Introduction to Stoichiometry Quiz & Post Lab Life Sciences Lab Report

Please watch these two files: lab video and the experiment 7 Lab manual and finish the Stoichiometry post-lab. (just answer the question on the file, please do not write a paper) Make sure you can open the video, and if you have any questions, please let me know immediately.

57 Experiment 7. Introduction to Stoichiometry Terms: • Acidic • Actual yield • Aliquot • Alkaline/basic • Excess reactant • Limiting reactant • Stoichiometry • Theoretical yield Introduction A balanced chemical equation gives you lots of very useful information if you’re trying to make a chemical product. Stoichiometry tells you the ratios between the number of moles of each reactant consumed in the reaction and the number of moles of product made by the reaction. Using stoichiometry, you can plan an experiment so that no reactants are left over at the end of the reaction. This information is valuable to manufacturers when their reactants are extremely expensive and they don’t want any of them to be left over. Here’s an analogy to help you understand stoichiometry. If you want to make one table, you need four table legs and one tabletop. You could have 8 table legs:2 table tops or 40 table legs:10 table tops but in both cases because the ratio is 4 table legs:1 table top, there would be no table tops or table legs remaining after all possible tables are made. A balanced chemical equation follows the same rule; if the stoichiometry or ratio between the moles of both reactants is the same as in the balanced chemical equation, then there will be no reactants left over after the product is made. If at the end of a chemical reaction, a portion of one of the reactants remains (didn’t react), it is called the excess reactant. In our table analogy, if we had 8 legs and one table top, 4 legs would be used up to make a table and 4 more will remain as “excess reactant”. We could not make 2 tables, even though we have plenty of table legs, but we have only one table top. So, in this case table top is the “limiting reactant”. The reactant which is completely consumed in the reaction is called the limiting reactant because it limits how much product is made. Today’s Reaction EQUATION 1: NaHCO3 (s) + HC2H3O2 (aq) à NaC2H3O2 (aq) + H2O (l) + CO2 (g) In this reaction, sodium bicarbonate (baking soda) reacts with aqueous acetic acid (vinegar). The balanced chemical equation shows the production of one mole of carbon dioxide gas from one mole of sodium bicarbonate and one mole of acetic acid. In this case, the stoichiometric ratio of sodium bicarbonate to acetic acid is 1:1 and no reactants will be leftover 58 at the end of the reaction. One mole of carbon dioxide would be the theoretical yield. The theoretical yield is the maximum amount of product which can be expected based on the starting amounts for the reactants. How to Identify the Limiting and Excess Reactant What would happen if a greater number of moles of acetic acid were used for the reaction but there was still only one mole of the sodium bicarbonate reagent? In this case the stoichiometric ratio would be greater than 1 mole HC2H3O2: 1 mole of NaHCO3 reactant. Would you get more product? No, you would get the same one mole of product because the NaHCO3 reactant is limiting the amount of product made. One mole of the NaHCO3 reactant would react with one mole of HC2H3O and the rest of the HC2H3O would remain at the end of the reaction. Because the amount of the NaHCO3 reagent is “limiting” how much product is formed it would be called the limiting reactant and the HC2H3O would be called the excess reactant because at the end of the reaction, there’s unreacted HC2H3O still present. Figure below shows how to identify the limiting and excess reactants using the stoichiometric ratio between the reactants. Test Your Understanding CaCO3(s) + 2 HCl(aq) à H2O(l) + CO2(g) + CaCl2(aq) a) For the reaction above, what would be the stoichiometric ratio between the reactants? b) If there were 4 moles of calcium carbonate and 10 moles of HCl, what would be the limiting reactant? c) What would be the theoretical yield of carbon dioxide gas in moles if the reaction began with 4 moles of calcium carbonate and 10 moles of hydrochloric acid? d) In addition to the products of the reaction, which reactant would still be present in the reaction beaker after the reaction is complete? Moles HC2H3O reactant: Moles NaHCO3 reactant 1:1 <1:1 >1:1 HC2H3O2 (numerator) is the limiting reactant and NaHCO3 (denominator) is the excess reactant. NaHCO3 (denominator) is the limiting reactant and HC2H3O2 (numerator) is No limiting or the excess reactant. excess reactant. 