LIMITING REAGENT LAB: THE REACTION BETWEEN VINEGAR AND BAKING SODA Goal: During this lab students will gain a quantitative understanding of limiting reagents. Safety: Safety goggles should be worn at all times. Background information: Ever notice how hot dogs are sold in packages of 10 while the buns come in packages of 8? What s up with that?! The bun is the limiting reactant and limits the hot dog production to 8 as well! The limiting reactant [or reagent] is the one consumed entirely in the chemical reaction and therefore limits the amount of product that can be produced. Let s use a famous process, the Haber process. This reaction is essentially making ammonia for fertilizer production from the nitrogen in the air reacted with hydrogen gas. The hydrogen gas is obtained from the reaction of methane with water vapor. This process has saved millions from starvation!! The reaction is shown below. N2 + 3H2 2NH3 If we look at the chemical equation we see that the combining molar ratio of nitrogen and hydrogen is 1:3 respectively. What this means is that for every 1 mole of nitrogen gas we, in turn, need 3 moles of hydrogen gas for each to completely react. Anything other than this ratio will give us both a limiting reactant as well as a limiting reactant Notice in situation 1, we have 15 moles of H2 and 5 moles of N2 (assume each molecule is 1 mole of substance). Based on the balanced chemical equation this will give us our desired 3:1 ratio of hydrogen to nitrogen and therefore we do not have a limiting reactant because we have a perfect molar ratio. However, in situation 2 we 1
have 9 moles of hydrogen and 5 moles of nitrogen (a 1:1.8 ratio respectively). This molar ratio is different from the one needed in the balanced chemical equation and therefore we will have both a limiting reactant as well as an excess reactant. Knowing that 1 mole of nitrogen reacts with 3 moles of hydrogen it is easy to see that I have do not have enough hydrogen to react with all of the nitrogen present, which is why I still have unreacted nitrogen after the reaction. Therefore, hydrogen will be my limiting reactant because it will run out before all of the nitrogen reacts and thus will limit the amount of NH3 that can be produced. Pre-Lab: 1. Consider the following reaction: Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) (a) What is the combining mole ratio of Mg to HCl? (b) Suppose this reaction is performed similarly to the acetic acid/baking soda reaction. A volume of HCl is put in the test tube and a mass of Mg metal is added. Hydrogen gas is produced and released. Complete the following table: (There are 0.0100 mol of HCl in 10.0 ml of HCl; Mg = 24.3 g/mole). Test Tube # Mg (grams) Moles of Mg Volume of HCl, ml Moles of HCl Mole ratio Mg:HCl Excess Reagent Limiting Reagent H2 released 1 0.122 10.0 2 0.500 10.0 3 0.100 10.0 Show one set of sample calculations you did to fill out the table in the space below: 2. Before mixing the chemicals in the erlenmeyer, make a prediction about which combination will produce the greatest amount of carbon dioxide gas. The reaction occurring is as follows: NaHCO3 (aq) + CH3COOH (aq) ----> CO2 (g) + H2O (l) + NaCH3COO (aq) 2
Prediction: Procedure 3. Students need to get together in teams of three to four students. 4. Students need to follow the directions below: Step 1: You will have six empty erlenmeyer flasks. Label each flask 1-6 and then record the mass of each flask empty and record the masses in your data table Step 2: Weigh the following six amounts of baking soda (sodium bicarbonate, NaHCO3): 1.8 grams, 3.5 grams, 5.2 grams, 7.0 grams, 10.0 grams, and 17.0 grams into an erlenmeyer flask making sure to record the mass for the correct erlenmeyer in the right spot in the data table Step 3: Step 4: Step 5: Obtain an empty, dry graduated cylinder and record the mass. Record the mass in your data table Using a graduated cylinder, accurately measure out 100.0 ml vinegar (0.83M acetic acid, HC2H3O2), take the mass of the graduated cylinder with the acetic acid and record the mass in your data table. Pour the 100 ml of acetic acid into the first erlenmeyer flask and mix the contents by swirling. You should see carbon dioxide gas being produced as the reaction proceeds (bubbles). When the reaction stops bubbling we can assume the reaction is complete. Time saving trick: Repeat step 4 and 5 for each of the erlenmeyer flasks that you have labeled before the first one finishes so you are not waiting on reactions. Step 5: Once the reaction has finished for each trial, weigh the Erlenmeyer and record the mass after the reaction. Data Table: Empty flask (g) flask and baking soda (g) baking soda (g) empty graduated cylinder (g) graduated cylinder and acetic acid (g) mixture before reaction (g) Final mass of mixture after reaction is complete (g) 3
Postlab: Answer the following about the lab and data you collected: 1. Fill out the following table completely using the data for each trial for each trial. baking soda added Moles of Baking soda added Moles of acetic acid added Mole ratio Limiting reactant Show a sample of one of your calculations performed in the table in the space provided below: 2. Explain why in trial 6, where we added the most baking soda, we did not get the most carbon dioxide produced. 3. Please fill out the following table using data collected during the lab: mixture before mixture after reaction (g) reaction Total mass of CO2 lost 4
4. On the axis shown, please graph the moles of baking soda added (x-axis) versus the mass of CO2 lost (yaxis) for each trial and to the right explain the shape of the curve 5. Using the limiting reactant for each trial, calculate the expected (theoretically) mass of carbon dioxide we expected to lose for each trial. Limiting Reactant Moles of Limiting reactant present Theoretical mass of CO2 expected to lose Actual mass of CO2 lost Show a sample of one of your calculations performed in the table in the space provided below: 5
Exp. 15 7