IV (after at least an hour has passed) so any precipitates can settle well. Part II. Filtration. 1. Obtain three medium-flow filter papers and label them in pencil on their edges with a small A1, A2 and A. Weigh each filter paper and record their masses in your Lab Notebook. Obtain three 50 ml beakers and label them A1, A2 and A. 2. Set up a filtration apparatus with a plastic funnel in a ring stand. Fold and insert the filter paper A1 into the funnel and place the 50mL beaker A1 beneath the funnel to catch the filtrate.. Pour the contents of test tube A1 into the funnel, using a D.I. wash bottle to wash any remaining precipitate out of the test tube into the filter paper. When the liquid had passed through filter, wash it with about ml of D.I. water. When this liquid had passed through the filter, repeated the wash with another ml of D.I. water. SAVE the filtrate for each trial to use in Part III. 4. Remove the filter paper and carefully open it up, placing it on a watch glass. Place the watch glass on a hot plate (set to heat setting 4, or 90 C) to dry. Record the time drying started and ended in your notebook under observations. While the filter paper A1 is drying, repeat steps 2-4 above with A2. While the filter paper A2 is drying, repeat steps 2-4 with A. (You may filter all three simultaneously with three ring clamps, three funnels, and three beakers if you wish. Please dry each on a separate hot plate, though.) Note: It is a good idea to complete all of your calculations while waiting for them to dry! 5. Weigh each filter paper when dry and record its new mass in your Lab Notebook. Calculate the actual yield of precipitate for each trial and record this in your Lab Notebook. 6. Write and balance the chemical equation for the reaction in your Lab Notebook. Identify the precipitate in this reaction. Calculate the theoretical yield of each of these three trials, and then determine the percent yield for each of these trials. Determine the limiting reagent and the reagent in excess for each trial. Be sure to clearly label each trial s calculation and results. Results will be graded in your lab notebook and in your formal report! Part III. Testing the Filtrates from Set A. 1. Test the ions dissolved in the filtrate solutions by transferring a small amount of the solution from filtrate trial A1 to two different clean test tubes with a disposable pipette. Use new disposable plastic pipettes for each test to prevent cross-contamination. 2. Add about 1 ml more CaCl2 (aq) solution to the first new test tube and about 1 ml more Na2CO (aq) to the second. Record your observations in your notebook in Data & Results Summary Table Part III (see page 4 as a reference).. Repeat steps 1-2 for the filtrates from A2 and A. Remember: use new pipettes for each test. GCC CHM 151LL: Limiting Reagent GCC, 2016 page of 6

4 Part IV. Observing the Precipitates in Set B. 1. Now, without disturbing the precipitates, examine the test tubes in test tube rack B that were set aside for at least an hour. Draw the three test tubes in your Lab Notebook, making sure to note the level of the precipitates and the level of the liquid surface in each. Observe and record the physical properties of the precipitates. Please copy the following Data and Results Summary Tables in your lab notebook. Your data should be neatly entered into your lab notebook as the experiment is carried out, not on this page. Example Data & Results Summary Table Parts I & II. Trial A1 A2 A B1 B2 B Volume of CaCl 2 (aq) Volume of Na 2CO (aq) Theoretical mass of precipitate Limiting reagent Reagent in excess Actual mass of precipitate Percent yield Example Data & Results Summary Table Part III. Trial A1 A2 A Predicted reagent in excess Observation after adding CaCl 2 (aq) Observation after adding Na 2CO (aq) Finally, complete the Data Page and Postlab Questions, page 6. GCC CHM 151LL: Limiting Reagent GCC, 2016 page 4 of 6

5 Name: Section: Pre-Lab Questions Write your answers on this page and turn it in to your instructor before starting this experiment. 1. Betty Joe Beaker is making little stick figure donkeys for her daughter s birthday party. Each donkey needs one head, one body, two ears, and four legs. Draw a stick figure donkey here How many donkeys can she make with 28 heads, 24 bodies, 44 ears and 68 legs? The limiting body part is The body parts in excess are How many of each excess piece will be left over? 2. Complete the beaker drawings for the two reagents used in this week s Limiting Reagent Experiment. Formula Reagent 1: Formula Reagent 2:. Write the balanced chemical reaction for the reagents used in this week s Limiting Reagent Experiment. Molecular Equation: Total Ionic Equation: Net Ionic Equation: GCC CHM 151LL: Limiting Reagent GCC, 2016 page 5 of 6

6 Name: Section: Discussion Questions Write your answers on these pages and attach them to your report. 1. (6 pts) Consider the following reaction: V 2O 5 (s) + 2NaOH (l) D ¾¾ 2NaVO (s) + H 2O (g) red colorless yellow colorless solid liquid solid gas When the red solid is mixed with the colorless liquid and heated, the mixture starts to give off a gas and change color. When the reaction has stopped, the result is a dry, yellow solid containing small, red particles. Which substance is the limiting reactant? Explain how you determined this. 2. (12 pts) In the beakers below, using your calculation results, draw what the filtrate solutions contained for each trial. Show the precipitate and the excess reagents in the beakers. (Recall how we showed solution contents in the pictures from the Chemical Reactions Lab.) Trial A1 Trial A2 Trial A. (6 pts) If sodium carbonate is the limiting reagent and calcium chloride is in excess: a. Which ions are present in the supernatant? b. What would happen if you added CaCl 2 (aq) to the supernatant? c. What would happen if you added Na 2CO (aq) to the supernatant? 4. (6 pts) If calcium chloride is the limiting reagent and sodium carbonate is in excess: a. Which ions are present in the supernatant? b. What would happen if you added CaCl 2 (aq) to the supernatant? c. What would happen if you added Na 2CO (aq) to the supernatant? GCC CHM 151LL: Limiting Reagent GCC, 2016 page 6 of 6