59 Answers to Test Your Understanding: a) The stoichiometric ratio would be 1 mole calcium carbonate:2 moles of hydrochloric acid. NOTE: Would it be correct to reverse the ratio and say 2 mole HCl:1 mole CaCO3? Yes. What’s important is that you always keep the two reactants in the same location (numerator or denominator) in the ratio. b) 4 moles of calcium carbonate to 10 moles of hydrochloric acid gives a stoichiometric ratio of 0.25 which is lower than the 1:2 (or 0.5) ratio for the balanced chemical equation. This indicates that the calcium carbonate (the numerator) would be the limiting reactant. c) According to the balanced chemical reaction, for every 1 mole of calcium carbonate, 1 mole of carbon dioxide gas is produced. Because calcium carbonate is the limiting reactant and the reaction started with 4 moles of calcium carbonate, the maximum amount of carbon dioxide gas which could be made is 4 moles. d) All of the calcium carbonate would have been consumed in the reaction because it is the limiting reactant. Some of the hydrochloric acid would remain after the reaction is complete since it was the excess reactant and not all of it reacted. Steps to identify the limiting and excess reactants for a reaction Sample Problem: Ethylene reacts with oxygen to produce carbon dioxide and water (as a gas). If you started with 4.95 grams of ethylene and 3.25 grams of oxygen, what would be the limiting reactant? 1. Write the balanced chemical equation. When balanced, this equation says that 1 mole of ethylene (C2H4) and 3 moles of oxygen gas react and produce 2 moles of carbon dioxide and 2 moles of steam. EQUATION 2: C2H4(g) + 3O2 (g) à 2CO2(g) + 2H2O (g) 2. Determine the number of moles present for each reactant using the molar mass to convert grams into moles. The molar mass of C2H4 is (2 * 12.011) + (4 * 1.008) = 28.054 g/mole and the molar mass for O2 is 15.999 X 2 = 31.998 g/mole. Ethylene: 4.95 g C2H4 * (1 mol C2H4/28.054 g) = 1.76 X 10-1 mol Oxygen: 3.25 g O2 * (1 mol/31.998 g) = 1.02 X 10-1 mol 3. Find the stoichiometric ratio between the two reactants from the balanced chemical equation. There are 3 mol O2: 1 mol of ethylene for 3:1 ratio. 4. Find the stoichiometric ratio between the two reactants for the number of moles available to react. Stoichiometric Ratio between reactants = 1.02 X 10-1 /1.76 X 10-1 = 0.58 60 5. Identify the limiting and excess reactants based on the ratio. Since the ratio is smaller than 3:1, the oxygen reactant (the numerator) is in the limiting reactant and ethylene (the denominator reactant) is the excess reactant. Experiment Part 1. Nuts and Bolts of Chemistry Let’s pretend that nuts (Nu) and bolts (Bt) are “atoms”, which make up a “molecule”: EQUATION 3. Nu + Bt à NuBt 1. Obtain a bag that contains a supply of nuts and bolts. 2. Later you will be assembling “molecules” by threading a nut onto a bolt. Without counting, predict whether you will run out of nuts or bolts or both. Prediction: ______________________________________________ 3. Now weigh all nuts from your supply bag; then weigh all the bolts. Mass of Nuts Available: ____________________ Mass of Bolts Available: ____________________ 4. Now assemble “molecules” using one nut and one bolt until you run out of one of the components. 5. Which substance did you run out of?___________________ 6. Was your prediction correct? _____________________ 7. Identify the limiting reactant and the excess reactant in this “reaction”. Limiting Reactant: _________________________ Excess Reactant: _________________________ 8. Is there a way to determine, before the reaction is started, which part will be the limiting reactant when you can’t count the “atoms” or see what’s happening? 9. Determine the average mass of a nut and the average mass of a bolt by weighing 10 nuts and then 5 bolts: The mass of 5 Nuts = _______________ The average mass of 1 Nut ____________ The mass of 5 Bolts = _______________ The average mass of 1 Bolt ____________ 61 10. Using the masses from step 3 and the respective average masses determine the number of nuts and the number of bolts in your supply. Do not count them….do the math. Number of Nuts Available: ____________________ Number of Bolts Available: ____________________ 11. From those numbers calculate how many BtNu molecules you should have made. Number of BtNu theoretically produced: ____________________ 12. Empty the bag and count the number of molecules you actually made. Does it match? 13. Disassemble the “molecules” and place the nuts on the and the bolts back in the bag. Part 2. Baking Soda and Vinegar Reaction 1. Wear a lab coat and your goggles for this part of the experiment. Weigh 1 gram of baking soda and place in a balloon. Now do the same for the next 3 balloons. You can re-use weighing paper for this part of the experiment. 2. Label 125 mL Erlenmeyer flasks 1 through 4. Put 2 mL of vinegar in the first flask, 10 mL into the second flask, 18 mL into the third flask and 26 mL into the fourth one. 3. Place one of the balloons over the top if the first Erlenmeyer flask. Turn the balloon over so that baking soda drops into the flask. Swirl the mixture. What happens to the balloon? 4. Repeat step 3 with the rest of the balloons and record your observations. Which balloon was inflated the least? Which one was inflated the most? Record your observations. Calculations: 1. Calculate the number of moles of HC2H3O2 used in each reaction. Remember, that vinegar is only 5% acetic acid, meaning that there are 5 g of acetic acid in 100 mL of vinegar. The molar mass of HC2H3O2 is 60.052 grams per mole. Record this in Table 1 on the Data Sheet. 2. Determine the stoichiometric ratio based on the mass of the NaHCO3 and HC2H3O2 reactants which you used for your reaction. 3. Identify the limiting and excess reactants for your reaction mixture.

Exp 7. Introduction to Stoichiometry Quiz and Post Lab

Name ___________________________

For the quiz, it is OK to look up answers in your lab manual, textbook, or other reliable sources. However, you CANNOT work on your answers with another student or copy answers from another student. That will be considered an academic integrity violation.

Pre-lab Quiz (1 point per question, total 5 points)

1. For the chemical reaction below, there are 3 moles of iron III and 3 moles of oxygen. What is the limiting reagent in this reaction?

6 Fe (s) + 3 O₂ (g) à 2 Fe₂O₃ (s)

1. Hydrogen gas and oxygen react to produce water (see the reaction below). If you have 2 moles of hydrogen and 2 moles of oxygen, how many moles of water can you make?

2 H₂ (g) + O₂ (g) à 2 H₂O (g)

1. Methane gas (CH₄) and oxygen (O₂) react to produce carbon dioxide and water. If 100 grams of both reagents were present, what would be the first step in determining the limiting reactant?

1. In your own words, what is a definition of excess reactant?

1. What is the theoretical yield?

Experimental Part

Part 1. Nuts and Bolts of Chemistry (5 points)

1. One bolt (a “reactant”) and one nut (another “reactant”), can make one bolt-nut “molecule”

Bolt + Nut à Bolt-Nut

as in this picture:

For the following “reaction” below, what would be

a)      The maximum number of molecules you could possible make?

b)      The excess “reactant”?

c)      The limiting “reactant”?

1. One bolt weighs 3 grams, one nut weighs 1 gram, and one molecule of Bolt-Nut weighs 4 grams. If you have 30 grams of bolts and 30 grams of nuts, what would be the

a)      The theoretical yield?

b)      The excess “reactant”?

c)      The limiting “reactant”?

Show your calculations to receive full credit!

Part 2. Baking Soda and Vinegar Reaction (10 points)

[For more details about this reaction and the experiment itself, please refer to your lab manual.]

Baking soda reacts with acetic acid (vinegar) to form CO₂ gas, water, and sodium acetate salt:

NaHCO₃ (s) + HC₂H₃O₂ (aq) à NaC₂H₃O₂ (aq) + H₂O (l) + CO₂ (g) 

In order to do this reaction by yourself in the lab, you would need the following:

Figure 1. Reagents and lab equipment that are necessary for stoichiometry reaction.

Then, you would have to measure 1 g of sodium bicarbonate and different volumes of vinegar:

Figure 2. Reagents are prepared for the reaction.

Then, you would have to perform the final few steps to make this (see lab manual):

Figure 3. After the reagents were mixed in Erlenmeyer flasks, sodium bicarbonate reacted with vinegar, producing CO₂ gas. Carbon dioxide gas inflated the balloons, which differ in size due to different stoichiometric ratios between baking soda and vinegar. The first flask (on your left) has the smallest amount of vinegar, while the one on the right (flask #4), has the largest.

You can also watch how this reaction is being performed in the following video:

https://www.youtube.com/watch?v=Uc4N7vrX6H4

Please keep in mind, that in video the amount of vinegar is constant, and the amount of baking soda varies. In your reaction it is the other way around – the amount of baking soda is constant; the amount of vinegar is different in each flask. But the overall principle is the same!

For your post lab, fill out the rest of the Table 1 and answer the questions below:

Table 1.  Amounts of the two reactants (shown in red) for the four reaction mixtures used to make carbon dioxide gas. Calculate the number of moles of HC₂H₃O₂ used in each reaction and then fill out the rest of the table. VERY IMPORTANT: vinegar is only 5% acetic acid, meaning that there are only 5 g of acetic acid in 100 mL of vinegar. Please use the correct number of significant figures in all your answers. For the balloon volumes, enter the relative/approximate sizes based on what you see in Figure 3.

Questions

1. (2 pts) Show how you calculated number of moles of acetic acid for flask # 4 reaction.

1.  (2 pts) What would be the theoretical yield, in grams, of sodium acetate (NaC₂H₃O₂) in reaction (flask) #4?

1. (3 pts) What would happen if you added more baking soda to flask #4? Would you produce more gas? Why or why not?

1. (3 pts) What would happen if you added more vinegar to flask #4? Would you produce more gas? Why or why not?

I have neither given nor received unauthorized help on this assignment.                                          

                                                                                                                       (Your Signature)

If you need a brief refresher on stoichiometry, here is a video from Khan Academy   https://www.youtube.com/watch?v=SjQG3rKSZUQ

And another: https://www.youtube.com/watch?v=rESzyhPOJ7I

You can also watch how the stoichiometry experiment is done here: https://www.youtube.com/watch?v=Uc4N7vrX6H4